it does three instruments in one, for the thermometer on the right hand
of the scale having its bulb covered with muslin kept moist by
communication with a cistern of water enables the two thermometers to
be employed as a Hygrometer, the use of which is described at page 50.
This barometer should be suspended in a place where it will be exposed
as much as possible to the external air, but not in sunshine.
[Illustration: 30.
Wheel Barometer. Scale
about 1/6.]
In Wheel Barometers the varying height of a column of mercury is shown
by the movement of a needle on a divided circular dial, by adopting the
syphon form of barometer tube, concealed behind the dial and frame. An
iron or glass float sustained by the mercury in the open branch (Fig.
31) is suspended by a counterbalance a _little_ lighter than itself.
The axis of the pulley has the needle attached to it, and consequently
moves the needle with the rise and fall of the mercury. It is obvious,
therefore, that if the atmospheric pressure increases the float falls
and the needle turns to the right, and if it diminishes the needle
turns in the opposite direction. The divisions on the scale represent
inches, tenths, and hundredths in the rise and fall of a column of
mercury, and these can be read with great facility, as one inch
occupies the space of six or more on this very open scale, according to
size of dial (Fig. 30). The wording is arbitrary, and indicates the
_probable_ weather that may be expected.
Important improvements have recently been effected in this form of
household barometers, so that they may be recommended as good weather
indicators where facility of reading is a desideratum.
[Illustration: 31.
Mechanism of Wheel
Barometer.
Scale about 1/8.]
Since the more scientific “Pediment” has attained so high a degree of
popularity, a certain amount of unmerited obloquy has attached itself
to the Dial or Wheel Barometer invented by Dr. Hooke. It must be
conceded that the standard form of pediment barometers in which the
height of the mercury is seen at a glance is more strictly an
“instrument of precision,” but it should not be forgotten, although a
delicate mechanism intervenes between the mercury and the observer, it
is so arranged that a tenth of an inch rise or fall causes a movement
of the index over an inch of space.
The Aneroid Barometer indicates variations in atmospheric pressure by
the elevation and depression of the sides of an elastic metallic box
from which the air is exhausted and which is kept from complete
collapse by a powerful spring. In cases where _extreme_ accuracy is not
indispensable, the portability and sensibility of this instrument
recommend it for use by tourists and fishermen. It is “quick in showing
the variations of atmospheric pressure.”[8] “The Aneroid readings may
be safely depended upon.”[9] “Its movements are always consistent.”[10]
“Atmospheric changes are indicated _first_ by the Aneroid.”[11] It is
especially adapted for determining mountain altitudes, some being
furnished with a scale of feet, enabling the observer to read off the
height by direct observation, and if adjusted once a year by comparison
with a mercurial standard is quite trustworthy. It is fully described
in a small pamphlet entitled “The Aneroid Barometer: How to Buy, and
How to Use it,” by a Fellow of the Meteorological Society.
[Footnote 8: Admiral Fitzroy.]
[Footnote 9: James Glaisher, Esq., F.R.S.]
[Footnote 10: James Belville, Esq., Royal Observatory, Greenwich.]
[Footnote 11: Sir Leopold McClintock.]
[Illustration: 32.
Aneroid Barometer. Full size.]
By a suitable arrangement of clockwork, revolving a cylinder bearing
prepared paper, the aneroid barometer forms an admirable self-recording
instrument, showing at a glance the height of the barometer: whether it
is falling or rising, for how long it has been doing so, and at what
rate the change is taking place, whether at the rate of 1/10th per
hour, or 1/10th in twenty-four hours—facts which can only be obtained
by very frequent and regular observations from an ordinary barometer,
but which are nevertheless essential to a reliable “weather
forecast.”[12]
[Footnote 12: _The Aneroid Barometer: How to Buy and How to Use it._
By a Fellow of the Meteorological Society. Post free for six stamps,
from any bookseller or optician.]
The height of mountains may also be determined by the temperature at
which water boils, as this depends on the pressure of the atmosphere,
and according to Deschanel, “just as we can determine the boiling-point
of water when the external pressure is given, so if the boiling-point
be known we can determine the external pressure,” and as this varies
with the elevation above sea-level, the boiling-point of water also
varies.
These facts induced Wollaston to attempt the determination of heights
of mountains by an apparatus which he called the Barometric
Thermometer, subsequently modified by Regnault and called a Hypsometer,
but now more generally known as a Boiling-point Thermometer.
[Illustration: 33.
Boiling-point
Thermometer.
Scale about 1/3.]
A portable form of boiling-point thermometer is shown at Fig. 33, which
is much used by Alpine travellers, and forms a trustworthy check on the
aneroid and barometer.
CONCISE TABLES FOR CALCULATING HEIGHTS BY MEANS OF BAROMETER OR
ANEROID, AND ALSO BY THE BOILING-POINT THERMOMETER.
+-----------+--------+---------------------------------------+
|Boiling- |Bartr. | |
|point of |at lower| BAROMETER AT UPPER STATION.--INCHES. |
|Water for |Station.| |
|pressure in| +----+----+----+----+----+----+----+----+
|next col. | In. | 30 | 29 | 28 | 27 | 26 | 25 | 24 | 23 |
+-----------+--------+----+----+----+----+----+----+----+----+
| 213·78 | 31 | 859| 873| 889| 905| 921| 939| 957| 977|
| 212·13 | 30 |.... 888| 904| 920| 937| 955| 974| 994|
| 210·43 | 29 |.... .... 919| 936| 953| 971| 991|1012|
| 208·67 | 28 |.... .... .... 952| 970| 989|1009|1028|
| 206·87 | 27 | Factor A. 988|1007|1028|1050|
| 205·01 | 26 |.... .... .... .... .... 1027|1048|1070|
| 203·09 | 25 |.... .... .... .... .... .... 1069|1092|
| 201·11 | 24 |.... .... .... .... .... .... .... 1115|
+-----------+--------+---------------------------------------+
+-----------+--------+---------------------------------------+
|Boiling- |Bartr. | |
|point of |at lower| BAROMETER AT UPPER STATION.--INCHES. |
|Water for |Station.| |
|pressure in| +----+----+----+----+----+----+----+----+
|next col. | In. | 22 | 21 | 20 | 19 | 18 | 17 | 16 | 15 |
+-----------+--------+----+----+----+----+----+----+----+----+
| 213·78 | 31 | 998|1020|1043|1068|1095|1124|1155|1188|
| 212·13 | 30 |1015|1038|1062|1087|1115|1144|1176|1210|
| 210·43 | 29 |1033|1056|1081|1107|1135|1165|1198|1233|
| 208·67 | 28 |1051|1075|1100|1127|1156|1187|1220|1257|
| 206·87 | 27 |1073|1097|1122|1150|1180|1211|1246|1283|
| 205·01 | 26 |1093|1118|1145|1173|1203|1236|1271|1309|
| 203·09 | 25 |1116|1141|1169|1198|1229|1262|1299|1338|
| 201·11 | 24 |1140|1166|1194|1224|1256|1290|1327|1367|
| 199·05 | 23 |1164|1191|1220|1251|1284|1319|1358|1399|
| 196·92 | 22 |.... 1218|1248|1280|1314|1350|1390|1433|
| 194·71 | 21 |.... .... 1278|1310|1346|1383|1424|1469|
| 192·41 | 20 |.... .... .... 1343|1380|1419|1461|1507|
| 190·00 | 19 |.... .... .... .... 1416|1457|1500|1548|
| 187·50 | 18 |.... .... Factor A. .... 1497|1542|1592|
| 184·87 | 17 |.... .... .... .... .... .... 1588|1639|
| 182·10 | 16 |.... .... .... .... .... .... .... 1690|
| 179·20 | 15 |.... .... .... .... .... .... .... ....|
+-----------+--------+---------------------------------------+
+--------------------------------------+
| Height | D. | | |
| in 1,000 |additive.|Latitude.| C. +---------------------+
| feet. | | | | Mean | Factor |
|----------+---------+---------+-------|Temperature.| B. |
| 2 | 5 | 0° | †2·7 |------------+--------+
| 4 | 11 | 10 | †2·5 | 10° | 0·951 |
| 6 | 17 | 20 | †2·0 | 20 | 0·973 |
| 8 | 23 | 30 | †1·4 | 30 | 0·996 |
| 10 | 30 | 40 | †0·6 | 40 | 1·018 |
| 12 | 37 | 45 | 0·0 | 50 | 1·040 |
| 14 | 44 | 50 | -0·5 | 60 | 1·062 |
| 16 | 52 | 60 | -1·3 | 70 | 1·084 |
| 18 | 60 | 70 | -2·0 | 80 | 1·127 |
+----------+---------+---------+-------+------------+--------+
RULE I.—If the temperature of boiling water be
observed at either or both Stations, find the
equivalent pressure in the 2nd column, and calculate
the height as for barometer.
RULE II.—The readings of the Barometer being
corrected and reduced to 32° F., multiply the
difference of pressure between the Stations by factor
A, found in line with pressure at lower Station, and
under that at upper Station; multiply again by factor
B, corresponding to the mean temperature of the air
at the Station; apply as many times C as there are
thousand feet in the height, corresponding to the
latitude; and add D, the correction for gravity.
EXAMPLE.—At the top of Snowdon, lat. 53° N., an
aneroid read 26·48, correction -0·18, the pressure
at sea-level was 29·91; the temperature of the
intermediate air was 57°; find the height.
Lower Station 29·91 inches.
Upper „ 26·30
-----
3·61
Factor A 933
-----
1083
1083
3249
-----
3368 (neglecting decimals.)
Factor B 1.055
-----
16840 N.B.—In taking out the quantities, if accuracy
16840 is aimed at, it will be necessary to
3368 proportion for parts in the usual manner
----- with such Tables.
3553
Cor. C = 3 × 1 = -3
Cor. D +10
----
Height 3560 feet.
The illustration (Fig. 33) shows the instrument with the telescopic
tube drawn out for use, and the thermometer surrounded by the vapour of
boiling water. The lamp is protected from wind by a perforated japanned
tin case covered with wire gauze. When the boiler is charged and the
lamp ignited the mercury ascends, and the point at which it becomes
stationary shows the temperature, which will give the elevation in feet
above the sea-level on reference to the table supplied by the optician
from whom the instrument is purchased.
[Illustration: 34.
Barograph. Scale about 1/6.]
A highly-refined automatic arrangement is adopted at some observatories
called a Barograph, which, by the aid of photography, becomes a
self-recording mercurial barometer. It is simpler in its arrangement
than the thermograph, and includes a clock of superior construction,
causing a cylinder bearing photographic paper to make one complete
revolution in forty-eight hours. A double combination of achromatic
lenses brings to a focus rays passing through a slit placed in front of
the mercurial column, behind which is a strong gaslight or paraffin
lamp, the rays of which are condensed upon the slit by a combination of
two plano-convex lenses.
Although a barometer is an instrument _artificially_ constructed by
man, it should not be forgotten that when once made the column of
mercury is placed in a passive or quiescent state in direct relation
with the great forces of nature, so that its indications become to some
extent _natural_ phenomena. This is aptly illustrated by what is called
the “daily fluctuation” of the barometer which occurs in all countries,
though the hours and extent vary with the latitude, diminishing as the
latitude increases, according to a definite law. The phenomena does not
admit of a satisfactory explanation, but is doubtless connected with
the daily variations of temperature and of vapour in the air. The
mercury falls _naturally_ (so to speak) from nine or ten to between
three and four p.m.; it then rises till between nine and ten p.m. It
falls again about four a.m., and rises again about ten a.m. It is
usually highest at nine a.m. and nine p.m., and lowest at three a.m.
and three p.m.
These natural elevations and depressions of the mercury should be
allowed for in reading the barometer, as any rise or fall in opposition
to the natural rise and fall possesses for that reason increased
importance. For instance, fine weather may be expected if the mercury
rises between nine a.m. and three p.m.; in like manner rain may be
expected should a fall take place between three p.m. and nine p.m.
It will be inferred from the preceding facts that there are certain
hours better suited for “taking a reading” than others. When one
observation only is made daily, noon is the best time, two observations
should be made at nine a.m. and nine p.m., and for three the best hours
are nine a.m. (maximum), noon (mean), and three p.m. (minimum).
The opinion generally entertained that a high barometer is an
indication of fine weather, and a low one a warning of bad weather, is
open to exception, and an increased value would attach to the
indications of the instrument in proportion as the following points are
noted and allowed for:—
1. The actual height of the mercury. 2. Whether it is rising or
falling. 3. The rate of rise and fall. 4. Whether the rise or fall has
been long continued.
The state of the barometer foretells _coming_ weather, and when the
present weather disagrees with the barometer a change will soon take
place. A fall of half a tenth, or more, in an hour is a sure warning of
a storm, a rapid rise is a warning of unsettled weather.
The barometer is generally lowest with wind from the S.W., and highest
with wind N.E., or with a calm. N.E. and S.W. may be called the wind’s
poles, and the difference of height due to _direction_ only from one of
these bearings to another amounts to about half an inch.
BAROMETER PRECAUTIONS.
If vacuum suspected, cause mercury to strike top of tube.
A clear metallic “click” indicates a good vacuum.
A dull “thud” indicates air or moisture.
In latter case return to optician, but if unable
Incline _very_ gently until nearly inverted, when
Air if present will ascend in a bubble into the cistern.
Suspend barometer in good light out of sunshine.
Let no heat of fire or lamp affect it.
Let no sudden changes of temperature affect it.
It _must_ hang _absolutely_ vertically.
Note temperature of attached thermometer before reading barometer.
Then adjust mercury in cistern to touch ivory point.
Then adjust vernier and take reading _quickly_.
Ascertain height above sea-level according to direction.
The Storm Glass (Fig. 36) is a glass bottle, ten inches long,
containing a mixture of camphor, nitre, sal-ammoniac, alcohol, and
water. As “temperature affects the mixture much,” an arrangement has
recently been designed in which the stem of a thermometer is immersed
in the fluid, as shown at Fig. 37, thus imparting a higher value to its
indications. The late Admiral Fitzroy says—
“Since 1825, we have generally had some of these glasses, as
curiosities rather than otherwise; for nothing certain could be made of
their variations until lately, when it was fairly demonstrated that if
fixed undisturbed in free air, not exposed to radiation, fire, or sun,
but in the ordinary light of a well-ventilated room, or, _preferably_,
in the outer air, the chemical mixture in a so-called storm glass
varies in character with the _direction_ of the wind—not its force.”
The quarter from which the wind or storm is blowing is indicated by the
substance adhering more closely to the bottom of the glass _opposite_
to the point whence the wind or tempest arises.
The Sympiesometer is an instrument used chiefly at sea for purposes of
comparison with the mercurial and aneroid barometers. Its indications
result partly from the pressure and partly from the temperature of the
atmosphere; it would, therefore, be more correctly named a
Thermo-Barometer.
[Illustration: 35. 36. 37.
Storm Glass, or Chemical Weather Glass. Scale about 1/5.]
The _height_ of the atmosphere has been variously estimated:—By
Bravais, from the duration of twilight, at 66 to nearly 100 miles; by
Dalton, in 1819, from observations of the auroral light, at 102 miles;
by Sir John Herschel, from similar observations in 1861, at 83 miles;
from observations of meteors, from 100 to 200 miles; by Liais, in 1859,
from observations on the polarisation of the sky, at no less than 212
miles.
The _density_ of the atmosphere diminishes with distance from the
earth’s surface, in accordance with the following rule:—“At a height
of seven miles the density of the atmosphere is reduced to one-fourth
the density at the sea-level, and for every additional seven miles, the
rarity of the air is similarly quadrupled.”
NOTE ON THE VERIFICATION OF INSTRUMENTS AT THE
KEW OBSERVATORY.
The Kew Committee of the Royal Society receive, for verification and
comparison with the standard instruments of the Kew Observatory,
barometers, thermometers, and other instruments intended for
meteorological observation or scientific investigations.
Any persons ordering instruments of opticians may direct them to be
previously forwarded to the observatory for verification.
A scale of charges is issued by the Committee which is exclusive of
packing and carriage, or of rail expenses, when a special messenger is
sent out. The Meteorological Office, Victoria Street, London, also
receives and forwards instruments for verification to the Kew
Observatory.
The Committee wish it to be understood that they cannot undertake the
verification of an inferior class of instruments (such as barometers
mounted upon wooden frames, and thermometers not graduated on the
stem), and that the superintendent of the observatory may at his
discretion decline to receive such instruments as he may consider unfit
for scientific observation.
BAROMETER WARNINGS.
May be Expected
-----
Increasing storm If mercury falls during a high wind from
S.W., S.S.W., W. or S.
Violent but short If the fall be rapid.
Less violent but of longer If the fall be slow.
continuance
A violent storm from the If the mercury falls suddenly while the
N.W. or N. wind is due W.
N.W., N., or N.E. winds, or If the mercury having been at its usual
less wind, or less rain, or height, 29·95, is steady or rising,
less snow _while the thermometer falls_ and the
air becomes drier.
Wind and rain from S.E., S., If the mercury falls, while the
and S.W. thermometer rises and the air becomes
damp.
A violent storm from N.W., When the mercury falls suddenly with a
N., or N.E. W. wind.
Snow If the mercury falls when the
thermometer is low.
Less wind, or a change to When the mercury rises, after having
N., or less wet been some time below its average height.
Strong wind or heavy squalls With the _first_ rise of the mercury
from N.W., N., or N.E. after it has been very low (say 29).
Improved weather When a gradual continuous rise of the
mercury occurs with a falling
thermometer.
Winds from S. or S.W. If the mercury suddenly rising, the
thermometer _also_ rises.
Heavy gales from N. _Soon_ after the _first_ rise of the
mercury from a very low point.
Unsettled weather With a _rapid_ rise of the mercury.
Settled weather With a _slow_ rise of the mercury.
Very fine weather With a continued _steadiness_ of the
mercury with dry air.
Stormy weather with rain (or With a rapid and considerable fall of
snow) the mercury.
Threatening, unsettled With an alternate rising and falling of
weather the mercury.
Much wind, rain, hail, or When the mercury falls considerably. If
snow, with or without the thermometer be low (for the season)
lightning the wind will be N., if high, from S.
Lightning _only_ When the mercury is low, the storm being
beyond the horizon.
Fine weather With a rosy sky at sunset.
Wind and rain When the sky has a sickly greenish hue.
Rain When the clouds are of a dark Indian
red.
Bad weather or much wind When the sky is red in the morning.
EXPLANATORY CARD.
BY THE LATE VICE-ADMIRAL FITZROY, F.R.S., ETC.
WEATHER GLASSES.
THE BAROMETER RISES | THE BAROMETER FALLS
for Northerly wind | for Southerly wind
(including from North-west, by | (including from South-east, by the
the _North_, to the Eastward), | _South_, to the Westward),
for dry, or less wet weather,--for | for wet weather,--for stronger
less wind,--or for more than one | wind,--or for more than one of
of these changes:-- | these changes:--
|
EXCEPT on a few occasions when | EXCEPT on a few occasions when
rain, hail, or snow comes from the | _moderate_ wind with rain (or
Northward with _strong_ wind. | snow) comes from the Northward.
--------------------------
For change of wind toward | For change of wind toward
Northerly directions,-- | Southerly directions,--
|
A THERMOMETER FALLS. | A THERMOMETER RISES.
|
Moisture or dampness in the air (shown by a Hygrometer) increases
BEFORE rain, fog, or dew.
--------------------------
On barometer scales the following | Add one-tenth of an inch to the
contractions may be useful:-- | observed height for each hundred
| feet the Barometer is above the
RISE | FALL | half-tide level.
| |
FOR | FOR | The _average_ height of the
| | Barometer, in England, at the
NORTH | SOUTH | sea-level, is about 29·94 inches;
| | and the _average_ temperature
N.W.--N.--E. | S.E.--S.--W. | of air is nearly 50 degrees
(London
| | latitude).
DRY | WET |
| | The Thermometer falls about
OR | OR | one degree for each three hundred
| | feet of elevation from the ground,
LESS | MORE | but varies with wind.
| | ------
WIND. | WIND. | “When the wind shifts against
| | the sun,
------ | ------ | Trust it not, for back it will
| | run.”
| | ------
EXCEPT | EXCEPT | _First_ rise after very low
| | Indicates a stronger blow.
WET FROM | WET FROM | ------
| | Long foretold--long last,
NORTH. | NORTH. | Short notice--soon past.
(_In South Latitude read South for North._)
IV.—CONDENSATION.
Dew is a deposition of moisture from the air, resulting from the
condensation of the aqueous vapour of the atmosphere on substances
which have become cooled by the radiation of their heat. This is, in
fact, the substance of Dr. Wells’s famous Theory of Dew, enunciated in
1814, and which, according to Dr. Tyndall, “has stood the test of all
subsequent criticism, and is now universally accepted,” and by which
all the phenomena of dew may be explained.
Dr. Wells’s experiments were interesting and conclusive. He exposed
definite weights (10 grains) of wool to the air on clear nights, one
_on_ a four-legged stool, the other _under_ it, the upper portion
gained 14 grains in weight, the lower only 4 grains. On an evening when
one portion of wool, protected by a curved pasteboard roof, gained only
2 grains, a similar portion on the top of the miniature roof gained 16
grains. A little reflection will suggest the explanation: radiation
from the wool was arrested by the pasteboard cover, while the portion
fully exposed to the sky lost all its heat, and thus condensation
ensued. Dr. Wells speaks with such candour, and so pointedly, on this
fact and its consequences, that his words may be advantageously quoted:
“I had often, in the pride of half-knowledge, smiled at the means
frequently employed by gardeners to protect tender plants from cold, as
it appeared to me impossible that a thin mat, or any such flimsy
substance, could prevent them from attaining the temperature of the
atmosphere, by which alone I thought them liable to be injured. But
when I had learned that bodies on the surface of the earth become
during a still and serene night colder than the atmosphere, by
radiating their heat to the heavens, I perceived immediately a just
reason for the practice I had before deemed useless.”
Familiar instances of the formation of dew will have been noted by many
“watchers;” _e. g._, breathing on a cold pane of glass, a tumbler of
cold water becoming dew-covered on being brought into a warm room, the
outside of a tankard of iced claret cup, &c. When, radiation is so free
and rapid that the temperature is below the freezing point, the dew
freezes as it forms, producing _hoar-frost_.