THE 
BAROMETER, THERMOMETER. HYGROMETER, 
AND 
ATMOSPHERIC APPEARANCES AT SEA AND ON LAND 
AS 
AIDS IN FORETELLING WEATHER, 
WITH 
BRIEF RULES FOR THEIR USE, AND THE PRACTICAL APPLICATION OF THEIR 
SEPARATE AND COMBINED INDICATIONS 
AS 
WEATHER GUIDES. 
COMPILED FROM THE LATEST AND MOST TRUSTWORTHY AUTHORITIES, 
BY 
COMMODORE THORNTON A. JENKINS, U. S. NAVY, 
CHIEF OF BUREAU OF NAVIGATION, NAVY DEPARTMENT. 
WASHINGTON: 
GOVERNMENT PRINTING OFFICE. 
1869. WEATHER GUIDES, 
THE WEATHER. 
1. Weather is the condition of the air at any time as regards heat, 
moisture, wind, rain, cloud, and electricity; 'and a change of weather 
implies a change in one or more of these atmospheric elements. 
2. Difficult as it is to fortell weather accurately, much useful foresight 
may be acquired by combining the indications of instruments (such as 
the barometer, thermometer, and hygrometer) with atmospheric ap- 
pearances. What is more varying than the aspect of the sky ? Color, 
tint of clouds, their soft or hard look, their outline, size, height, direc- 
tion, all vary rapidly, yet each is significant. There is a peculiar aspect 
of the clouds before and during westerly winds, which differs from that 
which they have previous to and during easterly winds, which is one 
only of the many curious facts connected with the differing natures of 
easterly and westerly currents of air throughout the world, which remain 
unchanged, whether they blow from sea to land or the reverse. 
3. Perhaps some of those who make much use of instruments rather 
undervalue popular knowledge, and are reluctant to admit that " a wise 
saw" may be valuable as well as a " modern instance ;" while less in- 
formed persons who use " weather glasses" unskilfully, too often draw 
from them erroneous conclusions, and then blame the barometer. 
4. Under the general head of weather must be included, as of the 
greatest interest to seamen, squalls* of wind, gales, hurricanes, and tor- 
nadoes. 
5. As a general rule squalls are preceded or accompanied or followed 
by clouds; but there is one of a very dangerous character called the 
" white squall," (of the West Indies and other regions,) which is only 
indicated by a rushing sound, and by what sailors call " white caps" 
(white wave crests.) 
6. " Williwaws" (whirl-awa ?) of the old sealers and whalers are " de 
scending squalls," and come slanting downwards off high land or from 
the upper regions of the atmosphere. They are very violently strong, and 
therefore dangerous. Vessels have been knocked down by these de- 
scending squalls before there was any previous warning of their approach. 
7. A squall cloud that can be seen through or under is not likely to 
bring or be accompanied by so much wind as a dark continued cloud 
extending beyond the horizon. How the comparative hardness or soft- 
*" First the rain and then the wind, 
Topsail sheets and halyards mind ; 
But when the wind's before the rain, 
Hoist the topsails up again." 4 
WEATHER GUIDES. 
ness of clouds foretells more or less wind or rain, may be seen by the 
following general rules: 
8. Soft-looking or delicate clouds foretell fine weather with moderate 
or light breezes ; hard-edged oily-looking clouds, wind. 
9. A dark gloomy, blue sky is windy, but a light, bright blue sky 
indicates fine weather. 
10. Generally, the softer clouds look, the less wind (but perhaps more 
rain) may be expected; and the harder, more "greasy" rolled, tutted, or 
ragged, the stronger the coming wind will prove. Also, a bright yellow 
sky at sunset presages wind; a pale yellow, wet. And thus by the 
prevalence of red, yellow, or gray tints, the coming weather may be 
foretold very nearly-indeed, if aided by meteorological instruments, 
almost exactly. 
11. Small inky-looking clouds foretell rain; a light scud driving across 
heavy clouds, wind and rain; but if alone, wind only.* 
12. High-up clouds crossing the sun, moon, or stars, in a direction 
different from that of the lower clouds, or wind then blowing, foretell a 
change of wind (beyond tropical latitudes.) In the trade-wind belts of 
the tropics there is usually a counter current of air with light clouds, 
which does not indicate any approaching change of weather. 
13. In middle latitudes such upper currents are not so evident, except 
before a change of weather. 
14. After fine clear weather, the first signs (in the sky) of change are 
usually small, curled, streaked, or spotty clouds, followed by an over- 
casting of vapor that grows into cloudiness.! This murky appearance, 
more or less oily or watery, as wind or rain will prevail, is a sure sign. 
The higher and more distant the clouds seem to be, the more gradual 
but extensive the coming change of weather will prove. 
15. Generally speaking, natural, quiet, delicate tints or colors, with 
soft undefined forms of clouds, foretell fine weather; but gaudy or unu- 
sual hues with hard definite outlines, presage rain and wind. 
16. Misty clouds forming or hanging on heights show wind and rain 
coming if they remain or descend. If they hang about the land or over 
it, sometimes flying inland, expect strong winds with stormy weather. 
17. Dew is an indication of fine weather. So is fog. Neither of these 
two formations (dew and fog) occurs under an overcast sky, or when there 
is much wind. One sees the fog occasionally rolled away, as it were, by 
wind, but not formed while it is blowing. 
18. T he expressions " hardening up," " softening," " or looking greasy," 
are familiar to seamen ; and such very sure indications are the appear- 
ances so designated that they can hardly be mistaken. 
*" When wind comes before rain, 
Soon you may make sail again; 
M hen rain comes before wind, 
Halyards, sheets, and braces mind." 
t" Mackerel skies and mares' tails, 
Make tall ships carry low sails." WEATHER GUIDES. 
5 
19. The rapid or slow rise of a squall cloud, its more or less disturbed 
look-that is, whether its body is much agitated and changing form con- 
tinually, with broken clouds or scud flying about, or whether the mass of 
cloud is shapeless and nearly quiet, though floating onwards across the 
sky-foretells more or less wind accordingly. 
20. An officer of the watch, with a good eye for clouds and signs of 
changing weather, may save his men a great deal of unnecessary expo- 
sure as well as work, besides economizing sails, spars, and rigging. 
21. It not unfrequently happens that good and careful seamen tack 
ship to get to windward of or to avoid a squall cloud rising in the track 
of or approaching his vessel. 
RULES TO EXPLAIN THE INDICATIONS OF THE INSTRU- 
MENTS. 
1. The barometer shows the pressure of the air. 
2. The thermometer (in the shade) shows heat and cold, or the tem- 
perature of the air. 
3. The hygrometer shows the degree of moisture or the dampness of 
the air. 
4. It should always be remembered that changes in weather almost 
always give signs of their coming, for the instruments are affected before 
the wind actually begins to blow or the rain to fall; thus the instruments 
may be said to enable us " to feel the pulse" of the atmosphere. It must 
not be forgotten that the length of time which passes between the first 
appearance of a change of weather and its actual setting in is not 
always the same. It is much greater when a southwest wind is going 
to succeed a northeast wind, than when the opposite change is going to 
take place. We shall see further on why this is the case, and also how 
the appearance of the sky will aid us in forming an opinion as to proba- 
ble weather. 
5. The general principles on which the following explanation is 
founded have been laid down by Professor Dove, and may be briefly 
stated, thus :* 
*"EXPLANATORY CARD. 
"(In south latitude read south for north ) 
" WEATHER GLASS. 
" The barometer rises for northerly wind (including from northwest, by the north, to the 
eastward,) for dry or less wet weather; for less wind, or for more than one of these changes, 
except on a few occasions when rain, hail, or snow comes from the northward with strong 
wind. 
"The barometer falls for southerly wind (including from southeast, by the south, to the 
westward,) for wet weather; for stronger wind, or for more than one of these changes, 
except on a few occasions when moderate wind with rain (or snow) comes from the north 
ward. 
"For change of wind toward northerly directions-a thermometer falls. 
" For change of wind toward southerly directions-a thermometer rises. 6 
WEATHER GUIDES. 
a. If the level of the mercury in the barometer is about at its average 
height, viz., 30 inches, at the sea-level, and is steady or rising while the 
weather gets colder and the air becomes drier, northwesterly, north- 
erly, northeasterly winds, or less wind, less rain or snow may generally 
be expected. 
1). On the contrary, if the level of the mercury falls while the weather 
gets warmer and the air becomes damper, wind and rain may be looked 
for from the southeast, south, or southwest. 
c. If these rules are broken in any way, there is reason to fear. 
d. If the weather gets warmer while the barometer is high and the 
wind northeasterly, (polar current,) we may look for a sudden shift of 
wind to the south. On the other hand, if the weather becomes colder 
while the wind is southwesterly (equatorial current) and the barometer 
low, we may look for a sudden squall or severe storm from the northwest, 
with a fall of snow if it be winter time. 
e. Slight exceptions to these rules sometimes happen; the commonest 
are with northeast winds, which sometimes bring rain, sleet, or snow, 
especially during heavy gales, although the barometer may be rising. 
On the other hand, when the wind is northeasterly and light, and the 
barometer begins to fall, rain may set in before the wind begins to blow 
from east or east-southeast. 
f- Besides these rules for the instruments, there is one about the way 
in which the wind changes, which is very important. It is well-known 
to every seaman, and is contained in the following couplet: 
" When the wind veers against the sun, 
Trust it not for back it will run." 
The wind almost 'always shifts with the sun, i. e., from left to right in 
front ot you. A change in this direction is called veering.* 
Moisture, or dampness, in the air (shown by a hygrometer,) increases before ru.iu, fog, 
or dew. 
Admiral Fitz Roy proposed the following words for barometer scales: 
Rise for northeasterly, NW., N., E.; dry or less wind, except wet from northeastward. 
Fall tor southwesterly, SE., S., W.; wet or more wind, except wet from northeastward. 
"When rise begins after low, 
Squalls expect and clear blow. 
Long foretold-.ong last, 
Short notice-soon past. 
First rise after low, 
Foretells stronger blow. 
"Also generally speaking: 
" When the glass falls low, 
Prepare for a blow; 
When it rises high, 
Let all your kites fly. 
These rules wdl suit the southern hemisphere exactly, if we put N. for S., and S. for N. 
throughout." 
* w 1th watch-hands in the northern hemisphere ; but the contrary in south latitude. This 
however, is only apparent, the wind is actually circulating in the contrary direction. WEATHER GUIDES. 
7 
g. Thus an east wind shifts to west through southeast, south, and 
southwest, and a west wind shifts to east through northwest, north and 
northeast. 
h. If tlie wind shifts the opposite way, viz., from west to southwest, 
south, and southeast, the change is called backing, and it seldom occurs 
unless when the weather is unsettled. 
i. However, slight shifts of wind do not follow this rule exactly; 
thus, for instance, the wind often shifts from southwest to south and back 
again. 
6. In Great Britain there are only two quarters from which the wind ever 
blows for many days together. They are opposite to each other, and are 
the southwest and northeast. In most other parts of the world the same 
remark is true as to the two wind currents, but they do not always blow 
from the same points as they do in Great Britain. What is it that causes 
these winds to blow and makes them so different from each other, as they 
are known to be? The simplest account of them is that the air is always 
flowing to the equator from the poles and back again. It then forms 
two great currents; one is called the polar current, as it flows from the 
pole, and is felt in Great Britain as a northeast wind. The other is called 
the equatorial current, as it flows from the equator and is felt there as a 
southwest wind. The equatorial current blows over Great Britain nearly 
ten times as many days as the polar current does. 
7. The air of the equatorial current has been heated, and so it is light, 
warm, and moist; while it is blowing the barometer is low and the weather 
usually wet. 
8. The air of the polar current has been chilled, and so it is heavy, 
cold, and dry; while it is blowing the barometer is high and the weather 
dry. 
9. If we keep the idea of these two great currents clearly in our 
heads, we shall easily understand most of the signs of weather which are 
noticed.* 
10. To know the state of the air, not only barometers, thermometers, 
and hygrometers must be noticed, but the appearances of the sky must 
be vigilantly watched also. 
11. When the barometer rises, owing to a change of wind, the weather 
gets colder; while when the barometer falls, owing to a change of wind, 
the weather gets warmer. ' 
12. If the barometer be high (above 30 inches) and remain steady for 
some days, the wind will probably be from some point between north 
and east, and the weather will be dry. 
13. If the barometer be low (below 29.5 inches) and remain steady, the 
wind will probably be from some point between south and west, and the 
weather will be cloudy and wet. 
14. If the barometer rises slowly, the weather will become drier and 
* See Dove's Law of Storms : London, 1862. 8 
WEATHER GUIDES. 
the wind will become lighter, and perhaps die away. There may also be 
ocal fogs. 
15. If the barometer falls gradually, the weather will become wetter 
and more unpleasant, and there will never be a certainty ot having a line 
day, though there need not be much wind. 
16. In general, whenever the level of the mercury continues steady, 
there is very little danger of a storm, but when it is unsteady great care 
is necessary to avoid being surprised by a squall or serious gale. 
17. When the barometer is steady, there is no great likelihood of a 
storm being near us, while, when it is unsteady, there is great danger of 
the wind freshening to a gale. This unsteadiness may be due to mere 
local causes, so that it is at times very hard to say whether it shows that 
a serious storm or only a slight squall is coming on. 
18. A sudden rise of the barometer is very nearly as dangerous as a 
sudden fall, because it shows that the level is unsteady. In an ordinary 
gale the wind often blows hardest when the barometer is just beginning 
to rise, directly after having been very low. 
19. When the barometer at anyplace rises very high (above 30.5 inches) 
and continues so for some days, it is because there is, so to speak, too 
much air at the place, and the winds will be very light. A gale can 
only set in when the wind flows away, and it will not at first be severe 
at the place. 
20. When the barometer is very low (below 29.0 inches) and continues 
so, there may be calm and even dry weather for a short time, what is 
called a "pet day" or "weather breeder," but there is great danger of a 
serious storm, because the air will try to force its way into the district 
where the readings are low, and increase the pressure there. The storm 
will probably be worst where the barometer has been lowest. 
21. The air of the equatorial current is lighter than that of the polar 
current, and so southerly winds will begin to blow aloft before they are 
felt below, while the northerly winds will begin to blow below. Accord- 
ingly southwest winds give much more warning of their coming than 
northeasterly ones. The southwest wind will show itself first by long 
streaks of cloud at a great height, called " mares'tails," or, when the gale 
is very near, by driving scud* 
- Signs ot weather, such as those first noticed, are very important 
to anj one watching for changes, as they will often enable him to confirm 
the opinions formed from the behavior of his instruments. 
23. The change from a clear sky to a cloudy one almost always begins 
with the appearance of long streaks of cloud, which show the track of 
the wind in the sky. At night we often see rings around the moon when 
such clouds as these are observed. If they stretch right across the sky, 
forming what is called a "Noah's ark," we know that the wind above us 
has in in earnest, and that wet weather is sure to follow. 
*" Mackerel skies and mares' tails 
Make tall ships carry low sails." WEATHER GUIDES. 
9 
24. The reason that the air clears and that mountains look near just 
before rain is that after a long spell of dry weather there is a great deal 
of dust in the air, and when the air gets damp, as it does just before rain, 
the moisture settles on the dust and makes it sink, leaving the air clear. 
25. In the morning the sun is to the east of us and the clouds which 
occasion the rainbow are to the west, and therefore likely to come over 
us. In the evening, if we see a rainbow, it must be in the east, and the 
sun must be shining in the west; hence the saying: 
"A rainbow in the morning 
Is the sailor's warning ; 
A rainbow at night 
Is the shepherd's delight." 
26. However, after a wet day the whole of the sky often clears at night. 
This is not a certain sign of tine weather. The clouds may, and often 
do, form just as heavily after sunrise next morning. 
27. Thunder-storms almost always occur when the weather is hot for 
the season; they are generally caused by a cold wind coming over a place 
where the air is much heated. They do not cool the air; it is the wind 
that brings them which makes the weather cooler. If a thunder-storm 
comes up from the east the weather will not be cooler after it. This will 
not happen till another storm comes up from the west. Thunder-storms 
are more violent the greater is the difference of temperature between the 
two currents of wind which produce them. 
28. If in winter the barometer suddenly rises very high and a thick 
fog sets in, it is a sure sign that the southwest and the northeast winds 
are " fighting each other." Neither of them can make head against the 
other, and there is a calm; but there is great danger of such a state of 
things being followed by a bad gale. 
29. The changes shown by the instruments in stormy weather are like 
those just described, but they take place more quickly, and are greater 
when they come. 
30. In winter, after a prevalence of easterly winds, if the barometer 
begins to fall and the thermometer to rise, a gale which commences to 
blow from southeast will veer to southwest, while the barometer falls 
constantly. As soon as the wind passes the southwest point, the baro- 
meter begins to rise, a heavy shower of rain falls, and a strong west- 
northwest or northwest wind may follow, after which the sky clears and 
the weather becomes colder. 
31. In some rare cases this northwest wind may be followed by a north 
or northeast wind. 
32. If the wind "backs" from northwest towards west and southwest, 
and continues strong, the bad weather is almost sure to continue. 
33. The (northeast) polar storms do not " veer" to the same extent as 
the equatorial (southwest) winds; they very seldom change their direc- 
tion more than two or three points, while the shift of wind with the sun 
through six or seven points is very common in the case of southwest 
storms. 10 
WEATHER GUIDES. 
34. A fall, with the thermometer low, for the season and place, foretells 
snow. 
35. Exceptions to these rules occur when a northeasterly wind, with 
wet (rain, hail, or snow) is impending, before which the barometer often 
rises (on account of the direction of the coming wind) and deceives per- 
sons who, from that sign only, (the rising,) expect fair weather. 
3G. When the barometer is rather below its ordinary height, say down 
to nearly 29A inches, (at the sea level,) arise foretells less wind, or a change 
in its direction towards the north, or less wet; but when it has been very 
low, about 29 inches, the first rising usually precedes or indicates strong 
wind, at times heavy squalls, from the northwestward, northward, or 
northeastward, after which violence a gradually rising glass foretells 
improving weather-if the thermometer falls. But, if the warmth con- 
tinue, probably the wind will back, (shift against the sun's course,) and 
more southerly, or southwesterly wind will follow, especially if the 
barometer rise is sudden. 
37. The most dangerous shifts of wind, or the heaviest northerly gales, 
happen soon after the barometer first rises from a very low point; or, 
if the wind veers gradually, at some time afterwards. 
38. Indications of approaching changes of weather, and the direction 
and force of winds, are shown less by the height of the barometer than 
by its falling or rising. Nevertheless, a steady height of more than 30.0 
inches at the level of the sea is indicative of fine weather and moderate 
winds, except occasionally those from east to north. 
39. A rapid rise of the barometer indicates unsettled weather. A slow 
movement the contrary; as likewise a steady barometer, which, when 
continued, and with dryness, foretells very fine weather. 
40. A rapid and considerable fall is a sign of stormy weather with 
thunder and lightning and rain, hail, or snow. Alternate rising and sink- 
ing indicates unsettled and threatening weather. 
41. The greatest depressions of the barometer are with gales from 
southeast, south, or southwest; the greatest elevations, with wind from 
northwest, north, or northeast, or with calm. 
42. Though the barometer generally falls with a southerly, and rises 
with a northerly wind, the contrary sometimes occurs; in which cases 
the southerly wind is usually dry with fine weather, or the northerly wind 
is violent and accompanied by rain, snow, or hail; perhaps witli lightning. 
43. hen the barometer sinks considerably, much wind, rain, (perhaps 
hail,) or snow will follow; with or without lightning* The wind will be 
from the northward, if the thermometer is low (for the season ;) from the 
southward if the thermometer is high. Occasionally a low glass is fol- 
lowed or attended by lightning only, while a storm is beyond the horizon. 
* " When the glass falls low 
Prepare for a blow ; 
When it rises high 
Let all your kites fly." WEATHER GUIDES. 
11 
44. A sudden fall of the barometer, with a westerly wind, is sometimes 
followed by a violent storm from northwest, or north, or northeast. 
45. If a gale sets in from the east or southeast, and the wind veers by 
the south, the barometer will continue falling until the wind is near a 
marked change, when a lull may occur; after which the gale will soon 
be renewed, perhaps suddenly and violently, and the veering of the wind 
toward the northwest, north, or northeast will be indicated by a rising 
of the barometer with a fall of the thermometer. 
4G. Three causes (at least)* appear to affect a barometer: 
47. I. The direction of the wind, the northeast wind tending to raise 
it most, the southwest to lower it the most, and wind from points of the 
compass between them proportionally as they are nearer one or the 
other extreme point. 
48. Northeast and southwest may appropriately be called the wind 
poles, (as Dove showed.) 
49. The range, or difference of height shown, due to change of direction 
only, from one of these bearings to the other, (supposing force and moist- 
ure to remain the same,) amounts im the latitudes of Great Britain to 
about half an inch (as read off.) 
50. II. The quantity, taken by itself, of vapor, moisture, rain, or snow, 
in the wind, or current of air, (direction and strength remaining the same,) 
seems to cause a change amounting in an extreme case to about half an 
inch. 
51. III. The force alone of wind from any quarter (moisture and direc- 
tion being unchanged) is preceded, or foretold, by a fall or rise, accord- 
ing as the strength will be greater or less, ranging in an extreme case to 
more than two inches. 
52. Hence, supposing the three causes to act together, in extreme cases, 
the height would vary from near thirty-one inches (30.9) to about twenty- 
seven (27.0) inches, which has happened, though rarely, (and even in 
tropical latitudes.) 
53. In general, the three causes act much less strongly, and are less in 
accord, so that ordinary varieties of weather occur much more frequently 
than extreme changes. 
54. It is not by any means intended to discourage attention towhat is 
called " weather wisdom." On the contrary, every prudent person will 
combine actual observation of the elements with such indications as he 
may obtain from instruments, and will find that the more accurately 
the two sources of foreknowledge are compared and combined, the more 
satisfactory their results will prove. 
55. A barometer bqgins to rise considerably before the conclusion of a 
gale, sometimes even at its commencement. Although it falls lowest 
before very high winds, it frequently sinks much before heavy rain. 
The barometer falls, but not always, on the approach of thunder and 
* Electrical effects are yet uncertain. 12 
WEATHER GUIDES. 
lightning.* Before and during the earlier part of settled weather it 
usually stands high, and is stationary; the air being dry, and compara- 
tively quiet. 
56. Instances of fine weather, with a low barometer, occur, however 
rarely, but they are always preludes to a duration of wind or rain, ij not 
both, or they indicate bad weather in the vicinity. 
57. After very warm and calm weather, a storm or squall, with rain, 
may follow, as likewise locally at any time when the atmosphere is heated 
much above the usual temperature of the season. 
58. Allowance should invariably be made for the previous state of the 
barometer during some days,-as well as some hours, because its indica- 
tions may be affected by distant causes, or by changes close at hand. 
Some of these changes may occur at a greater or less distance, influencing 
neighboring regions, but not visible to each observer whose barometer 
feels their effect. 
59. There may be heavy rains or violent winds beyond the horizon and 
the view of an observer, by which his instruments may be affected con- 
siderably, though no particular change of weather occurs in his imme- 
diate locality. 
60. It may be repeated, that the longer a change of wind or weather is 
foretold before it takes place, the longer the presaged weather will last; 
and, conversely, the shorter the warning, the less time whatever causes 
the warning, whether wind or a fall of rain or snow, will continue. 
61. Sometimes severe weather from the southward, not lasting long, 
may cause no great fall, because followed by a duration of wind from the 
northward; and at times the barometer may fall with northerly winds 
and fine weather, apparently against these rules, because a continuance 
of southerly wind is about to follow. By such changes as these one may 
be misled, and calamity may be the consequence, if not duly forewarned. 
62. A few of the more marked signs of weather, useful to seamen, are 
the following: 
63. \\ hether clear or cloudy, a rosy sky at sunset presages fine weather ;t 
a sickly, greenish hue, wind and rain ; tawny, or coppery clouds, wind; 
a dark (or Indian) red, rain; a red sky in the morning, bad weather, or 
much wind, perhaps also rain; a gray sky in the morning, fine weather; 
a high dawn, wind; a low dawn, fair weather.| 
64. Soft-looking or delicate clouds foretell fine weather, with moderate 
or light breezes; hard-edged oily-looking clouds, wind. A dark, gloomy 
blue sky is windy; but a light, bright blue sky indicates fine weather. 
Generally, the softer clouds look, the less wind (but perhaps more rain) 
* Thunder clouds rising from northeastward, against, or above the lower wind* do not 
usually cause a fall of the barometer. 
t " When the sun sets in a clear, 
An easterly wind you need not fear." 
t A "high dawn" is when the first indications of daylight are seen above abank of clouds. 
A ' low dawn " is when the day breaks on or near the horizon, the first streaks of light being 
very low down. WEATHER GUIDES. 
13 
may be expected; and the harder, more " greasy," rolled, tufted, or rag- 
ged, the stronger the coming wind will prove. Also, a bright yellow sky 
at sunset presages wind; a pale yellow, wet; orange or copper-colored, 
wind and rain; and thus, by the prevalence of red, yellow, green, gray, 
or other tints, the coming weather may be foretold very nearly-indeed, 
if aided by instruments, almost exactly. 
65. Light, delicate, quiet tints or colors, with soft, indefinite forms of 
clouds, indicate and accompany fine weather; but gaudy or unusual 
hues, with hard, definitely outlined clouds, foretell rain, and probably 
strong wind. 
66. Small inky-looking clouds foretell rain; light scud clouds driving 
across heavy masses show wind and rain, but if alone, may indicate wind 
only, proportionate to their motion. 
67. High upper clouds crossing the sun, moon, or stars, in a direction 
different from that of the lower clouds, or the wind then felt below, fore- 
tell a change of wind toward their direction.* 
68. After fine clear weather, the first signs in the sky, of a coming 
change, are usually light streaks, curls, wisps, or mottled patches of 
white distant cloud, which increase, and are followed by an overcasting 
of murky vapor that grows into cloudiness. This appearance, more or 
less oily, or watery, as wind or rain will prevail, is an infallible sign. 
69. Usually the higher and more distant such clouds seem to be, the 
more gradual, but general, the coming change of weather will prove. 
70. Misty clouds forming or hanging on heights, show wind and rain 
coming, if they remain, increase or descend. If they rise, or disperse, 
the weather will improve, or become fine. 
71. When sea birds fly out early, and far to seaward, moderate wind 
and fair weather may be expected. When they hang about the land, or 
over it, sometimes Hying inland, strong winds with stormy weather are 
probable. As, besides birds, many creatures are affected by the approach 
of rain or wind, their indications should not be slighted by an observer 
who wishes to foresee changes. 
72. There are other signs of coming change in the weather known less 
generally than is desirable; and therefore worth notice, such as when 
birds of long flight fly up and down, or low, rain or wind may be expected. 
Also, when smoke from chimneys does not ascend readily, (straight 
upwards during calm,) unfavorable change is probable. 
73. Dew is an indication of coming fine weather; so is fog. Neither 
of these two formations begins under an overcast sky, or when there is 
much wind. One occasionally sees fog rolled away, as it were, by wind, 
but seldom or never formed while it is blowing with any considerable 
force, though it exists with wind. 
* In the tropics, or regions of trade winds, there is generally an upper and counter current 
of air, with very light clouds, which is not an indication of any approaching change. In 
middle latitudes such upper currents are not so frequent (or evident ?) except before a change 
of weather. 14 
WEATHER GUIDES. 
74. Remarkable clearness of atmosphere, especially near the horizon ; 
distant objects, such as hills, unusually visible or well defined, or raised 
(by refraction,*) and what is called "a good hearing day," may be men- 
tioned among signs of wet, if not wind, to be expected in a short time. 
75. More than usual twinkling or apparent size of the stars; indis- 
tinctness or apparent multiplication of the moon's horns; haloes, "wind- 
dogs,"! and the rainbow,| are more or less significant of increasing wind, 
if not approaching rain, with or without wind. 
76. Near land, in sheltered harbors, in valleys, or over low ground, 
there is usually a marked diminution of wind and a dispersion of clouds 
during the early part of the night. At such times an eye on an over- 
looking height may see a body of vapor extending below (rendered visi- 
ble by the cooling of night) which seems to check the wind. 
77. The dryness or dampness of the air, and its temperature, (for the 
season,) should always be considered-with other indications of change, 
or continuance of wind and weather. 
78. Much inequality of electricity, atmospheric pressure (tension,) or 
temperature; great fall or rise of the barometer; sudden or rapid alter- 
nations; great falls of rain or snow, foretell more or Aess strong wind, 
with its usual accompaniments, either in some places only, or throughout 
an extensive area of hundreds, if not thousands, of miles ; some tracts, 
however, remaining almost unaffected, unless by rain or squalls. 
79. Speaking generally, southerly gales are commonly preceded by 
notable signs in the atmosphere, such as a falling barometer and a tem- 
perature higher than usual at the season; whereas, on the contrary, 
dangerous storms from a polar quarter (northwest to north and easterly) 
are sometimes sudden, and preceded by a rising barometer, which may 
mislead persons, especially if accompanied by a temporary lull of a day 
or two, with a fallacious appearance of fine weather. This fallacy is 
caused by a circuitous movement of wind following; influencing by 
checking and then overpowering, or uniting with, a preceding similar 
cyclonic sweep. 
80. Occasionally, however, a southerly gale begins with a high ba- 
rometer, and only as it increases does the barometer fall. This occurs 
when the mercury has fallen notably in the north, and is still falling 
there. 
81. M hen a gale occurs with a high barometer, which does not fall, 
but remains steady, or rises, a northerly (polar) wind, or a duration of 
fine weather, may be expected, and more of either as the delay of ap- 
proach is greater. 
* Much refractiou is a sign of easterly wind. 
t Fragments or pieces (as it were) of rainbows (sometimes called " wind-galls,") seen on 
detached clouds. 
t " A rainbow in the morning, 
Sailors take warning; 
A rainbow at night 
Is the shepherd's delight." WEATHER GUIDES. 
15 
82. The gorgeous aerial landscapes of red and golden-colored clouds 
which fire the western sky at sunset, " the day's dying glory" of the poet, all 
admire. They are observed to be the accompaniment of cumulus clouds 
(the cloud of the day during fine weather,) while in the act of dissolving 
as they sink slowly down into the lower and warmer parts of the atmo- 
sphere consequently they disappear from the sky shortly after sunset. 
Such sunsets are therefore universally regarded as prognostics of fine 
weather. 
83. Frequently small thin clouds appear high up in the eastern sky 
before sunrise, or when 
"The dappled dawn doth rise." 
And when the sun has risen they disappear. They may be regarded as 
sure prognostics of fine weather. 
84. A green or yellowish green-tinted sky, on the other hand, is one 
of the surest prognostics of rain in summer and snow in winter. An 
attentive consideration of the changing tints of the evening sky after 
stormy weather, supplies valuable help in forecasting the weather ; for 
if the yellow tint becomes of a sickly green, more rain and stormy 
weather may be expected ; but if it deepen into orange and red, the atmo- 
sphere is getting drier and fine weather may be looked forward to. In 
the morning when the sky is red and lowering, it is regarded as a prog 
nostic of unsettled weather. 
85. There are three important causes which contribute to the produc- 
tion of wind: 
I. Unequal atmospheric pressure. 
II. Unequal specific gravity of the air; and 
III. The rotation of the earth. 
86. Unequal pressure tends to produce motion in the atmosphere. If 
the weight of one column exceeds that of the other, the air must tiow 
from the heavier to the lighter column. The wind must therefore blow 
from places where the barometer is highest, towards places where it is 
most depressed. Unequal specific gravity of the air may result from 
unequal temperature or from unequal humidity. 
87. Observations of the clouds often disclose the existence of currents 
in the atmosphere flowing in various and perhaps opposite directions* 
We sometimes notice a stratum of clouds moving nearly in the direction 
of the air at the earth's surface, while at a greater elevation we observe 
a stratum moving in a different direction, and sometimes a third and 
perhaps a fourth moving in still other directions. Such cases are of 
frequent occurrence near the commencement or during the progress of 
a great storm. 
88. The expansion of the air in forming cloud, particularly after rain 
begins to fall, causes the air to spread out in all directions above, caus- 
ing a barometer under the middle of the cloud to fall below its mean 
height, and beyond the limits of the cloud to rise above its mean height. 
89. Storms are often experienced simultaneously over large portions 16 
WEATHER GUIDES. 
of the earth's surface. The storms of winter are particularly severe and 
extended. Storms are generally accompanied by a considerable depres- 
sion of the barometer below its mean height, and are succeeded by a 
rise of the barometer above its mean height. At the centre of the storm 
the barometer sometimes sinks an inch below its mean height. Beyond 
the area of low barometer the barometer rises above its mean height 
frequently to the amount of half an inch, sometimes to the amount of 
an entire inch, and occasionally still higher. 
90. Winter storms commence gradually, and generally attain their 
greatest violence only after a lapse of several days. After a certain 
period their violence gradually diminishes, and at length they disappear 
entirely. 
91. Throughout the middle latitudes of the American continent, when 
violent storms advance with considerable rapidity, the direction of pro- 
gress is always from west to east. This direction is not absolutely uni- 
form, but has been observed to vary from about due east to north 51° 
east. The rate of progress of storms has been observed to vary from 
zero to forty-four (44) miles per hour. They generally travel from St. 
Louis to New York in about 24 hours, and from New York to New- 
foundland in another 24 hours. Generally when the barometer is unu- 
sually low at New York, it is usually high at St. Louis, and also high in 
Newfoundland. The winter storms of the United States are said to 
move side foremost. 
92. In the West Indies the great storms called cyclones are almost 
exclusively confined to the months from July to October, being most 
common in the month of August; in the China sea and Bay of Bengal 
they are most prevalent at about the same period of the year. In south- 
ern latitudes they are most common from January to March. 
93. It has been found that the West India Islands cyclones first make 
their appearance completely developed at the Windward islands, and it 
may be possible to trace the origin of some of the hurricanes with which 
these islands are so frequently visited to regions of the ocean to the 
eastward.* 
A very concise but sure rule may be added for avoiding the central or strongest part of a 
hurricane, cyclone, typhoon, tornado, or circling storm: 
XX ith your face towards the wind, in north latitude, the centre of the circling or rotatory 
storm will be square to your right. In south latitude, square to your left. 
Ihe apparent veering of the wind, and the approach or retreat of the dangerous central 
circle, depend on your position in the curvilinear whirl or sweep. 
Draw a circle, mark the direction of the rotation or circulation, by an arrow with the head 
towards the left hand (against the movement of a watch's hands) in north latitude; but 
towards the right, (or with the bands of a watch,) if in south latitude. The direction of the 
wind, and the bearing of the centre, show your position in the meteor, for such it is, though 
perhaps hundreds of miles in diameter; and the veering of the wind, or the contrary, and 
its change in strength, will show how the meteor is moving bodily over an extensive region, 
revolving horizontally, or inclined at a certain angle to the horizontal plane. 
If the observer be stationary, in north latitude, and the centre pass on his polar side, he 
will experience a change of wind from southward by the west towards north; but if it pass 
between him and the equator, the change will be from southward by the east towards north; WEATHER GUIDES. 
17 
94. Previous to the commencement of a cyclone the air is observed to 
be close, sultry, and oppressive, and the wind is moderate or calm. 
95. A fresh breeze sets in from the east, and rises and falls with a 
moaning sound; after a few hours it is succeeded by a lull, which may 
last for an hour or more, after which the wind changes to the west, often 
with great suddenness, and blows with increased violence, and this is 
usually the time of the greatest danger to vessels. 
90. The approach of a cyclone is often announced by a swell of the 
ocean, resulting from the action of the wind upon a neighboring sea, 
while the waves thus excited advance more rapidly than the storm. 
97. During the passage of the cyclone the barometer oscillates in a 
remarkable manner, rising and falling rapidly, so that a great barometric 
oscillation almost always announces the approach of a tempest. The 
most rapid fall begins from three to six hours before the passage of the 
centre. The barometer is lowest near the middle of the storm area, and 
begins to rise before the strength of the cyclone is over. 
98. The fall of the barometer during the passage of the cyclone varies 
according to the intensity of the storm. It frequently amounts to an inch, 
and has been known to exceed two inches. The rise of the barometer after 
the storm is usually as rapid as was its fall on the approach of the storm. 
99. The character of the weather at any place is affected by so many 
circumstances which may transpire at distant parts of the world, and 
which can be but very imperfectly known to us, that it is impossible to 
predict, except very imperfectly, what weather may be expected at a 
given time and place. To a limited extent, however, such predictions are 
possible. 
100. When restricted to observations at one locality, the predictions 
of the weather must needs be more uncertain, and the conclusions to be 
derived from a motion of meteorological instruments are not the same 
for all parts of the world. Along the Atlantic coast of the United States 
the approach of a violent northeast storm is generally indicated by the 
barometer rising above its mean height; at the same time the wind 
veers to the northeast, and the atmosphere grows hazy. After the rain 
or snow commences, the barometer begins to fall; when the barometer 
reaches it lowest point, the wind changes to north or northwest, after 
which the barometer begins to rise. 
101. If a gale sets in from the east or southeast, and the wind veers 
by the south, the barometer will continue falling until the wind becomes 
southwest, when a comparative lull may occur, after which the gale 
will be renewed and the change of the wind toward the northwest will 
be accompanied by a fall of the thermometer, as well as a rise of the 
barometer. 
but otherwise in south latitude, as his place in circles sketched will show more cleat ly than 
words. The roughest sketch or diagram, indicating the various directions of wind, and the 
course of the meteor's centre, will show more plainly than descriptions, which must neces- 
sarily vary with each case, and are tedious. 
2 W G 18 
102. A considerable and rapid depression of the barometer for 
instance, a fall of three-fourths of an inch in twenty-four hours-indi- 
cates an approaching storm, with rain or snow. The wind will be from 
the northward if the barometer is low for the season; from the south- 
ward if the barometer is high. If the barometer falls with a rising ther- 
mometer and increased dampness, wind and rain may be expected from 
the southward. A rapid rise of the barometer indicates unsettled 
weather; a slow rise indicates fair weather. The result of all rapid 
changes in the weather, or in any of the instrumental indications, is brief 
in duration, while that of a gradual change is more durable. When the 
upper clouds move in a direction different from that of the lower clouds, 
or that of the wind then blowing, they foretell a change of wind. 
103. When the outlines of cumulus clouds are sharp, it indicates a dry 
atmosphere, and therefore presages fine weather. Small inky-looking 
clouds foretell rain. A light scud driving across hazy clouds indicates 
wind and rain. 
104. Remarkable clearness of the atmosphere near the horizon, and 
an unusual twinkling of the stars, indicate unusual humidity in the upper 
regions of the atmosphere, and are therefore indications of approaching 
rain. 
105. Haloes, coronae, &c., presage approaching rain or snow. 
106. Dew and fog are indications of fine weather. 
1. Sir J. F. W. Herschel, (than whom there is no more competent or 
reliable authority on the subject,) has said that, "There is no branch of 
physical science which can be advanced more materially by observations 
made during sea voyages than meteorology,* and that for several dis- 
tinct reasons: 
2. First. That the number and variety of the disturbing influences at 
sea are much less than on land, by reason of the uniform level and homo- 
geneous nature of its surface. 
3. Second. Because, owing to the penetrability of water by radiant 
heat, and the perpetual agitation and intermixture of its superficial 
strata, its changes of temperature are neither so extensive nor so sud- 
den as those of the land. 
4. Third. Because the area of the sea so far exceeds that of the land, 
and is so infinitely more accessible in every part, that a much wider 
field of observation is laid open, calculated thereby to afford a far more 
extensive basis for the deduction of general conclusions. 
5. Fourth. The sea being the origin from which all land waters are 
WEATHER GUIDES. 
* The variety of interesting and useful, if not always important, subjects included within 
the lange of meteorology, is not perhaps sufficiently realized in the minds of active partici- 
pators in the world s stirring work. Irrespective of any scientific object, how much utility 
is there to all classes in what is commonly called "weather wisdom ?" Even animals, birds, 
and insects have a presaging instinct, perhaps a bodily feeling, that warns them; but man 
often neglects his perceptive and reasoning powers; neither himself observes, nor attends to 
the observations ot others, unless special inclination or circumstances stimulate attention to 
the subject. WEATHER GUIDES. 
derived, in studying the hygrometrical conditions of the sea atmosphere 
we approach the chief problems of hygrology in their least involved 
and complicated form, unmixed with those considerations which the 
perpetually varying state of the land (as the recipient at uncertain inter- 
vals of derivative moisture) forces on the notice of the meteorologist 
of the continents. Nor ought it to be left out of consideration that 
this, of all branches of physical knowledge, being that on which the 
success of voyages and the safety of voyagers are most immediately 
and unceasingly dependent, a personal interest of the most direct kind 
is infused into its pursuit at sea, greatly tending to relieve the irksome- 
ness of continued observations, to insure precision in their registry, and 
to make their partial or complete reduction during the voyage an agree- 
able, as it always is a desirable, object." * * * * * 
6. * * * * * " it is to the regular meteorological register, 
steadily and perseveringly kept throughout the whole of every voyage, that 
we must look for the development of the great laws of this science. 
7. To the eminent meteorologists whose interesting works are con- 
sulted and whose directions are followed by careful and intelligent sea- 
men, commerce, navigation, and all those who trust themselves upon 
the ocean, are much more indebted than the world seems to recognize at 
present; but great as these benefits have been and are now, to all navi- 
gating interests, much still remains to be made known, and it is to the 
seamen of all nations that we must look for reliable facts to solve the 
questions bearing upon atmospheric currents under the varied conditions 
and states of the weather at sea. 
8. The navigator who has the means at his command and will devote 
the time necessary for keeping a continuous meteorological register dur- 
ing the entire cruise or voyage, or mere over sea passage from one port 
or place to another more or less distant, will find at its close ample com- 
pensation for himself in the experience he will have gained, independ- 
ently of the resulting benefits consequent upon the reduction and discus- 
sion of the observations by experts in the office at home. 
9. Next in point of importance to the accurate and carefully kept 
meteorological register of the entire cruise, voyage, or passage, is the 
record of the meteorological observations during extraordinary or abnor- 
mal conditions of the weather at sea. It is the duty and interest of 
every intelligent seaman entrusted with command, if no continuous 
register is kept by him, to keep or cause to be kept a correct register of 
the barometric, thermometric, and hygrometric readings preceding, 
during the continuance of, and succeeding all hurricanes, storms, and 
other extraordinary states of the weather at sea, and forward these 
observations at the termination of the cruise or voyage to persons en- 
gaged in the discussion of subjects relating to meteorology. 
10. Log-books and meteorological registers kept on board of vessels 
of the navy are sent to the Bureau of Navigation, Navy Department; 
those kept on board of vessels of commerce will be received also if sent, 20 
WEATHER GUIDES. 
and such examinations and discussions of them will be made as the 
means at command will permit, and results published for the informa- 
tion and benefit of mariners generally. 
THE BAROMETER 
1. The principle of the barometer is now so generally known, being 
too in almost universal use at sea, that it need hardly be mentioned 
here. While the mercury in the cistern falls, or is pressed down by the 
air, the column rises in the exhausted tube, and when the mercury in 
the cistern rises, owing to diminished pressure of the air, the column of 
mercury in the tube falls. But the actual length of the column of mer- 
cury is required, (in order to know the weight or pressure of the atmos- 
phere,) as measured from the actual surface of the mercury, which is 
variable; therefore a correction is required for most instruments on 
'account of the difference from the neutral line or point, namely, that 
defining the base of the column when the scale attached to it was gradu- 
ated. 
2. In some barometers the graduated scale can be adjusted so that the 
lower point just touches the mercury, in which case no correction for 
the relative capacities of the tube and cistern is necessary. In small 
tubes an error is caused by what is called capillarity and by friction. 
Unlike other fluids mercury is depressed by capillary action. Other 
errors to be noticed and allowed for are caused by the expansion or con- 
traction of the mercury by higher or lower temperature than that of the 
mercury when the scale was graduated, and by the dilatation or con- 
traction of the tube and scale. 
3. Also, the corrected height of the mercury in the tube must be 
reduced to the standard of observation, or what it would have been had 
the temperature of the air and mercury been at 32° Fahrenheit, with 
the barometer at the level of the sea. Moreover, there are errors in 
most barometers, occasioned by the variation of the scale, or incorrect 
graduation, or both, besides that occasioned by air, moisture, or gas in 
or above the column, which affects its height. Some of these may be 
comprehended as index errors, and are so in instruments on the Kew 
Observatory principle. 
4. The barometer requires very careful handling, the glass tube being 
exceedingly liable to fracture. When moved from one place to another 
the cistern end should be kept uppermost, the instrument having been 
gently reversed. 
5. When suspended for use it should hang freely in a vertical position, 
where neither the sun's rays, nor a fire, nor any local cause of heat or 
cold may be likely to affect it. Every observation of the height of column 
should be accompanied by a notation of the degree of temperature shown 
by the attached thermometer, (which indicates the temperature of the 
mercury,) as well as by those of the air. In reading off, the edge of the 
vernier scale should appear to touch (or be tangent to) the uppermost WEATHER GUIDES. 
21 
point of the mercury when the eye is at an equal height and looking 
horizontally at the tube. A card or a piece of white paper behind or at 
the side of the tube aids the eye by reflecting light, or showing the edge 
of the quicksilver more plainly. 
6. The vernier scale enables one to read off hundredths of an inch, or 
in the instruments used for the most scientific purposes, thousandths of 
an inch. Its principle is the same as that of the sextant, substituting 
scale for arc, and divisions of inches for parts of arc. An extreme degree 
of minuteness in reading is scarcely necessary in marine barometers at 
sea, which are unavoidably liable to errors exceeding even hundredths, 
partly from defective construction or adjustment, partly from the motion 
of the vessel, which causes oscillation (often called pumping) of the mer- 
cury in the tube. To prevent the barometer from acquiring too much 
motion, (from the movement of the vessel,) from acquiring a momentum 
which augments its swinging and gives impetus to the mercury, soft 
cushions, India-rubber straps, or weak springs may be employed. 
7. As the column of mercury rises with increase of pressure by the 
atmosphere, and descends when the pressure diminishes, it indicates a 
greater or less accumulation of air, which, like other fluids, such as water, 
when heaped above its average level or reduced below it, (from whatever 
cause,) will have a tendency to fall or rise until the general equilibrium 
is restored. 
8. An observer may be under the centre of such accumulation or 
depression, or he may be more or less distant from it, though within the 
influence of whatever horizontal movement of air may be caused by such 
temporary increase or diminution of pressure. Hence, the barometer 
shows, and generally foretells, changes of wind; but as complications 
often occur, and changes are of a greater or less extent, affecting or 
extending through a wider or a more limited area, and are accompanied 
by hygrometric and electrical alterations, it is extremely difficult at times 
to say beforehand what particular change of weather is to be expected, 
and at what interval of time; although after the event the correspond- 
ence of barometric changes with those of the weather can be readily 
traced. However, notwithstanding occasional perplexity, the general 
character of the weather during the next following few days may be pre- 
dicted by an observer who has watched the barometer in the few imme- 
diately preceding days, and who understands the nature and use of the 
instrument. 
9. The general peculiarity should always be remembered in endeavoring 
to foretell weather that the barometric column usually stands higher with 
easterly than it does with westerly winds, all over the oceans; and with 
winds from the polar regions also higher than with those from the direc- 
tion of the equator. Hence, the highest barometric columns are observed 
with northeast winds in northern latitudes and with southeast winds in 
the southern hemisphere. In middle latitudes there is an average differ- 
ence (unreduced or observed height as read off) of nearly half an inch, 
other things being similar. 22 
WEATHER GUIDES. 
10. The column of mercury ranges from about 28 to near 31 inches in 
the higher latitudes, but except before or during storms, when it occa- 
sionally falls below 28 inches. The range in tropical latitudes extends 
but few tenths of an inch above or below 30 inches at the level of the 
sea. 
11. As a general rule wind affects the column of mercury in the 
barometer more than rain, (hail, or snow,) but either has influence; and 
the longer the interval between the alteration of the barometer and the 
foretold change of the weather the longer the new state of the weather 
will last, and conversely. 
12. Moreover, the more gradual the column moves, the more settled 
in character will the weather be, and the reverse; also, when the baro- 
metric column moves contrary to the natural movement, according to 
the daily tidal change (however masked) inferior weather may be expected, 
and conversely. 
13. Movements of the atmosphere maybe illustrated by reference to 
the motion of water drawn off from a reservoir by a small opening below 
or increased by a gradual pouring in at one point of the upper surface. 
14. From a slight motion at the commencement, affecting only that 
portion of the fluid adjoining either of those points of diminution or 
repletion, gradually all the water becomes influenced and acquires more 
or less rapid movement. But suppose a long reservoir or canal of fluid 
which has two such points of exhaustion or two of repletion, (as imagined 
above,) and that one of either is near each end of the vessel, if each 
aperture be opened at the same moment equal effects will be caused in 
each half of the fluid towards either end of the vessel, but in the middle 
there must be a neutral point at which the water falls, but has no hori- 
zontal motion. The reverse takes place in raising the level. And in the 
case of fluid drawn off or diminished in weight at one end while increased 
by repletion at the other, the whole body of water will move similarly 
to that in the former vessel, but unequally. Hence, it is evident that 
before horizonal motion occurs an augmentation or a diminution of pres- 
sure must take place somewhere not very remote, and so it is with the 
atmosphere, which is not only a light fluid, but extremely elastic. 
15. These considerations show in some degree why the barometric 
changes usually precede, but sometimes only accompany, changes of 
weather; and, though very rarely, occur without any sensible altera- 
tion in the state of the atmosphere. 
16. An observer may be near a central point, towards which the sur- 
rounding fluid tends or from which it diverges. He may be at the very 
furthest limit of the portion of water or air that is so influenced. 
17. He may be at an intermediate point, or he may be between bodies 
of aeriform fluid tending towards opposite directions. 
18. Ctcteris paribus, the column of mercury falls a tenth of an inch 
for about one hundred feet of elevation near the sea level, and gradually 
less for each hundred feet as it becomes more elevated. (The ratio of WEATHEIl GUIDES. 
23 
diminution, however, is slightly variable.) Allowance, therefore, should 
be made in correcting or reducing close observations for any difference 
between a fixed limit (usually the edge of the copper sheathing) and 
the actual water line of the vessel at the time of observation, which 
should be noted if considerably different. 
19. Before observation gently tap the barometer near the top, as the 
mercury will occasionally adhere to the sides of the glass, which would 
prevent its showing a very slight change that may have taken place in 
the atmosphere. 
20. A rail, bar, or frame of some kind should guard the barometer 
from an accidental blow or displacement; and, indeed, generally speak- 
ing, the greatest care should be taken of this invaluable monitor. 
21. It should not be forgotten that it is not from the point at which 
the mercury may stand in the barometer tube that we are alone to form 
a judgment of the state of the weather, but from its rising or falling. 
The laws of storms are daily becoming more developed, not only by the 
investigations of the philosopher, but likewise by the careful observa- 
tions of some of the most intelligent navigators, who, from experience, 
affirm the truth of the laws and the value of the barometer.* 
22. The principal use to seament of the barometer is to indicate 
atmospheric changes, and by so doing give timely warning of approach- 
ing storms and gales, thereby enabling them to have their vessels 
well prepared to withstand the shocks of tempests and seas, and to 
take advantage of the earliest indications of changes from bad to good 
weather, to prosecute their voyages in comparative safety. 
23. It has frequently happened that the motions of the barometric 
column have given warning of approaching tempests and gales when 
there was little or nothing in the appearance of the sea or sky to excite 
the least alarm. 
24. And as the tides are also affected by atmospheric pressure to such a 
degree that a rise of one inch in the barometer will cause a corresponding 
fall in the tides of nine to sixteen inches, or an average of about one foot 
for each inch. The barometer will also serve as a guide to vessels in 
entering docks and harbors, and in crossing bars where the depth of 
water is scant for their draught. 
25. As the docking and launching of vessels of great draught render 
•Taking the state of the barometer in connection with the appearance of the weather one 
could make a satisfactory conclusion as to the quarter from which any violent wind would 
come. And the barometer, after very little practice, can be used by any man. There is no 
difficulty in using it sufficiently to know that danger is coming on; and if danger is coming 
on, a man refrains, of course, from exposing himself to it; the quarter from which the wind 
comes being of minor consequence. 
tl think that the neglect of the use of the barometer has led to the loss of many ships. 
From a want of attention to the barometer, they have either closed the land (if at sea) or 
have put to sea (being in harbour in safety) at improper times; and in consequence of such 
want of precaution the ships have been lost, owing to bad weather coming on suddenly, 
which might have been avoided had proper attention been paid to that very simple instru- 
ment. 24 
WEATHER GUIDES. 
it necessary to make close, careful ami accurate calculations of the height 
of the water at the time, the state of the barometer not only becomes a 
matter of considerable importance, but one of the elements ot calcula- 
tion that should not be neglected on those occasions. 
26. Mr. Findly observes, in his Directory for the Pacific Ocean: "The 
barometer measures the weight of the atmosphere above its place in 
exactly the same manner that other ponderables are weighed in a bal- 
ance. The weight of the column of •mercury in the tube is exactly 
equivalent to that of a column of air of similar diameter the height ot 
the atmosphere; therefore, any change in the one affects or is affected 
by the other. Now, it has been proved that stormy winds lower the 
barometric column, therefore there is less superincumbent air at those 
times; and this principle indicates some of the atmospheric conditions 
which precede the approach of or alteration in a storm. The mean 
height of the barometer varies considerably in various parts of the 
world, and therefore its changes, rather than its absolute height, is the 
most important consideration.* 
27. There is also an hourly variation in the barometer which gives the 
greatest height at about 10 a. m., daily, and the least height at about 
4 p. m. Smaller fluctuations will be observed during the night, the 
barometer attaining a second maximum at about 10 p. in., and a second 
minimum at about 4 a. m. The amount of this diurnal oscillation is 
greatest at the equator, where it is 0.1 inch, and diminishes towards 
either pole. 
28. Many persons have been misled in regard to the supposed indica- 
tions of weather by the barometer, by regarding the words, "Fair," 
" Change," " Set Fair," " Stormy," " Rain," &c., on the scales of barom- 
eters made for use in particular localities, but consulted with the same 
signs in tropical or other far distant regions from which they were 
intended to be used, and this misconception has no doubt led to the 
often repeated remark that " the barometer is useless in these parts." 
29. These words being omitted from the scales of the marine barome- 
ters of the present day no such misconception can occur. The marine 
barometer differs from the ordinary barometer for use on land in having 
its tube very much narrowed the greater part of its length to prevent 
pumping by the motion of the vessel. 
30. It will be observed that the surface of the mercury in the barome- 
ter tube is convex when rising, and concave when falling; a difference 
arising from friction, or a tendency of the particles of quicksilver to 
adhere to the sides of the glass tube at all times, and to a greater degree 
when the quicksilver has become impure from long use and exposure in 
the cistern. 
31. The barometer in use in the United States navy at present does 
not require any adjustment of the cistern or screw to make it portable. 
*See Appendix A. WEATHER GUIDES. 
25 
32. When moved or carried, it may be held horizontally, or, which is 
better, with the cistern end up. 
33. The box in which it is packed or placed for being transported is 
marked " this end up" When the barometer is taken out of the box and 
hung in its place it will be ready for observation as soon as sufficient 
time shall have elapsed to allow the mercury to fall, and settle to the 
temperature of the place. 
34. When a barometer is taken down from its place more care will be 
required. Before laying it in its box it must be held at an angle of about 
45° long enough for the mercury to run to the top of the tube. This will 
ordinarily require from five to ten minutes, depending upon the larger 
or smaller size of the interior or bore of the tube, or plan of the means 
of observation may vary. 
35. The selection of the place for the barometer on board ship must be 
left to the judgment of the commander, as he is often restricted to a very 
limited available range to select from. 
36. Observations recorded are reduced by a table to the temperature 
of 32° Fahrenheit, but for ordinary observations a correction of 0.01 
inch for 4° of temperature can be remembered and will be sufficiently 
near the truth. This correction must be added (+) when below 32°, but 
(-) when above 32° of Fahrenheit. 
37. The aneroid barometer, from its small sizq and portability with 
comparative safety, is an admirable adjunct to, and can be usefully 
employed where a mercurial one cannot be taken. It can, however, only 
be relied upon when it can be frequently compared with a standard 
mercurial barometer. The advantage of size and easy portability has 
brought it into a general use far beyond its real merits as a proper 
measurer of the weight of the atmosphere. As a " weather glass," (if 
corrected for temperature,) it will generally do good service. It however 
requires some care, as its safe transportation is rather apparent than real. 
Slight shocks wii not ordinarily, but a jar or knock that would break a 
mercurial barometer will quite likely, change the reading an unknown 
quantity that may vary from one-tenth to one inch, and if this amount 
can be found by a direct comparison (after travel or rough handling) 
with a standard barometer it can be allowed for, or the aneroid may be 
adjusted; but if there is no standard at hand it will require a series of 
observations to obtain from a mean of them an approximate correction. 
It will, however, be useful even without this for its relative reading, but may 
without great care lead to mistakes. If the instrument is on shipboard, 
and will remain undisturbed in its place, then the most important point 
is to get its correction for temperature. The correction for temperature 
in the mercurial barometer is a known quantity, and a table short enough 
to be convenient can be used, but in the aneroid it must be found for 
each instrument. The temperature of the aneroid may be obtained 
THE ANEROID BAROMETER 26 
approximately by placing it in the coldest place nearly, and then in the 
warmest, and noting the change, leaving it in each place long enough 
to acquire the temperature, say about half an hour. If possible this 
operation should be repeated. During a stable condition ot the air, it a 
mercurial or another aneroid can be set and kept at a uniform tempera- 
ture during the time, it will correct for any change of pressure, and 
fewer trials will be required. 
The aneroid may be hung up or placed flat on its back, but changing 
from one position to another ordinarily changes the readings sensibly, 
and it should therefore be kept while in use constantly in its selected 
position and place. It is usual to adjust them to the standard mercurial 
barometer while they are lying flat in their cases. 
38. An additional merit which the aneroid possesses is, that being 
more sensitive than the mercurial one', its variations take place simul- 
taneously with their causes. Its mechanism, however, like other pieces 
of mechanism, is liable to derangement, which can only be detected by 
frequent comparison with a correct standard. 
Well-constructed and thoroughly tested aneroids, if issued for use 
with a record of their variable index errors, and properly cared for there- 
after, will not only be found, even with mercurial barometers, very use- 
ful on board ship, but in the event of accident to the mercurial one, 
which frequently happens, an indispensable instrument to the careful 
and intelligent seaman. 
39. Aneroid barometers, if often compared with good barometric col- 
umns, are similar in their indications and valuable ; but it must not be 
forgotten that they are not independent instruments, that they are set 
originally by a mercurial barometer, by the application of a small turn- 
screw gently to the screw-head at the back. They require adjustment 
occasionally in the same way that they are in the first i>lace set. They 
may also deteriorate in time, though slowly. 
40. The aneroid barometer is quick in indicating the variation of 
atmospheric pressure, and to the navigator, who knows the difficulty in 
using mercurial barometers at times, this instrument is a great boon, 
for it is steady under all circumstances of weather, at the same time 
giving indication of increased or diminished pressure. 
41. In ascending or descending elevated land, the hand of the aneroid 
may be seen to move like the hand of a watch, showing the height above 
the level of the sea or place of comparison by allowing 0.0011 of an inch 
for each foot. 
42. The principle on which the aneroid is constructed may be ex- 
plained in a few words, without going into a scientific or minute detail 
of its various parts. The weight of a column of air, which in the mer- 
curial barometer acts on the mercury, in the aneroid (a name intended 
to express " without fluid") presses on a small round metal box, from 
which nearly all air is extracted; to this box is attached by nice mechan- 
ical arrangement the hand seen to work over the face of the instru- 
WEATHER GUIDES. WEATHER GUIDES. 
27 
ment. When the atmospheric pressure is lessened in the vacuum box, 
a spring, acting on a lever, turns the hand to the left, and when the 
pressure increases the spring is pressed down by the lever, and the hand 
is turned to the right. It acts in any position, but may vary several 
hundredths with such a change. 
43. The known expansion and contraction of metals under varying 
temperatures might cause a doubt as to the accuracy of the aneroid 
under such changes; but the inventor has met this difficulty by intro- 
ducing into the vacuum box a small quantity of gas as a compensation 
for the effects of heat and cold ; the gas in the box changing its bulk on 
a change of temperature, is intended to compensate for the effect on the 
metal of which the aneroid is made. Additional compensations have 
also been devised. 
44. Metallic barometers, as well as sympiesometers,* are likewise de- 
pendent or secondary instruments, and are liable to deterioration. 
45. For limited employment, when sufficiently compared, they are 
very useful, and are justly considered to be more sensitive than the 
marine barometer, falling sooner and rising earlier, beginning to move 
before the mercury in a barometer, being affected by some cause besides 
pressure and temperature. 
4G. In the sympiesometer a gas is used, which presses on the con- 
fined surface of the liquid with an uniform pressure at an equal state of 
temperature. 
47. The liquid is raised or depressed by any increase or diminution in 
the density of the atmosphere, and the change of temperature is allowed 
for by the sliding scale being always set to agree with the height of the 
mercury in the thermometer attached to the instrument in the follow- 
ing manner: Bring the pointer on the sliding scale of the sympiesome- 
ter to the same degree on the inverted scale over which it slides, as is 
indicated by the thermometer; the height of the oil as then shown by the 
sliding scale will indicate the pressure of the atmosphere. 
48. Suppose the temperature marked by the thermometer to be 52, 
then slide the movable scale until the pointer is at 52 on the inverted 
scale, and the top of the red fluid stands opposite the second division 
above the third tenth, higher than the number 30; the height at which 
the instrument then stands will be 30 inches 3 tenths and a half; the 
tenths are drawn longer to distinguish them from the half tenths, and 
the quarters still shorter than the halves. In some instruments the 
tenths are again subdivided to show hundredths, but the above method 
is the one usually adopted as being sufficiently minute. 
49. The circular register at the bottom of the frame is set by turning 
the division on it, which corresponds with that indicated by the sym- 
piesometer to the index. As the instrument is delicate, great care 
should be taken in carrying or handling it (keeping the top upwards) in 
* I think that every ship ought to have either a barometer or sympiesometer, which is an 
efficient substitute tor a barometer. 28 
WEATHER GUIDES. 
preserving it from casual rays of the sun, and likewise in adjusting the 
sliding scale to the change of temperature. 
50. Commonly familiar as the practical use of the barometer as a 
weather-glass is at sea as well as on land, only those who have long 
watched its indications and compared them carefully are really able to 
conclude more than that a rising glass (column) usually foretells less 
wind or rain, a falling column (glass) more rain or wind, or both ; a high 
column fine weather, and a low one the reverse. But useful as are these 
general conclusions in most cases, they are sometimes erroneous, and 
then depreciating remarks are rather hastily made, tending to discour- 
age the inexperienced. 
51. By attending to the following brief observations, (the results of 
many years' practice and many persons' experience,) any one not accus- 
tomed to use a barometer may do so with less hesitation and with imme- 
diate advantage: 
52. The column of mercury in a good barometer usually stands on an 
average some tenths of an inch higher with or before polar and easterly 
winds than it does with or before equatorial and westerly winds (of equal 
strength and dryness or moisture) in all parts of the oceans. 
53. (The terms polar and equatorial are used with reference to winds 
blowing from the nearest polar direction, or from the direction of the 
equator.) 
54. This peculiarity causes many mistakes. 
55. The glass is high, perhaps, but falling. Wind, or rain, or both, 
are expected in consequence, yet neither follow to any remarkable degree. 
A change of wind only from one quarter to another takes place. Reversely 
the glass is low, but rising. Finer weather is expected, yet instead of 
that a strong wind, accompanied perhaps by rain, hail, or snow, rises 
from the polar direction. By such changes as these, seamen are often 
misled, and much calamity occurs sometimes. 
56. An impending fall of rain or snow affects the column of mercury; 
wind still more, (having regard to the quarter whence it blows, and the 
average difference.) Electricity in the atmosphere, lightning and thunder, 
sometimes affect the sympiesometer, or a metallic barometer, such as 
Bourdon s, remarkably, but not the aneroid, or a mercurial barometer. 
57. Allowance should always be made for the previous state of the 
column during some days, as well as some hours, because its indications 
maj be affected by remote causes, or by changes close at hand. Some 
of these changes may occur at a greater or less distance, influencing 
neighboring regions, but not visible to each observer whose barometer 
feels their effect. Moreover, as a general rule, the longer a change of wind 
oi weather is foretold by the barometer before it takes place, the longer 
the piesaged weather will last; and, reversely, the shorter the warning, 
the less time whatever causes the warning, whether wind, or a fall of 
rain or snow, will continue. 
58. There maj be hea\ y rains or violent winds beyond the horizon, and WEATHER GUIDES. 
29 
even the view of an observer, by which his instruments may be affected 
considerably, though no particular change of weather occurs in his imme- 
diate locality. 
59. Sometimes severe weather from an equatorial (southerly in north 
latitude, northerly in the southern hemisphere) direction, not lasting long, 
may cause no great fall of the glass, because followed by a duration of 
wind from polar regions; and at times the column may fall considerably 
with polar winds and fine weather, apparently against the rule (or law,) 
because a continuance of equatorial wind is about to follow. 
GO. Although wind and its direction affect the barometer more than 
rain or snow, as certain winds usually bring more moisture than others, 
the apparent cause of barometric movement is by some attributed chiefly 
to wind, by others (on land) generally to rain or snow. The hygrometer 
(or moist and dry thermometer) indications are useful in these cases. 
61. It may be repeated here, that as in the higher latitudes the mer- 
cury usually ranges from an inch to two or even three inches; but in 
tropical regions a few tenths of an inch are seldom exceeded, except 
before hurricanes, or similar tempests, (when the quicksilver has been 
known to fall below 28, even to 27 inches,) allowance should always be 
made, in forecasting weather, for the range in the latitude ; and that in 
the middle and higher latitudes storms are much more frequent and of 
longer duration than in tropical or low latitudes, where their fury, though 
extreme occasionally, lasts only for a short time. 
62. There is little variation of the barometer between the tropics, 
because the wind blows generally in the same direction and with equal 
force, and no contending currents of air cause any considerable change 
in the temperature or density of the atmosphere. For great storms or 
hurricanes, however, within the tropics, the barometer falls very low, 
but soon returns to its usual state. 
G3. It has been observed on some coasts that the barometer is differ- 
ently affected by the w ind according as it blow's from the sea or from the 
land, the mercury rising on the approach of the sea breeze, and falling 
previously to the setting in of the land wind. 
64. Continued series of observations have proved the inter-tropical 
diurnal rise and fall of the barometer, the mercury falling from near 10 
till about 4 in the afternoon, when it reaches the lowest point of depres- 
sion; and rising from betw een 3 and 4 till 9 or 10, at which time it 
reaches the highest point of elevation, after which it falls until about 
2 a. m. From this time it rises till past 9 o'clock, w hen it reaches the 
highest point of elevation. 
65. If the mercury moves contrary to this regular course, compara- 
tively unfavorable weather may be expected, in any inter-tropical region. 
66. These atmospherical tides depend upon the sun's influence and the 
rotation of earth, and do not follow the place of the moon. 
67. The rise and fall of the mercury is about 6 or 7 hundredths of an 
inch. Its regularity is disturbed by land, but in the ocean it prevails to 30 
WEATHER GUIDES. 
latitude 26° north and south; and in fine steady weather may be perceived 
even in the middle latitudes. 
68. The mercury begins to rise occasionally before the conclusion of a 
gale, sometimes even at its commencement, as the equilibrium of the 
atmosphere begins to be restored. Although the mercury falls lowest 
before high winds, it frequently sinks considerably before a heavy fall 
of rain. The barometer falls, but not always, on the approach of thunder 
and lightning, or when the atmosphere is highly charged with the electric 
fluid. Before and during the earlier part of serene and settled weather 
the mercury commonly stands high, and is stationary. 
69. Instances of fine weather with a low barometer occur exceptionally, 
but they are preludes to a duration of wind or rain, if not both. 
70. After very warm and calm weather a storm is likely to follow, or 
at any time when the atmosphere is heated much above the average tem- 
perature. 
71. It has been mentioned that wind and weather may be foretold by 
the sympiesometer or aneroid, as well as .by a (mercurial) barometer, 
provided that these dependent or secondary contrivances are set from 
time to time by the independent and more reliable instrument, a good 
barometer. 
72. They may foretell changes in the atmosphere better, because they 
are more sensitive than a marine barometer, (with a contracted tube,) 
though not so with reference to a good land barometer, with a large and 
uncontracted tube. Oil sympiesometers, or metallic barometers, may 
mislead at times, because more peculiarly affected by lightning or an 
electrical state of atmosifliere, but they are very useful in warning on all 
occasions. 
73. Some young seamen hardly appreciate sufficiently common rules 
about weather, which are as true as they are trite; namely, that a red 
sky at sunset presages fine weather; a red sky in the morning bad 
weather, or much wind (if not rain;) a gray sky in the morning, fine 
weather; that soft-looking or delicate clouds foretell fine weather, with 
moderate or light breezes; hard-edged, oily-looking clouds, wind; that 
a dark, gloomy blue sky is windy, but a light, bright blue sky indicates 
fine weather; that, generally, the softer the clouds look, the less wind 
(but perhaps more rain) may be expected; and the harder, more " greasy," 
rolled, tutted, or ragged, the stronger the coming wind will prove. Also, 
that a bright yellow sky at sunset presages wind; a pale yellow, wet; 
and that by the preponderance of red, yellow, or gray tints, the coming 
weather may be foretold very nearly; indeed, if aided by instruments, 
almost accurately. 
74. These indications of weather, afforded by colors, seem to deserve 
more critical study than has yet been given to the subject. 
75. \\ hy a rosy hue at sunset, or a gray, neutral tint at that time, 
should presage the reverse, of their indications at sunrise ; why bright 
yellow should foretell wind at either time, and pale yellow, wet; why WEATHER GUIDES. 
31 
clouds seem soft, like water-color, or hard-edged, like oil-paint, or Indian 
ink, or an oily plate; and why such appearances are infallible signs, are 
yet to be shown satisfactorily. 
THE THERMOMETER. 
1. The thermometer is an instrument founded on the principle that 
most bodies, fluids especially, expand by heat and contract by cold. It 
is used for the purpose of measuring the amount of heat in the atmo 
sphere, or other substances brought into contact with or the vicinity of it. 
In construction it differs from the barometer in having the tube closed 
at each end. The mercury contained in the tube moves in a vacuum 
caused by the expulsion of the air by boiling the mercury and then closing 
the top of the tube by means of the blow-pipe. There are three descrip- 
tions of thermometers in common use, all constructed on the same prin- 
ciple, but differing in the divisions or graduations of their respective 
scales. Fahrenheit's thermometer is generally used in England and 
America. 
2. Upon Fahrenheit's thermometer melting ice is marked 32° and boil- 
ing water 212°, the interval being divided into 180 equal parts. The 
same graduation is extended downward from 32° to zero, (0°,) and may 
be continued below zero as far as desired. Degrees below zero are 
distinguished by the minus (-) sign. 
3. For the purpose of measuring the temperature of air on board ship, 
the thermometer should be exposed in the open air, where the circulation 
is unobstructed; it should be always in the shade, removed at least a foot 
from the bulkhead or other material from or near which it is hung, pro- 
tected against the heat reflected from the neighboring objects and kept 
sheltered from the rain and spray. 
4. If the thermometer should happen to become moistened by rain or 
spray, the bulb should be carefully dried about five minutes before 
reading and recording the observations. A wooden frame of open lat- 
tice-work will be found to be a good covering for a thermometer, provided 
it is so constructed as to admit a perfectly free circulation of air about 
the instrument. 
5. The thermometer is an invaluable instrument to the careful naviga- 
tor in making observations of temperature, simultaneously, of the air 
and the surface of the ocean. The difference in the temperature of the 
air and the surface of the sea gives warning to the seaman of his approach 
to icebergs, banks, shoals or land, and, being on soundings, by being 
lower than where there are none of these obstructions to navigation; 
and by showing a higher temperature of the surface water upon entering 
the Gulf stream, the Brazil current, the equatorial currents of the Atlantic 
and Pacific oceans, the Japan current, &c., and a lower temperature on 
leaving them, which are now more or less accurately laid down on our 
charts. 
6. In approaching land at night, or in navigating in the vicinity of 32 
WEATHER GUIDES. 
shoals, observations for temperature of the air and water should be fre- 
quently made, and at regular intervals of time, with the greatest care 
and precision, and the differences and changes observed and noted. 
7. It has been found that the Guinea current, with a temperature of 
80° or 90° Fahrenheit, sets to the eastward, while in close proximity to 
it on its southern edge the equatorial current is met, setting to the west- 
ward with a temperature of 70° or lower. 
8. A temperature of only 66° in the Guinea current itself has been 
recorded by a very competent observer, showing that variations in sur- 
face temperature similar to those known to exist in the Gulf stream are 
traceable in this current, so close to the equator. 
9. The resulting deductions obtained from observations of the surface 
temperatures of the ocean are of the greatest importance to the naviga- 
tor, and should, therefore, never be neglected to be taken by those having 
the necessary means and opportunities for doing so. Every additional 
fact discovered and reported in surface temperature of the ocean is an 
advance towards the solution of the great problem of those currents, 
which is of so much interest to all seafaring persons. 
THE HYGROMETER-PSYCHROMETER OR AVET-BULB THER- 
MOMETER. 
1. The hygrometer is a most valuable and useful instrument to the 
seaman, especially as an adjunct to the barometer and thermometer in 
foretelling weather. 
2. Any instrument (and there are several instruments of the kind) 
adapted to measure the amount ot moisture in the air is called a hygro- 
meter, (or psychrometer, " cold-measuring moisture measurer,") but the 
one best suited to use on board ship is the wet-bulb thermometer. 
3. The psychrometric hygrometer consists of two good equal thermom- 
eters, mounted on the same frame of wood, six inches wide by twelve 
inches long, or thereabouts, the bulb of one thermometer being naked, 
while the bulb of the other is wrapped in some thin, absorbent covering, 
such as a little muslin bag, with a kind of wick reaching from it into a 
small cistern of water, such as a small preserve pot, or a short-necked 
bottle. 
4. For the instrument to act truly, great care must be taken to choose 
two thermometers, which correspond exactly, degree for degree, from 
about 15° up to 90°. This is not at all an easy matter, for thermometers 
a ary in the most tiresome manner, even when both are superior instru- 
ments. Both the bulbs (naked and covered, or "dry" and "wet") should 
project an inch or two, clear all round, below their frame, for the action 
of the air to be exerted on them more perfectly. 
5. The little cistern ot water should be suspended, so that the surface 
of the water may be from one to three inches away from the bulb to 
which it is connected by the wick, and it should be placed on the side 
farthest from the dry bulb, so that its evaporation may not affect the dry 
bulb, as well as the wet bulb. WEATHER GUIDES. 
33 
G. The evaporation of the water produces cold, and this (the wet bulb) 
thermometer habitually (with very rare exceptions) stands lower than a 
dry-bulb thermometer similarly exposed. This dei>ression, strictly, meas- 
ures only the evaporating power of the air; yet, as the latter depends 
upon the amount of moisture present in the air, the depression of the 
wet-bulb thermometer measures the humidity of the air. 
7. When the temperature is in the neighborhood of the freezing-point, 
the observation of the psychrometer requires very peculiar care. 
During fog the wet bulb thermometer may sometimes be higher than the dry 
bulb; then the air is over-saturated, and contains, besides the vapor at its 
'maximum of tension, water suspended in a disseminated liquid state. This 
is, however, not a frequent occurrence. 
8. If the temperature of the air (i. e., the dry bulb) should have 
descended below 32°, it will often happen that the wet bulb thermome- 
ter will for a time read higher than the dry bulb. Such observations must 
not be recorded ; but when the water surrounding the wet bulb has begun 
to freeze, the proper readings will take place. 
9. If the water in the muslin covering the wet bulb be frozen, the 
readings will be perfectly correct. 
If the muslin be found dry, it should be wetted with a brush or small 
sponge, and then be left a little while for the water in the muslin to be 
frozen; and when satisfied that such is the case, the observer may pro- 
ceed to take the readings in the usual way. Unless this caution is 
attended to, the wet bulb will read as high or higher than the dry bulb. 
When the weather is frosty, the muslin should be thoroughly wetted 
some time (say an hour) before the usual and chief hour of observation. 
10. If the temperature should have ascended above 32° (in frosty 
weather) immerse the wet-bulb thermometer in warm water for a minute 
or so, that any ice remaining on the muslin may be melted. Unless this 
be attended to, the wet bulb thermometer will continue to read 32°, so 
long as any ice remains in contact with it. 
11. The muslin and wick should so act as, by capillarity, to keep the 
wet bulb always wet, but not actually dripping, so that rapid evaporation 
may be constantly going on.* 
12. If the air is very still, it is well to increase the evaporation by 
setting the air in motion by a fan. If the wind is too strong, the instru- 
ment should be protected by the movable blind. The reading must be 
made rapidly, and, as much as possible, at a distance, and without open- 
ing the window; for the proximity of the observer, either by the heat 
radiating from his body, or by his breath, as well as the temperature 
and hygrometrical state of the air issuing from the chamber, which is 
always different from that of the external air, especially in winter, 
would infallibly act upon the instruments, and would falsify the obser- 
vation. 
*Tbe little niusliu bag and its cotton wick should be changed about once a month. 
3 W G 34 
WEATHER GUIDES. 
13. Verification.-The two thermometers must be carefully compared 
from time to time, and if a difference is found, the instruments must be 
adjusted, 01* it must be taken into account, and the observations cor- 
rected when entered in the journal. 
14. The more the dry bulb is elevated in temperature above that of 
the wet bulb, the less is the amount of moisture in the air, in proportion 
to the temperature of the air, and vice versa. The process of evapora- 
tion lowers the temperature of the wet bulb beneath that of the dry 
bulb, either by some whole degrees or some decimal parts of a degree. 
15. If the two thermometers correspond exactly, when both of them 
are dry, they can never stand alike, when one of them is wetted, except 
when the atmosphere is so completely saturated that it can take up 
no more moisture. In this condition ot the air, the dry bulb is really as 
much wetted by the surrounding air as the wet bulb is wetted by the 
surrounding moistened muslin. The thermometric variation of the 
hygrometer is therefore said to be in an inverse ratio to the amount of 
atmospheric moisture. 
16. Popularly, the air is said to be the most damp at about sunrise; 
and in the sense of dew, or palpable moisture, the idea is correct, because, 
the temperature being very low at that hour, there is rapid condensation. 
But at a much hotter period of the day there may be absolutely more 
moisture than at sunrise, for the higher temperature causes it to 
be retained invisibly, and without imparting any sensation of moisture. 
The simple inspection of the two thermometers will often afford a better 
criterion of the weather, and of the probability of rain, than the barom- 
eter itself; regard, however, being had to the time of the day, and of 
the year, when the observation is made. 
17. In summer, when the diurnal range of temperature is great, if in 
the early morning the difference between the air temperature and the 
dew-point temperature be small, and the rise of temperature during the 
day be considerable, it is probable that the difference will increase; and 
it the temperature ot the dew point at the same time decrease, it is an 
indication ot very fine weather. If, on the contrary, the temperature 
of both should increase as the day advances, in nearly equal propor- 
tion, rain will almost certainy follow, as the air cools with the declining 
sun. 
18. In winter, when the diurnal range of temperature is small, the 
indication ot the weather is shown by the increase or decrease in the 
temperature of the dew-point, rather than by the difference between the 
temperature of the dew-point. 
1J. In showery weather, the indications vary rapidly'; and a person 
making observations, at short intervals, may predict the approach of 
a storm, particularly if he take simultaneous observations with the 
barometer. 35 
WEATHER GUIDES. 
APPENDIX A. 
THE BAROMETER AT SEA. 
Latitude. 
Barometer. 
Latitude. 
Barometer. 
Equator 
Inches. 
29. 92 
Between 0° and 5° S 
Inches. 
29.94 
Between 0° and 5° N 
29. 915 
5° S 
29. 96 
5° N 
29.91 
Be i ween 5° and 10° S 
29. 981 
Between 5° and 10° N 
29. 922 
10° S 
30 
10° N 
29. 94 
Between 10° and 15° S 
30. 028 
Between 10° and 15° N 
29. 964 
15° S 
a 30.05 
15° N 
29. 99 
Between 15° and 20° S 
30. 06 
Between 15° and 20° N 
30. 018 
20° S 
30.09 
20° N. 
30 05 
Between 20° and 25° S 
30. 102 
30. 081 
25° S 
30. 11 
25° N 
30. 12 
Between 25° and 30° S 
30. 095 
30. 149 
30° S. 
30. 08 
30° N 
30. 18 
Between 30° and 36° S 
36. 052 
Between 30° and 35° N 
30.210 
35° S 
30. 02 
35° N 
30.16 
40° S 
29. 92 
Between 35° and 40° N 
30. 124 
Between 40° and 43° S 
29. 88 
40° N 
30. 11 
Between 43° and 45° S 
29. 78 
Between 40° and 45° N 
30. 077 
45° S 
29. 75 
45° N 
30.05 
Between 15° and 48° S 
29.63 
Between 45° and 50° N 
30. 06 
Between 48° and 50° S 
29. 62 
50° N 
30 
50° S 
29.55 
Between 50° and 55° N., (England) 
29. 95 
Between 50° and 53° S 
29. 48 
55° N 
29. 99 
Between 53° and 55° S 
29. 36 
In 59° 51' N., (St. Petersburg) 
29.88 
55° S 
29. 34 
60° N 
29.88 
56|° S 
29.29 
65° N 
29. 84 
60° S 
29.15 
28. 96 
70° N 
29. 80 
65° S 
In 78° 37' N., (Dr. Kane) 
29. 759 
70° S 
28. 69 
MEAN HEIGHT OF THE BAROMETER AT SEA. 
MEAN HEIGHT OF THE BAROMETER BETWEEN THE PARALLELS OF 40° AND 50° N., 
IN 1 HE ATLANTIC WEST OF 45° WEST. 
Wind from- 
Winter. 
Spring. 
Summer. 
Fall. 
First quarter, or north to east 
30. 03 
29. 92 
29. 98 
29. 98 
Second quarter, or east to south 
29. 98 
29. 87 
30. 04 
29. 98 
Third quarter, or south to west 
29.86 
29. 86 
29. 89 
29. 92 
Fourth quarter, or west to north 
29.85 
29. 78 
29.91 
29. 92 
Mean 
29.89 
29.84 
29.93 
29. 94 
MEAN HEIGHT OF THE BAROMETER IN HIGH SOUTHERN LATITUDES, 
Between the parallels of- 
% 
w 
As observed between the meridians of- 
20° W. and 
140° E. 
140° E. and 
80° W. 
Off Cape 
Horn. 
Mean. 
40° S. and 43° S 
Inches. 
29. 90 
Inches. 
29. 84 
Inches. 
29.86 
Inches. 
29.88 
43° S. and 45° S 
29. 80 
29. 73 
29. 75 
29. 78 
45° S. and 48° S 
29.58 
29.71 
29. 68 
29. 63 
48° S. and 50° S 
29.53 
29. 56 
29. 61 
29.57 
50° S. and 53° S 
29.35 
29.45 
29.56 
29. 48 
53° 8. and 55° S 
29. 17 
29. 35 
29. 37 
29. 36 
S. of 55° 
29. 27 
29. 42 
29. 28 
29. 29 36 
WEATHER GUIDES. 
MEAN OF BAROMETRICAL OBSERVATIONS SOUTH OF 40° S., AND BETWEEN THE 
MERIDIANS OF 20° W. AND 140° E. 
J CO CO CO CO CO CO 
Between the 
parallels of- 
: : Jagggg 
: : g£gS2e 
January. 
S88SB83 
February. 
30.01 
29. 74 
29. 51 
29. 57 
March. 
ggggg 
£E$2g£ 
April. 
ggggs 
gssss 
May. . 
ggggg 
June. 
88ZS213 
July. 
: gggg 
: gguS 
August. 
: : 9JHI 
: : 
September. 
: gggg 
: 5S23 
October. 
: gggggg 
: 
November. 
: SSSggS 
: 
December. 
MEAN OF BAROMETRICAL OBSERVATIONS SOUTH OF 40° S., AND BETWEEN THE 
MERIDIANS OF 140° E. AND 80° W. 
r= 
"mm 
• GO GO GO GO GO GO 
ft 
t?5£ir£25 
January. 
ggsssss 
5582282 
February. 
ggsssss 
March. 
SSSKSSg 
2383823 
April. 
?W.888.B 
8258883 
May. 
8S2S888 
2828325 
June. 
*W£?W 
8288333 
July. 
:gggggjg 
August. 
:£32338 
ggggggs 
8358232 
September. 
: 88S8SS 
: 88'8338 
October. 
Iggggsss 
888. £889 
November. 
S8S82SS 
8522283 
December. 
The barometer in high southern latitudes. 
MEAN OF BAROMETRICAL OBSERVATIONS 
SOUTH OF 40° S., AND BETWEEN THE 
OR IN THE CAPE HORN REGIONS. 
MERIDIANS OF 80° W. 
AND 20° W., 
Between the 
parallels of- 
January. 
February. 
! March. 
April. 
£ 
c3 
•S 
June. 
July. 
August. 
September. 
October. 
November. 
December. 
40° and 43° S .. 
29. 85 
29. 93 
29. 94 
29.82 
29.78 
29. 78 
29. 98 
29. 93 
29. 89 
29. 87 
29. 84 
29. 80 
43' and 45° S .. 
29. 60 
29.74 
29. 84 
29. 70 
29. 73 
29. 66 
29. 83 
29. 97 
29. 77 
29 87 
29.60 
29. 72 
45° and 48° S .. 
29. 61 
29 70 
29. 73 
29. 72 
29. 55 
29. 76 
29.61 
29.84 
29. 78 
29. 78 
29. 40 
29. 60 
48° and 50° S .. 
29. 46 
29. 54 
29. 73 
29. 56 
29. 65 
29. 83 
29. 74 
29. 66 
29. 69 
29. 70 
29. 37 
29. 55 
50° and 53° S .. 
29. 44 
29. 57 
29. 46 
29. 75 
29. 57 
29.58 
29. 30 
29. 60 
29. 53 
29. 49 
29. 23 
29. 43 
53° and 55° S .. 
29. 33 
29. 43 
29. 26 
29. 49 
29. 35 
29. 45 
29. 33 
29. 47 
29. 50 
29. 30 
29. 00 
29. 47 
S. of 55° 
29.27 
29. 28 
29. 34 
29. 33 
29. 43 
29. 25 
29. 62 
29. 25 
29. 17 
29. 33 
29.08 
29.41 
MEAN HEIGHT OF BAROMETER AS OBSERVED IN GALES OF WIND SOUTH OF THE 
PARALLEL OF 40° S. 
Between the parallels of- 
A. 
1st quarter. 
2d quarter. 
3d quarter. 
4th quarter. 
40° and 43° S 
43° and 45° S 
29. 86 
29. 59 
29 65 
29. 50 
45° and 48° S 
29. 58 
29. 59 
29. 49 
29.50 
48° and 50° S 
29. 42 
29. 51 
29. 48 
29. 38 
50° and 53° S 
29. .15 
29.46 
29.33 
29. 27 
53° and 55° S 
29. 46 
29. 34 
29. 34 
29.18 
S. of 55° 
29. 31 
29. 07 
29. 23 
29. 19 
29. 24 
29. 45 
29.17 
29. 14 WEATHER GUIDES. 
37 
APPENDIX B. 
DIRECTIONS FOR USING THE INSTRUMENTS AT SEA. 
BAROMETER. 
1. The barometer should be fixed in a good light for observing, but 
out of the reach of sunshine or the occasional heat of a fire or lamp. It 
should hang where it can swing freely, and be out of the way of persons 
passing near it, so as to be carefully protected from injury. The height 
of the cistern of the barometer above the level of the sea should be 
ascertained, and noted in the register at the beginning of the cruise or 
voyage. 
How to fix a barometer in position. 
2. A bracket and screws for suspending the barometer are in its box. 
Screw up the bracket where the barometer is to hang. Then lift the 
instrument carefully out of its box, bend back the hinged part of the 
suspension arm, and slip it into the bracket. (The holding screws should 
not be driven quite home until the instrument is in position.) The mer- 
cury will then fall gradually and the instrument will usually be ready 
for observation in about an hour. The box should be safely stowed 
away. 
Hoiv to take dozen a barometer. 
3. Whenever it may be necessary to take down a barometer and stow 
it in its box, the vernier should be brought doivn to the bottom of the scale. 
Then, having lifted it out of the bracket, place or hold it in an inclined 
position for a few minutes so as to allow the mercury to flow very gently 
up to the top of the glass tube. It should then be taken lengthwise and 
laid in its box. It is now portable, without any other adjustment what- 
ever; and may be carried with the cistern end upwards or lying flat, but 
it must not be subjected to jars or concussions. 
How to pack a barometer for travelling. 
4. Experience shows that it is advisable to give some directions as to 
packing barometers. The instrument having been taken down and 
placed in its box as directed, if it is to be sent by rail or other convey- 
ance, and will probably have to be handled by persons unacquainted 
with its delicate and peculiar construction, should be placed in a pack- 
ing case with two or three inches of soft elastic packing all round it, as 
hay, straw, shavings, tow, or paper-cuttings. The lid of the case should 
never be nailed down, but should always be fastened with screws. The 
address label should pasted (not nailed) on the end of the case which 
is next the cistern, or lower end of the barometer, and it should be 
marked " Glass and fragile instruments. Keep this box lying flat or 
carry it this end upwards." Of course, if two or more barometers are 38 
WEATHER GUIDES. 
packed together, the cisterns should all be placed at this marked end of 
the case. Barometers should not be transmitted in freight trains, but 
always by that route and that conveyance which afford the most easy 
transit. Transshipment, or change of conveyance should be avoided if 
possible. 
Barometer vernier. 
5. The vernier, as usual in standard barometers, reads to the one five 
hundredth part of an inch (.002.) Every long line cut on the vernier 
corresponds to one hundreth (.010,) each small division on the scale is 
five hundredths (.050.) When the lower edge of the vernier is next 
above one of the short lines of the scale, the hundredth parts read oft 
from the vernier must be added to .050. When the lower edge of the 
vernier is next above one of the divisions marked tenths, the figures 
read on the vernier, only, are to be written down. 
G. The general principle of this moveable dividing scale is that the 
total number of the smallest spaces or subdivisions of the vernier are 
made equal, taken together, to one less or more than that number of the 
smallest spaces in an equal length of the fixed scale. In these barome- 
ters the twenty-five spaces of the vernier are equal to any twenty-four 
spaces of the scale, which are each half a tenth or five-hundredths of an 
inch; therefore a space on the scale is larger than a space on the vernier 
by the twenty-fifth part of .05, which is .002 inch, consequently the ver- 
nier exhibits differences of .002 of an inch. 
7. It may be added that in these barometers the diameter of the cis- 
tern is about an inch and a quarter, and that of the tube about a quar- 
ter of an inch. The scale, instead of being divided into inches in the 
usual way, is shortened in the proportion of about 0.04 of an inch for 
every inch. The object of shortening the scale is to avoid the necessity 
of applying a correction for difference of capacity between the cistern 
and the tube. 
How to set the vernier. 
8. The vernier is moved by a rack and pinion. Turn the milled head 
of the pinion so as to bring the lozcer edge of the vernier on a level with 
the top of the mercurial column. When set properly, the front edge of 
the vernier, the top of the mercury, and the back edge of the vernier 
should be in the line of sight, which line will thus just touch the middle 
and uppermost point of the column. Great care should be taken to 
acquire the habit of reading with the eye exactly on a level with the top 
of the mercury, that is with the line of sight at right angles to the scale. 
9. A piece of white paper held at the back of the tube so as to reflect 
the light, assists in accurately setting the vernier. A small bull's eye 
lamp held behind the instrument enables the observer to get a correct 
reading at night. When observing the barometer it shouldhang/reeZy, 
not being inclined by holding or even by a touch ; because any inclina- 
tion will cause the column to rise in the tube. WEATHER GUIDES. 
39 
10. The mode of reading off may be learned from a study of the fol- 
lowing diagrams, in which A B represents part of the scale, and C D 
the vernier, the lower edge D denoting the top of the mercurial column. 
The scale is readily understood; B is 29.00 inches ; the first line above 
B is 29.05 ; the second line 29.10, and so on. The first thing is to note 
the scale line just below D, and the next is to find out the line of the 
vernier which is in one and the same direction with a line of the scale. 
In figure 1 the lower edge of the vernier D is represented in exact coin- 
cidence with scale line 29.5 ; the barometer therefore reads 29.5C0 inches. 
Studying it attentively in this position it will be perceived that the ver- 
nier line a is .002 inch below the next line of the scale. If, therefore, 
the vernier be moved so as to place a in line with z, the edge D would 
read 29.502. In like manner it is seen that b is .001 inch away from the 
How to read the barometer. 
Fig. 1. 
Fig. 2. 
line next above it on the scale ; c, .006 inch apart from that next above 
it; d, .008 inch from that next above it; aid 1 on the vernier is .010 
below y. Hence, if 1 be moved into line with y, D would read 29.510. 
Thus the numbers 1, 2, 3, 4, 5, on the vernier indicate hundredths, and 40 
WEATHER GUIDES. 
the intermediate lines the even thousandths of an inch. Beferring now 
to figure 2, the scale line just below D is 29.65. Looking carefully up 
the vernier the third line above the figure 3 is seen to lie evenly with a 
line on the scale. The number 3 indicates .03, and the third subdivision 
.006; and thus we get 
Reading on scale 
29.650 
Reading on vernier 
i .030 
( .006 
Actual reading 
inches, 29.686 
Sometimes two pairs of lines will appear to be coincident ; in which 
case the intermediate thousandth of an inch should be set down as the 
reading. Thus suppose the reading appears to be 29.681 or 29.686, the 
mean 29.685 should be adopted. 
THERMOMETERS. 
11. Two thermometers should be suspended in a screen, (provided for 
the purpose,) which should be fixed in a shady place on the upper deck, 
about four feet from the deck, and freely exposed to the external air. 
Extra precaution is sometimes requisite to protect them from the influ- 
ence of the sun's rays. 
12. One is intended to give the temperature of the air, and the other 
that of evaporation. They should be fixed in the screen, without cases 
or guards, near each other, but not within a less distance than two or 
three inches. They should be placed away from all influences of stoves 
or furnaces, and of draughts of hot air from the cabins or engine room. 
13. A piece of the finest muslin or cambric should be tied round the 
bulb of one thermometer, and a few threads of cotton wick tied round 
the glass stem close to the bulb, (see a, Fig. 3,) touching the muslin, and 
long enough to reach two or three inches below the lowest part of the 
bulb, should be carried down so as to dip into and remain in a small 
vessel of water. By this arrangement the water is slowly conducted, 
by capillary attraction, to the bulb and evaporated there. See Fig. 3. 
14. The cup, glass, or other small holder of water ought not to be 
under or too near the dry thermometer. This little reservoir should be 
on the off-side of the wet thermometer, that is, as far as possible from the 
dry thermometer, which of course should not receive any moisture 
either from rain or otherwise. The water should be either distilled or 
rain water, or if this be not procurable, the softest pure water which 
can be had. The water vessel should be replenished after, or some little 
time before, observing; because observations are incorrect if made while 
the water is either colder or warmer than the air. 
15. The muslin and be well washed before being applied, 
and occasionally while in use. They should be changed once or twice a 
month, according to quality and exposure to dust or blacks. Accuracy WEATHER GUIDES. 
41 
depends much on the care taken for cleanliness, and for a proper supply 
of fresh water. The temperature of evaporation is a very important 
observation, and therefore especial care should be taken to make it cor- 
rectly. 
Fig. 3. 
1G. When the wet bulb is frozen, some cold water should be taken 
from under ice, being cautions to raise its temperature as little as possi- 
ble, and the thermometer bulb should be wetted with it by means of a 
camel-hair brush or feather. After waiting a few minutes, the tempera- 
ture of evaporation may be observed. 
17. A thermometer protected by a copper case should be used for 
taking the temperature of the sea. The water should not be pumped, 
but drawn up by a bucket (and in steamers before the ejection pipe) as far 
as possible from the ship's side, and placed in the shade. The thermom- 
eter should be immediately immersed in the water, left there two or three 
minutes, and then read while the bulb is in the water. 
Hydrometers. 
To obtain the specific gravity of the sea-water a hydrometer should be 
carefully immersed in the bucket of water drawn for the temperature 
observation. When the instrument has become steady, more or less of 
its stem will stand out of the water, according as the water is more or 
less salt. The scale in the stem is divided into 40 parts, and the read- 
ing is to be taken at the surface of the water, bringing the eye as nearly 42 
WEATHER GUIDES. 
on a level with that surface as possible. In distilled water at the tem- 
perature of 62° F., the instrument should indicate 0. In sea-water it 
ranges between 14 and 36. The hydrometer should be kept free fiom 
contact with the vessel and should be kept scrupulously clean, because 
dirt or grease not only adds to the weight of the instrument, but also 
interferes with the capillary attraction between the glass and the water. 
General directions. 
The official numbers of all the instruments used should be inserted in 
the register, and any change of any kind, such as the substitution of one 
instrument for another, should at once be noted in the register. * 
All observations should be entered as made, no alterations or correc- 
tions of any kind are to be applied. 
APPENDIX C. 
MARINE BAROMETER, ADOPTED BY H. B. MAJESTY'S GOVERNMENT, ON 
THE RECOMMENDATION OF THE KEW OBSERVATORY COMMITTEE OF 
THE BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE. 
1. This instrument should be suspended in a good light for reading, 
but out of the reach of sunshine or the heat of a fire or lamp. It 
should be as nearly amidships, and exposed as little to sudden changes 
of temperature, gusts of wind, or injuries, as possible. In a ship of war 
it should be below the lowest battery or gun deck. Light should have 
access to the back of the tube, to admit of setting the index so as to 
have its lower edge a tangent to the surface of the mercury-the eye 
being on the same level, which is known by the back and front edges of 
the index being in one line with the mercury surface. White paper or 
card will reflect light for setting the vernier correctly. Tlie height of 
the cistern above or below the ship's water line should be ascertained, 
and entered on the register. 
2. It is desirable to place the barometer in such a position as not to 
be in danger of a side blow, and also sufficiently far from the deck above 
to allow for the spring of the metal arm in cases of sudden movements 
of the ship. 
3. If there is risk of the instrument striking anywhere when the ves- 
sel is much inclined, it will be desirable either to put some soft padding 
on that place, or to check movement in that direction by a light elastic 
cord; in fixing which, attention must be paid to have it acting only 
where risk of a blow begins, not interfering otherwise with the free 
swing of the instrument ; a very light cord attached above, when possi- 
ble, will be least likely to interfere injuriously. 
4. The vernier, as usual in standard barometers, reads to the two-thou- 
santh (.002) part of an inch. Every long line cut on the vernier corre- 
sponds to .01 part; each small division on the scale is .05; the hundredth WEATHER GUIDES. 
43 
parts on the vernier being added to the five when its lower edge is next 
above one of the short lines; or written down as shown by the figures on 
the vernier only, when next above one of the divisions marking tenths. 
5. In placing this barometer, it is only necessary to fix the instrument 
carefully, as indicated in the above directions, and give a few gentle taps 
with the fingers on the bottom, to move the mercury. Without further 
operation it will usually be ready for observation in less than an hour. 
6. When moving the barometer, or replacing it in its case, the mer- 
cury should be allowed to run gently up to the top of the tube, by hold- 
ing the instrument for a few minutes inclined at an angle. The vernier 
should be brought down to the bottom of the scale. No other adjust- 
ment for portability is required. During carriage, it ought to be kept 
with the cistern end uppermost, or lying flat, the former position being 
preferable. 
7. If the mercury should not descend at first by a few gentle taps, 
use sharper, (but of course without violence,) by which, and two or three 
taps, with the finger ends, on the tube-between the scale and the tan- 
gent screw-the mercury will be made to begin to descend. 
8. In reading off from a barometer, it should hang freely, not inclined 
by holding, or even by touch. 
9. Sometimes, though rarely, at sea the mercury seems stopped. If so, 
take down the instrument, (after sloping,) reverse it, tap the tube gently 
while the cistern end is upwards, and then replace as before. 
1. In the year 1853 a conference of maritime nations was held at 
Brussels, on the subject of meteorology at sea. The report of this con- 
ference was laid before Parliament, and the result was a vote of money 
for the purchase of instruments and the discussion of observations, 
under the superintendence of the Board of Trade. Arrangements were 
then made, in accordance with the views of the Royal Society and the 
British Association for the Advancement of Science, for the supply of 
instruments properly tested. 
2. In the barometers now in general use by meteorologists on land, 
fhe diameters of the tubes are nearly equal throughout their whole 
length, and a provision is made for adjusting the mercury in the cistern 
to the zero point, previous to reading the height of the top of the col- 
umn. The object of the latter arrangement, it is well known, is to avoid 
the necessity of applying a correction to the readings for the difference 
of capacity between the cistern and the tube. At sea, barometers of 
this construction cannot be used. Part of the tube of the marine bar- 
ometer must be very much contracted to prevent " pumping," and the 
motion of the ship would render it impracticable to adjust the mercury 
in the cistern to the zero point. In the barometer usually employed on 
shore, the index error is the same throughout the whole range of scale 
readings, if the instrument be properly made; but in nearly all the bar- 
ometers which have till recently been employed at sea, the index correc- 
TESTING BAROMETERS, HYDROMETERS, AND THERMOMETERS. 44 
WEATHER GUIDES. 
tion varies through the range of scale readings, in proportion to the 
difference of capacity between the cistern and the tube. To find the 
index correction for a land barometer, comparison with a standard at 
any part of the scale at which the mercury may happen to be, is gene- 
rally considered sufficient. To test the marine barometer is a work of 
much more time, since it is necessary to find the correction for scale 
readings at about each half inch throughout the range of atmospheric 
pressure to which it may be exposed; and it becomes necessary to have 
recourse to artificial means of changing the pressure of the atmosphere 
on the surface of the mercury in the cistern. 
3. The barometers intended to be tested are placed, together with a 
standard, in an air-tight chamber, to which an air pump is applied, so 
that, by partially exhausting the air, the standard can be made to read 
much lower than the lowest pressure to which marine barometers are 
likely to be exposed ; and by compressing the air it can be made to read 
higher than the mercury ever stands at the level of the sea. The tube 
of the standard is contracted similarly to that of the marine barometer, 
but a provision is made for adjusting the mercury in its cistern to the 
zero point. Glass windows are inserted in the upper part of the iron 
air-chamber, through which the scales of the barometers may be seen; 
but as the verniers cannot be moved in the usual way from outside the 
chamber, a provision is made for reading the height of the mercury 
independent of the verniers attached to the scales of the respective bar- 
ometers. At a distance of some live or six feet from the air-tight cham- 
ber a vertical scale is fixed. The divisions on this scale correspond 
exactly with those on the tube of the standard barometer. A vernier and 
telescope are made to slide on the scale by means of a rack and pinion. 
The telescope has two horizontal wires, one fixed, and the other mov- 
able by a micrometer screw, so that the difference between the height of 
the column of mercury and the nearest division on the scale of the 
standard, and also of all the other barometers placed by the side of it 
for comparison, can be measured either with the vertical scale and ver- 
nier or the micrometer wire. The means are thus possessed of testing 
barometers for index error in any part of the scale, through the whole 
range of atmospheric pressure to which they are likely to be exposed, and 
the usual practice is to test them at every half inch from 27.5 to 31 inches. 
4. In this way barometers ot various other descriptions have been 
tested, and their errors found to be so large that some barometers read 
halt an inch and upwards too high, while others read as much too low. 
In some cases those which were correct in one part of the scale were 
found to be from half an inch to an inch wrong in other parts. These 
baiometers vere of the old and ordinary construction. In some the 
mercury would not descend lower than about 29 inches, owing to a fault 
very common in the construction of the marine barometer till lately in 
gem ral use, that the cistern was not large enough to hold the mercury 
which descended from the tube in a low atmospheric pressure. 
5. The practice which has long prevailed of mounting the marine bar- WEATHER GUIDES. 
45 
ometer in wood is objectionable. The instrument recently introduced 
agreeably to the recommendation of the Kew committee is greatly 
superior to any other description of marine barometer which has yet 
been tested, as regards the accuracy with which it indicates the pres- 
sure of the atmosphere. The diameter of the cistern is about an inch 
and a quarter, and that of the tube about a quarter of an inch. The 
scale, instead of being divided into inches in the usual way, is shortened 
in the proportion of about 0.04 of an inch for every inch. The object of 
shortening the scale is to avoid the necessity of applying a correction 
for difference of capacity between the cistern and the tube. The per- 
fection with which this is done may be judged of from the fact, that of 
the first twelve barometers tested at the Liverpool Observatory with an 
apparatus exactly similar to that used at Kew, (whence these instruments 
were sent by railway, after being tested and certified,) the index correc- 
tions in the two pressures of 28 and 31 inches in three of them were the 
same; two differed 0.001 of an inch; and for the remainder the differ- 
ences ranged from 0.002 to 0.006 of an inch. The corrections for capacity 
were therefore considered perfect, and, with one unimportant exception, 
agreed with those given at Kew. 
6. In order to check the pumping of the mercury at sea, the tubes of 
these barometers are so contracted, through a few inches, that, when 
first suspended, the mercury is perhaps twenty minutes in falling from 
the top of the tube to its proper level. When used on shore, this con- 
traction of the tube causes the marine barometer to be always a little 
behind an ordinary barometer, the tube of which is not contracted. The 
amount varies according to the rate at which the mercury is rising or 
falling, and ranges from 0.00 to 0.02 of an inch. As the motion of the 
ship at sea causes the mercury to pass more rapidly through the con- 
tracted tube, the readings are almost the same there as they would be if 
the tube were not contracted, and in no case do they differ enough to be 
of importance in maritime use. 
TESTING THERMOMETERS. 
The method of testing thermometers is so simple as scarcely to require 
explanation. For the freezing point, the bulbs ami a considerable por- 
tion of the tubes of the thermometers are immersed in pounded ice. For 
the higher temperatures, the thermometers are placed in a cylindrical 
glass vessel containing water of the required heat; and the scales of the 
thermometers intended to be tested, together with the standard with 
which they are to be compared, are read through the glass. In this 
way the scale readings may be tested at any required degree of tempera- 
ture, and the usual practice is to test them at every ten degrees from 32° 
to 92° of Fahrenheit. For this range of 60° the makers who supply gov- 
ernment are limited to 0.6 of a degree as a maximum error of scale read- 
ing ; but so accurately are these thermometers made that it has not been 
found necessary to reject more than a very few of them. 46 
WEATHER GUIDES. 
APPENDIX D. 
1. Placing. The barometer should be placed in a room of a tempera- 
ture as uniform as possible; not heated, nor too much exposed to the 
sun. The instrument must be suspended at the height of the eye, near 
a window, in such a manner as to be lighted perfectly, without exposure 
either to the direct rays of the sun, or to the currents of the air which 
always take place at the joinings of the windows. When the barometer 
has to be fixed to the wall, as is the case with all the common stationary 
and wheel barometers, care must be taken to secure the tube in a posi- 
tion perfectly vertical, regulating it by the plumb-line, first in front, then 
at the sides, at least in two vertical planes cutting each other at right 
angles. When the instrument is so constructed as to 
take its equilibrium itself, as the Fortin barometers and 
those of J. Green, recently made under the direction of 
the Smithsonian Institution, it is enough to hang it on a 
strong hook. These conditions being fulfilled, the rest 
of the arrangement may be varied according to the 
nature of the localities. For the Fortin and Green 
barometers, the following arrangement is convenient, 
and may be almost everywhere adopted. (See Fig. 3.)* 
2. A small oblong box, (a &,) some inches longer than 
the barometer, and a little broader than its cistern, is 
firmly set against the wall (zc w') near the window, in 
such a manner as to open in a direction parallel to the 
panes; at the summit (a) it has a strong hook, (/i A',) 
which extends beyond the box about two or three inches, 
and on which the barometer is suspended. The instru- 
ment remains generally in a box, which is closed by a 
movable cover, and which protects it from external 
injuries, from dust, and from the direct radiation of 
warm bodies, or the currents of air from the window, 
and diminishes the effect of the too sudden variations of 
temperature. When it is to be observed, the barometer 
is taken by the upper end of the tube, and the suspend- 
ing ring is made to slide toward the end of the hook. 
The instrument is then in the full light of the window, 
in front of which the observer places himself; the sum- 
mit of the mercurial column, as well as the surface of 
the mercury in the cistern, are completely lighted, and 
the reading becomes easy and certain. Moreover, the 
slight oscillating movement impressed on the instrument 
by changing its place, breaks the adherence of the mercury to the glass, 
BAROMETER. 
Fig. 3. 
* The standard barometer at the Smithsonian Institution is stationary and enclosed within 
a narrow case, the fiont and two sides of which open out by means of binges so as fully to 
expose the instrument at the time of the observation. WEATHER GUIDES. 
47 
and thus prepares a good observation. After the reading, the barom- 
eter is again slipped gently into the box, and this is closed. 
3. Observation.-The different operations of the barometer of constant 
level should be made in the following order: 
a. Before all, incline the instrument gently, so as to render the mer- 
curial column very movable; then, after having restored it to rest, strike 
several slight blows upon the casing, in such a manner as to impress on 
the mercury gentle vibrations. The adherence of the mercury to the 
glass will thus be destroyed, and the column will take its true equilibrium. 
b. Note the degree and the tenths of degrees of the thermometer 
attached to the instrument ; for it will be seen that the heat of the 
observer's body soon makes it rise. 
c. Bring, by means of the adjusting screw, (Figure 4,) the surface of 
the mercury to its constant level. In Green's first barometers, the metallic 
envelope of the cistern is pierced through, (o o',) and allows the surface of 
the mercury contained in the glass cistern to be seen. The plane which 
passes through the upper edge (e e') of this opening is the true level, or 
the zero of the scale, to which the surface of the mercury must be restored. 
4. For this, take 
hold, with the left 
hand, of the lower 
edge of the cistern, 
(Z Z',) taking great 
care not to disturb its 
vertical position; ap- 
ply the right hand to 
the adjusting screw, 
(s,) and, turning it 
gently, bring by de- 
grees the level surface 
of the mercury to the 
upper edge (e e') of. 
the opening cistern, 
until there remains 
between the two only 
an almost impercepti- 
ble line of light, as in 
Fig. 5, (ee'.) Then 
leave the instrument 
to itself, to re-establish its verticality, if it had been accidentally deranged, 
and placing the eye exactly at the height of the mercury, examine whether 
the contact is exact. For this operation, it is important to have a good 
light; the cistern ought to be placed higher than the lower edge of the 
window, so that the light may reach it directly. It is necessary also to 
take care not to confound the slight line of light which marks the oppo- 
site edge of the cistern with the light reflected by the surface of the mer- 
cury against the inner walls; the former is always sharp and well-defined, 
Fig- 4. 
Fig- 5. 48 
WEATHER GUIDES. 
the latter vague and indefinite. When, before adjusting the level, the 
mercury is higher than the upper edge, it is necessary to begin by 
lowering it beneath the level, (see Fig. 4,) so as to leave an interval of 
light, which is then gradually shut out, as has been described. When 
the observation is to be made in the night, place the lamp before, and 
not behind, the instrument, and somewhat higher than the eye; and if 
the wall itself is not light enough, place behind the cistern or the top of 
the column a piece of white paper, which reflects the light. 
5. In the barometers with an ivory point, as the Fortin, Newman, and 
Green barometers, the extremity of this point is the zero of the scale, 
which must be brought into exact contact with the surface of the mercury. 
We commonly judge that this takes place when we see the actual rounded 
summit of the point coincide exactly with its image reflected below by 
the mercury. This method may be very good when the surface of the 
mercury is perfectly pure and brilliant ; but this is very rare. It is gene- 
rally dimmed by a slight layer of oxide, which makes the coincidence of 
the point with its image uncertain. It is safer to judge of the contact 
in a different manner. From the moment when the point does more than 
touch the surface, it forms around itself, by capillary action, a small 
depression, which, breaking the direction of the reflected rays, becomes 
immediately very easy to discover. It is enough, then, to raise the mer- 
cury so as slightly to immerse the point; then to lower it gradually until 
the little depression disappears. If care is taken to make a good light 
fall on that portion of the mercury which is under the point, and to use 
the aid of a magnifier, the adjustment of the point thus made becomes 
not only easy, but very certain, and the errors to which we are liable are 
almost insensible, for they do not exceed two or three hundredths of a 
millimetre, or a thousandth of an inch. 
d. The level being thus adjusted to the zero of the scale, we proceed 
to observe the height of the summit of the column. Take hold of the 
instrument with the left hand, above the attached thermometer, without 
moving it from the vertical; strike several slight blows in the neighbor- 
hood of the top of the column; then, by means of the screw, lower the 
slide which carries the vernier, until the plane passing through the two 
lower opposite edges of it is exactly tangent to the summit of the meniscus, 
that is, the convexity which terminates the column. We know that this 
is the case when, placing the eye exactly at the height of the summit of 
the column, we still see the summit of the column without there being 
any trace of light between the summit and the edge of the ring. To con- 
vince ourselves that the barometer has remained quite vertical during 
its operation, we leave it to itself, and when it is at rest, we look again 
to see whether the ring has remained tangential to the summit of the 
column. If it has not, the vertically has been disturbed; it must be 
adjusted anew. It is necessary, at the same time, to examine if the 
adjustment of the surface of the mercury in the cistern has remained the 
same. The attached thermometer will also be read anew, and if it indi- 
cates a temperature noticeably higher than at the commencement of the WEATHER GUIDES. 
49 
observation, a mean value between the two indications must be adopted. 
An exact observer can never dispense with these verifications. 
e. Nothing more, then, remains than to read the instrument. In the 
English barometers, the inches and tenths of inches are read directly on 
the scale, the hundredths and thousandths on the vernier. In the French 
barometers, with the metrical scale, the centimetres and millimetres are 
read on the scale, and the fractions of millimetres on the vernier. We 
begin by reading on the scale the number of inches and tenths of an inch, 
or of millimetres, there are, as far up as the line which corresponds to 
the lower edge of the vernier, and which marks the summit of the column. 
In the Green barometers, this line marks at the same time the zero of the 
vernier. If this line does not coincide with one of the divisions of the 
scale, we read the fraction of the following division on the vernier. 
G. The principle of the vernier is very simple. If we wish to obtain 
tenths, we divide into ten parts a space on the vernier comprising nine 
parts of the scale, (see Fig. G;) each division of the vernier is thus found 
shorter by a tenth than each division of the scale. Now, if we start 
from the point where the zero of the vernier and its tenth division coin- 
cide exactly with the first and the ninth division of the scale, and if we 
cause the vernier to move gradually from the ninth to the tenth division 
of the scale, we shall see the first, the second, the third, and the other 
divisions of the vernier as far as the tenth, coincide successively with 
one of the divisions of the scale. Now, the divisions of the scale to which 
those of the vernier correspond, being equal parts, it follows that the 
space in question has been successively divided into ten parts, or tenths, 
by these successive coincidences. If the scale bears millimetres, the 
vernier will give tenths of millimetres; if it has tenths of an inch, the 
vernier will give hundredths. By changing the proportions, it may be 
made to indicate by the vernier smaller fractions, as twentieths of milli- 
metres, or five-hundredths of an inch, &c. • 
7. To read the vernier, we must look out for the line that coincides 
with one of the divisions of the scale; the number of this division of the 
vernier, proceeding from zero, indicates the number of tenths of milli- 
metres, or of hundredths of an inch, which must be added to the whole 
number given by the scale. If none of the divisions of the scale coincides 
exactly, we estimate by the eye, in decimals, the quantity by which the 
vernier must be lowered to obtain a coincidence, and this is added to the 
traction already obtained. 'This will be hundredths of millimetres in the 
metrical barometer, and thousandths of inches in the English barometers. 
8. The following figures will serve as an example; the instrument is 
an English barometer. 
9. In Fig. G the regulating line, which is the lower edge of the vernier 
ring, coincides exactly with the line of thirty inches on the scale. The 
zero and the tenth division of the vernier are also in exact coincidence; 
that is to say, there is no fraction. We shall read, then, 30.000 inches. 
10. In Fig. 7 the regulating line does not fall upon any of the divisions 50 
WEATHER GUIDES. 
of the scale, but between twenty-nine inches and two-tenths and twenty- 
nine inches and three-tenths of an inch. There is then a fraction which 
must be read on the vernier. Seeking which of these divisions coincides 
with that of the scale, we find that it is the fifth; we shall write then 
29.250 inches. 
Fig. 6. 
Fig. 7. 
Fig. 8. 
11. In Fig. 8 we see that the height falls between thirty inches and 
thirty inches and one-tenth; no line of the vernier also coincides exactly; 
but the line 7 is a little above, the line 8 is a little below, one of the lines 
of the scale; the fraction falls, then, between seven and eight hundredths. 
Estimating in tenths the distance the vernier passes over between the 
coincidence of seven and that of eight, we thus obtain the tenths of an 
hundredth or the thousandths. In this latter case, the distance above 
seven is less than the half; we shall then read 30.073. It will always be 
easy to judge whether the top approaches nearer the upper coincidence 
than the lower coincidence; in the former case, the fraction is greater 
than .005; in the latter, it is smaller than .005. The error which will be 
committed in this estimate will remain less than .005; with practice and 
a little skill, it will hardly ever exceed .002, always supposing the scale 
is well graduated. For this reading, as well as for the others, it is par- 
ticularly important to have the eye exactly at the height of the line to 
be determined. 
12. The same process of reading is applied to the metrical scale; the 
vernier then gives tenths directly, and by estimate the hundredths of 
millimetres. In the English instruments the inches must be separated 
by a (.) and three decimals written, even when the last is a zero; e. g., 
30.250, and not 30.25; the zero indicates that the thousandths have been WEATHER GUIDES. 
51 
taken into account, but that there are none. In the metrical scale put 
the (.) after the millimetres and admit two decimals, e. g., 761.25.* 
13. During the whole time of the observation of the barometer, the 
observer must endeavor to protect it as much as possible from the heat 
which radiates from his-body. But the best way is to learn to observe 
rapidly. All the operations of which we have just spoken take longer 
to describe than to execute; one or two minutes if the instrument be 
in place, three minutes if it is to be taken from its case and put back 
again, are sufficient for a practiced observer to make a good observation. 
14. Altitude.-The height of the barometer above the ground, or above 
some fixed point, which may serve as an invariable point of reference, 
ought to be exactly determined. 
APPENDIX E. 
1. The following is an account of Green's 
improved standard barometer, adopted by the 
Smithsonian Institution, for observers of the 
first class. 
2. The barometer consists of a brass tube, 
(Fig. 1) terminating at top in a ring A, for sus- 
pension, and at bottom in a flange B, to which 
the several parts forming the cistern are at- 
tached. 
3. The upper part of this tube is cut through 
so as to expose the glass tube and mercurial 
column within, seen in Fig. 5. Attached at 
one side of this opening is a scale, graduated 
in inches and parts; and inside this slides a 
short tube c, connected to a rack-work arrange- 
ment, moved by a milled head D: this sliding 
tube carries a vernier in contact with the scale, 
which reads off to (002) of an inch. 
4. In the middle of the brass tube is fixed 
the thermometer E, the bulb of which being 
externally covered, but inwardly open, and 
nearly in contact with the glass tube, indicates 
the temperature of the mercury in the barometei* 
tube, not that of the external air. This central 
position of the thermometer is selected that 
the mean temperature of the whole column may 
be obtained; a matter of importance, as the 
temperature of the barometric column must be 
taken into account in every scientific applica- 
tion of its observed height. 
GREEN'S STANDARD BAROMETER. 
* For the method of reading the vernier of Green's standard barometer, see the description ot the instrument. 52 
WEATHER GUIDES, 
Fi<P 53 
WEATHER GUIDES. 
5. The cistern (Fig. 2) is made up of a glass cylinder F, which allows 
the surface of the mercury q to be seen, and a top plate G, through the 
neck of which the barometer tube t passes, and to which it is fastened 
by a piece of kid leather, making a strong but flexible joint. To this 
plate, also, is attached a small ivory point h, the extremity of which 
marks the commencement or zero of the scale above. The lower part, 
containing the mercury, in which the end of 
the barometer-tube t is plunged, is formed of 
two parts ij, held together by four screws and 
two divided rings I m, in the manner shown 
in Figs. 2 and 3. To the lower piece J is fast- 
ened the flexible bag N, made of kid leather, 
furnished in the middle with a socket fc, which 
rests on the end of the adjusting-screw O. 
These parts, with the glass cylinder F, are 
clamped to the flange B by means of four long 
screws P and the ring R; on the ring B screws 
the cap S, which covers the lower parts of the 
cistern, and supports at the end the adjust- 
ing-screw O. G, i, j, and k, are of boxwood; the other parts of brass or 
German silver. The screw O serves to adjust the mercury to the ivory 
point, and also, by raising the bag, so as to completely fill the cistern 
and tube with mercury, to put the instrument in condition for transpor- 
tation. 
6. In Fortin's barometer, and also Delcro's modification of it, a cement 
is used to secure the mercury against leakage at the joints. This, sooner 
or later, is sure to give way; and tested under the extremes of the ther- 
mometrical and hygrometrical range of this climate especially, has made 
this defect more evident. This was removed by the substitution of iron in 
the place of wood; but it was soon found impracticable, in this form of 
cistern, to prevent damage from rust. These objections led to the pres, 
ent plan of construction, which effectually secures the joints without the 
use of any cement. The surfaces concerned are all made of a true figure, 
and simply clamped together by the screws, a very thin leather washer 
being interposed at the joints. This would not be permanent, however, 
but for the especial care taken in preparing the boxwood. The boxwood 
rings are all made from the centres of the wood, and concentric with its 
growth. They are worked thin and then toughened, as well as made 
impervious to moisture, by complete saturation with shellac. This is 
effected by immersing them in a suitable solution in vacuo. The air 
being withdrawn from the pores of the wood is replaced by the lac. This, 
however, with the after-drying or baking, requires care; but when properly 
done, the wood is rendered all but unchangeable. 
7. Another peculiarity consists in making the scale adjustable to cor- 
rect for capillarity, so that the barometer may read exactly with the 54 
WEATHER GUIDES. 
adopted standard, without the application of any correction; and this, 
too, without destroying the character of the barometer as an 
original and standard instrument. Near the 30 inches line, 
Figure 4, is a line v, on the main tube; this last line is dis- 
tant exactly 30 inches from the tip of the ivory point; there- 
fore, when these lines coincide, or make one line, the scale is 
in true measurement position; or the 30 mark is exactly 30 
inches from the tip of the ivory point in the cistern. In this 
position, the amount of correction due to capillarity being 
ascertained, the scale is then moved that quantity and 
clamped firm. The barometer will now give the readings 
corrected for capillarity, and thus avoid at once the labor of 
applying a correction, and the risk of error from an accidental 
neglect of it. 
8. It must be borne in mind that this correction applies only to the 
particular tube, and while preserved in good condition. 
9. If this tube is injured and again used, or another tube put in its 
place, the scale should then be moved until the lines coincide, the amount 
of correction for the repaired or the new tube being estimated until a 
good comparison can be made directly or intermediately with the Smith- 
sonian standard. 
10. The connecting the parts i and J by rings and screws, Figs. 2 and 
3, rather than by a single screw cut on the edge, is an improvement, 
as the single wood-screw is apt, after a time, to adhere so firmly that it 
is often difficult, and sometimes impossible, with safety to the parts, to 
separate it. 
11. It is not advisable to disturb the cistern, unless it becomes difficult, 
from the oxide of mercury which gradually forms, to make the adjust- 
ment of the mercury to the ivory point, as there is more or less risk in 
doing so. Any one accustomed to such mechanical affairs, with due 
attention to the plan, can, however, take out the mercury from the cis- 
tern, refilter, clear the parts of adhering oxide, and replace them; the 
instrument all the time being kept vertical, with the cistern at top, as 
the mercury must not be allowed to come from the tube. 
12. To insure a good vacuum by the complete expulsion of all air and 
moisture, the boiling of the mercury in the tube is done in vacuo; and 
care should be taken to preserve it in good condition. 
13. To put up the barometer for observation, suspend the barometer 
by the ring A in a good light, near to and at the left side of a window, 
and, when practicable, in a room not liable to sudden variations of tem- 
perature. Record the temperature, and then by the screw O lower the 
mercury in the cistern until the surface is in the same plane with the 
extremity of the ivory point. As this extremity of the point is the zero 
of the scale, it is necessary, at each observation, to perfect this adjust- 
ment. It is perfect when the mercury just makes visible contact. If the 
surface is lowered a little, it is below the point; and if raised a small 
Fig. 4. WEATHER GUIDES. 
55 
amount, a distinct depression is seen around the point. This depression 
is reduced to the least visible degree. A few trials will show that this 
adjustment can always be made to a thousandth of an inch. 
14. The adjustment effected, bring the lower edge of the vernier C, 
Fig. 5, by means of the milled head D, into the same plane with the 
convex summit of the mercury in the tube. Looking through the open- 
ing, with the eye on a level with the top of the mercury in the tube, 
when the vernier tube is too low, the light is cut off; when too high, the 
light is seen above the top of the mercury. It is right when the light 
is just cut off from the summit, the edge making a tangent to the curve. 
A piece of white paper placed behind, and also at the cistern, will be 
found to give a more agreeable light by day, and is, besides, necessary 
for night observations; the lamp being placed before the instrument 
and above the eye, to reflect the light. 
Fig. 9. 
Fig. 10. 
15. The method of reading off will perhaps be best explained by a 
few examples. Suppose, after completing the adjustments, the scale and 
vernier to be in the position shown in Fig. 9, on this page, it will be seen 
that the lowest or index line of the vernier coincides exactly with the 
line marked 30 on the scale. The reading, therefore, is 30.000 inches. 
16. If, as in Fig. 10, we find the line of the vernier coinciding with the 
third line of the tenths above 29, we read 29.300. 56 
WEATHER GUIDES. 
17. If, as in Fig. 11, on this page, we find the index at 29 inches three- 
tenths and five-hundredths, we read 29.350. 
18. If, as in Fig 12, we find the index at 30 inches no tenths five-hun- 
dredths and something more, this additional quantity we shall find by 
looking up the vernier scale, until we come to some one line on it, coin- 
Fig. 11. 
Fig. 12. 
ciding with a line on the other scale. In this instance it is the line 
marked 2, and indicates two-hundredths, to be added to the other num- 
bers, making 30.070. 
19. If, as in Fig. 13, we And 29 inches no tenths five-hundredths, and 
on the vernier the second line above that marked 2, is found to coincide 
with the scale, each of these short lines indicates two thousandths, con- 
sequently are so counted; the reading is therefore 29.074. 
20. Or it may be, as in Fig. 14, where we have 30 inches one-tenth, and 
the line on the vernier mark 3 coinciding nearly, but not perfectly, with 
a line on the scale, it is a little too high; the two-thousandths short line 
next above is, however, a like quantity too low; so the true reading 
must be the number between them-that is, one-thousandth, making 
together 30.131. 
21. These examples include all the combinations the scale allows. A 
little practice with the barometer, with reference to the examples, will 
soon enable the learner to read off the scale with facility. At first it will WEATHER GUIDES. 
57 
be best to write down the inches and parts in full, as in the diagrams, 
not trusting the memory with the whole, until experience shall have 
given confidence. 
Fig. 13. 
Fig. 14. 
22. Be careful never to lower the mercury in the cistern much below 
the necessary quantity, as it increases the risk of air entering the tube. 
23. When the barometer is to be removed for transportation, or change 
of position, before taking it down, the mercury is to be screwed up until 
the cistern and tube are just full. If it is screwed more than this, the 
mercury may be forced through the joints of the cistern. It should then 
be inverted and carried cistern end upwards. 
24. This instrument is well adapted for service as a mountain barom- 
eter, and when used as such is packed in a leather case, with suitable 
straps for convenient carriage. 
APPENDIX F. 
A new marine barometer, which is stronger and better suited to the 
condition of ships of war, intended for use on board of vessels of the 
British navy (if found to be well adapted to general purposes) may be 
described as follows: 
1. The glass tube is packed with vulcanized India-rubber, which checks 
A NEW MARINE BAROMETER FOR THE BRITISH NAVY. 58 
WEATHER GUIDES. 
vibration from concussion, but does not hold it rigidly or prevent ex- 
pansion. 
2. It does not oscillate, (or " pump,") though extremely sensitive. 
3. The scale is porcelain, very legible, and not liable to change. 
4. There is no iron anywhere to rust. 
5. Every part can be unscrewed, examined, or cleaned by any careful 
person used to nautical instruments. 
6. There is a spare tube fixed in a spare cistern filled with boiled mer- 
cury, and marked for adjustment in this or any similar marine barome- 
ter. Similar instruments have been tested at Kew Observatory satisfac- 
torily. Although, for various reasons, these new barometers are gradu- 
ated to hundredths only, they will be found acurate to that degree, the 
second decimal of an inch. 
7. They are "packed" with vulcanized India-rubber, in order that by 
this contrivance and a peculiar strength of glass tube, gunsmay be fired 
near these instruments without causing injury to them by ordinary con- 
cussion. 
8. Each instrument box carries such a tube fitted to a spare cistern, 
filled with boiled mercury, marked at 27 inches, and verified so that it 
may be placed duly in any such barometer by adjusting the lower inner 
screw. 
9. To ship a tube.-Incline slowly and take down the barometer, 
allowing the mercury to fill the upper part. Lay the instrument on a 
table, unscrew the outer cap at the joining just below the cistern swell, 
then unscrew the tube and cistern by turning the cistern gently against 
the sun or to the left, and draw out the tube very carefully without in 
the least bending it, turning it a little if required as moved. Then un- 
screw the cistern collar at the place next to the swell, joining the brass 
tube. Take off the packing by cutting the threads, and, if necessary, 
slitting the whole length with scissors; (all the packing rubber should be 
so cut.) 
10. After putting the brass collar on the spare glass tube, tie the 
packing to it closely at three places. Then insert the new tube very 
cautiously, screw on the collar, and adjust to the 27-inch mark. Attach 
the cap and suspend the barometer for use. 
11. In about ten minutes the mercurial column will be nearly right, 
but as local temperature affects the brass as well as the mercury slowly 
and very unequally, it may be well to defer any exact comparisons with 
other instruments for some few hours. 
APPENDIX G. 
MEASURING HEIGHTS. 
1. The aneroid will be found useful for measuring heights (approxi- 
mately) by comparing it with another barometer or itself at a higher or 
lower station. WEATHER GUIDES. 
59 
2. If the measure of a height rather greater than the aneroid will 
commonly show be required, it may be reset thus: When at the upper 
station (within its range,) and having noted the reading carefully, touch 
the screw behind so as to bring back the hand a few inches (if the instru- 
ment will admit,) then read off and start again. Reverse the operation 
when descending. This may add some inches of measure approximately. 
3. In the following table the difference between the number of feet 
opposite the height of a barometer at one station, and that at another 
station, is the approximate difference of height: 
TABLE. 
Barometer 
inches. 
Height in 
feet. 
Barometer 
inches. 
Height in 
feet. 
Barometer 
inches. 
Height in 
feet. 
31.0 
0 
26.8 
3,829 
22.7 
8, 201 
30.9 
85 
26.7 
3, 927 
22.6 
8,317 
30.8 
170 
26.6 
4,025 
22.5 
8, 434 
30.7 
255 
26.5 
4, 124 
22.4 
8,551 
30.6 
341 
26.4 
4, 223 
22.3 
8, 669 
30.5 
427 
26.3 
4,323 
22.2 
8,787 
30.4 
513 
26.2 
4,423 
22.1 
8,906 
30.3 
600 
26.1 
4, 524 
22.0 
9,025 
30.2 
687 
26.0 
4, 625 
21.9 
9, 145 
30.1 
774 
25.9 
4, 726 
21.8 
9,266 
30.0 
862 
25.8 
4, 828 
21.7 
9, 388 
29.9 
950 
25.7 
4, 930 
21.6 
9,510 
29.8 
1, 038 
25 6 
5, 033 
21.5 
9, 632 
29.7 
1, 126 
25.5 
5, 136 
21.4 
9, 755 
29.6 
1,215 
25.4 
5, 240 
21.3 
9, 878 
29.5 
1,304 
25.3 
5, 344 
21.2 
• 10, 002 
29.4 
1,393 
25.2 
5, 448 
21.1 
10, 127 
29.3 
1, 482 
25.1 
5,553 
21.0 
10, 253 
29.2 
1,572 
25.0 
5, 658 
20.9 
10, 379 
29.1 
1, 662 
24.9 
5, 763 
20.8 
10,506 
29.0 
1,753 
24.8 
5, 869 
20.7 
10,633 
28.9 
1, 844 
24.7 
5, 976 
20.6 
10, 760 
28.8 
1, 9115 
24.6 
6,083 
20.5 
10, 889 
28.7 
2, 027 
24.5 
6,190 
20.4 
11,018 
28.6 
2, 119 
24.4 
6, 297 
20.3 
11,148 
28.5 
2,211 
24.3 
6, 405 
20.2 
11,278 
28.4 
2, 303 
24.2 
6,514 
20.1 
11,409 
28.3 
2, 396 
24.1 
6, 623 
20.0 
11, 541 
28.2 
2, 489 
24.0 
6,733 
19.9 
11, 673 
28.1 
2,582 
23.9 
6, 843 
19.8 
11, 805 
28.0 
2, 675 
23.8 
6, 953 
19.7 
11, 939 
27.9 
2,769 
23.7 
7, 064 
19.6 
12, 074 
27.8 
2,864 
23.6 
7, 175 
19.5 
12, 210 
27.7 
2, 959 
23.5 
7, 287 
19.4 
12,346 
27.6 
3, 054 
23.4 
7, 399 
19.3 
12, 483 
27.5 
3, 149 
23.3 
7, 512 
19.2 
12, 620 
27.4 
3,245 
23.2 
7, 625 
19.1 
12, 757 
27.3 
3, 341 
23.1 
7, 729 
19.0 
12, 894 
27.2 
3, 438 
23.0 
7,854 
18.9 
12, 942 
27.1 
3, 535 
22.9 
7, 969 
18.8 
13, 080 
27.0 
26.9 
3,633 
3,731 
22.8 
8,085 
18.7 
13,219 
APPENDIX H. 
HOW TO MEASURE HEIGHTS BY THE BAROMETER. 
The following table on Baily's formula is copied from the article Mete- 
orology, by J. F. W. Herschel, in the Manual of Scientific Inquiry, pp. 
168,169, in which "/? /Sz represent the height of the barometer as read oft 
at the lower and upper stations, respectively, in parts of any one scale; 
11' the temperatures in degrees of Fahrenheit's thermometer at the same 
respective stations of the air, as shown by the detached thermometer; 60 
WEATHER GUIDES. 
r t' the temperatures (also in degrees Fahrenheit) of the mercury at the 
same stations, as shown by the attached thermometer:" 
Thermometers in open air. 
Thermometers attached to 
the barometer. 
Latitude of place. 
Argument = < + «'. 
Argument = r - r7. 
Argument = 0. 
1+ t'. 
A. 
t + t'. 
A. 
T - T'. 
B. 
0. 
C. 
o 
o 
o 
o 
40 
4. 76891 
102 
4. 79860 
0 
0. 00000 
0 
0. 00117 
42 
4. 76989 
104 
4. 79953 
1 
0. 00004 
5 
0. 00115 
44 
4. 77089 
106 
4. 80045 
2 
0. 00009 
10 
0. 00110 
46 
4. 77187 
108 
4. 80137 
3 
0. 00013 
15 
0. 00100 
48 
4. 77286 
110 
4. 80229 
4 
0. 00017 
20 
0. 00090 
50 
4. 77383 
112 
4. 80321 
5 
0. 0U022 
25 
0. 00075 
52 
4. 77482 
114 
4. 80412 
6 
0. 00026 
30 
0. 00058 
54 
4. 77579 
116 
4. 80504 
7 
0. 00030 
35 
0 00040 
56 
4. 77677 
118 
4. 80595 
8 
0. 00035 
40 
0. 00020 
58 
4. 77774 
120 
4. 80687 
9 
0. 00039 
45 
0. OOUGO 
60 
4. 77871 
122 
4. 80777 
10 
0. 00043 
50 
9. 99980 
62 
4. 77968 
124 
4. 80869 
11 
0. 00048 
55 
9. 99960 
64 
4. 78065 
126 
4. 80958 
12 
0. 00052 
60 
9. 99942 
66 
4.78161 
128 
4.81048 > 
13 
0. 00056 
65 
9. 99925 
68 
4. 78257 
130 
4. 81138 
14 
0. 00061 
70 
9. 99910 
70 
4. 78353 
132 
4. 81228 
15 
0. 00065 
75 
9. 99900 
72 
4. 78449 
134 
4. 81317 
16 
0. 00069 
80 
9. 99890 
74 
4. 78544 
136 
4.81407 
17 
0. 00074 
85 
9. 99885 
76 
78 
80 
4. 78640 
4. 78735 
4. 78830 
138 
140 
142 
4. 81496 
4.81585 
4. 81675 
18 
19 
20 
0. 00078 
0. 00083 
90 
9. 99883 
0. 00087 
82 
84 
4. 78925 
4. 79019 
144 
146 
4. 81763 
4.81851 
21 
22 
0. 00091 
0. 00096 
Take D equal to log /? - 
(log /?'-(- B) : then will 
the logarithm of the differ- 
ence of altitudes in Eng- 
lish feet be equal to A + 
C + log D. 
86 
88 
90 
92 
4.79113 
4. 79207 
4. 79301 
4. 79395 
148 
150 
152 
154 
4.81940 
4. 82027 
4.82116 
4. 82204 
23 
24 
25 
26 
0.00100 
0.00104 
0. 00109 
0.00113 
94 
4. 79488 
156 
4. 82291 
27 
0.00117 
96 
4. 79582 
158 
4. 82379 
28 
0. 00122 
98 
4. 79675 
160 
4. 82466 
29 
0. 00126 
100 
4. 79768 
162 
4. 82553 
30 
0. 00130 
Example. 
Iii latitude 21°, on the mountain of Guanaxato, in Mexico, Baron Von 
Humboldt observed as follows: 
Lower station. Upper station. 
O O 
Thermometer in open airt=77. 6 t,=70. 4 
Thermometer attached to barometerr=77. G t'=70. 4 
In. In. 
Barometer£=30.05 £'=23. GG 
What was the difference of level between the stations? 
Log £'=1.37401 
B=0. 00031 
1. 37432 
Log £=1. 47784 
D=0.10352 Log D=9. 01502 
C=0. 00087 
A=4. 81940 
Log G843. 7=3. 83529 
Consequently, the difference required = G843.7 English feet. WEATHER GUIDES. 
61 
This table mariners will find useful; by it the height of islands, head- 
lands, and mountains above the sea may be determined. With a mer- 
curial barometer and a good aneroid the height of all such places, when 
not far from the ship, may be measured within a few feet, provided the 
aneroid be carefully compared w ith the mercurial barometer immediately 
before setting out and after returning from the height to be measured; 
and provided, also, the observations on the height and on board be 
made, according to previous arrangement, at the same hour or within a 
few minutes of it. 
APPENDIX I. 
ON FILLING BAROMETER TUBES. 
[From the Smithsonian Reports, 1859.] 
I.-By James Green, of New York. 
One of the greatest difficulties with the inexperienced is to get the 
tube itself clean and free of moisture. If the tube is foul, the common 
way is to clean it with a covered copper wire, wrapped with additional 
cotton at the end to fit the tube, and moistened with alcohol and whiting 
at first, afterward with dry cotton. If the tube can be heated and air 
blown in dry, so much the better. 
The mercury and tube should be heated as much as will be allowable 
to handle them, to keep all the water in a state of vapor. The mercury 
is filtered into the tube in a long paper funnel, in a fine stream, until 
within a quarter of an inch of the top. The tube will now be found 
covered with small air bubbles. Stop the end of the tube with the 
finger, and run a large air bubble up and down the tube. This will col- 
lect the small ones together. Provided the tube be clean and dry, and 
mercury pure, a pretty good result is obtained. 
To boil the mercury in the tube, fill within three inches of the top. 
Then, with a clear charcoal fire or long spirit lamp, warm the whole tube 
as much as you can without inconvenience. The tube being held by a 
cloth, (with woollen gloves on hands is well,) then commence at the top 
or open end and hold the tube over the fire until the mercury boils, 
moving the tube a little in all directions all the time to equalize the dis- 
tribution of the heat from the fire. Continue the boiling downwards 
until you reach the end, and then return boiling up to the top again. 
Some begin at the closed end, (for economy of risk and labor,) and boil 
up only. This may answer the purpose, but not so well as the other, 
particularly if the tube is not perfectly dry and clean. The part of the 
tube unoccupied will be well prepared by the boiling mercury bubbling- 
up ; so that to complete the filling, filter hot mercury to the top. 
The more perfect methods of boiling are impracticable out of the 
workshop and hands of the glass-blower. 
One of the best tests for the purity of the mercury is, that after once 
filtering in a long paper funnel to get it clean, in filtering again slowly 
no lines or marks are left on the paper by the receding surface, and in 62 
WEATHER GUIDES. 
motion no strings or tails are made, but the mercury will be rounded at 
its edges. 
The best method ordinarily practicable for purifying mercury is to put 
it in a large bottle with some very dilute nitric acid, and shake it fre- 
quently. It should then be left for some days, and shaken occasionally; 
then well washed with pure water and dried. I distil first, and then 
wash with acid, and this will take out the metals likely to be found 
in it. 
II.-By W. Wurdeman, of Washington. 
In compliance with your request, as contained in your note received 
this day, I will give some notes in regard to my usual method of filling 
barometers. 
First, let me premise that I have so far filled only such as have a 
straight tube, without bend or contraction, and to such alone the method 
below explained is applicable; nor ought the tubes to be of a less bore 
than T3K of an inch. 
Besides the requisites stated, those of a clean tube and perfectly pure 
mercury are equally indispensably necessary with this method as well 
as any other, where a perfect instrument is desired. The purification of 
mercury is best accomplished by means'of perchloride of iron, with 
which it is shaken in a diluted state; then carefully washed with pure 
water, and again freed from moisture by heating. The glass tube must 
have its open end ground straight and smooth, so that it can be closed 
air-tight with the finger, or better with hard caoutchouc, as the former 
is liable to introduce moisture or grease. Warm well both mercury and 
glass tube, and fill in through a clean paper funnel with a very small 
hole (about of an inch) below, to within about one-fourth of an inch 
of the top. Shut up the end and turn the tube horizontal, when the 
space left will form a bubble that can be made to run from one end to 
the other by change of inclination, which will gather all the small air 
bubbles visible that adhered to the inside of the glass tube during filling. 
Now let that bubble, which has grown somewhat larger, pass to the open 
end. Fill up this time with mercury entirely, and shut up tightly. 
Then reverse tube over a basin, when, by slightly relieving the pressure 
against the end, the weight of the column of mercury will force some 
out, forming a vacuum above, which ought not to exceed one-half an 
inch. Closing up again tightly, let this vacuum bubble traverse the 
length of the tube on the several sides, when it will absorb those minute 
portions of air, now greatly expanded from removed atmospheric pres- 
sure, that were not drawn at the first gathering. 
The perfect freedom from air is easily recognized by the sharp concus- 
sion with which the column beats against the sealed end, when, with a 
large vacuum bubble, the horizontally held tube is slightly moved. 
Note.-If the tube requires cleaning, which is generally the case in filling, a stiff wire 
must be completely covered with thread, (to prevent scratching the tube,) to which affix a 
piece of clean buckskin and run it in and out until the tube is perfectly clean, and removing 
the buckskin, if necessary, until it comes out perfectly clean. ''V Str 'S*'*' Cirrus- (Ctr.) ''nnribis I Cum..) Nimbus iNimbJ 
©LOUIE)©. 
PRIMARY FORMS 
PLATE IV * Engraved. 3- Print edlyy JM. Butler, 
Ar Cirrus •*-\<xCirro cumulus >"'><~xCirrostratus ' umulo stratus 
(Cir-CumJ (Cir-StxJ ICuin-ui'j 
va/'ivus forms 
SECONDARY