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Title: Weather Warnings for Watchers
Author: Clerk, The
Language: English
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[Illustration: THE TWO OCEANS.
(1) Aërial Ocean.
(2) Greatest height attained by Messrs. Glaisher and Coxwell,
being 36,960 feet, or seven miles above the sea level.
(3) Aërial Alps, or stratum of clouds 15,000 feet in depth.
(4) Highest bird-region.]
WEATHER WARNINGS
FOR
WATCHERS
BY THE “CLERK”
HIMSELF.
WITH CONCISE TABLES FOR CALCULATING HEIGHTS
“The actuating force of every wind that blows; of every mighty current
that streams through ocean depths; the motive cause of every particle
of vapour in the air, of every mist and cloud and raindrop, is
SOLAR RADIATION.”—_George Warington._
LONDON
HOULSTON AND SONS
PATERNOSTER SQUARE, E.C.
1877
[_The right of translation is reserved. Entered at Stationers’ Hall._]
LIST OF WORKS OF REFERENCE.
BOUTAN ET D’ALMEIDA. Cours Eléméntaire de Physique.
BUCHAN, A. Introductory Text-book of Meteorology. _W. Blackwood and
Sons_, 1871.
CAZIN, ACHILLE. La Chaleur. _Hachette and Co._, 1868.
CRAMPTON, REV. JOS., M.A. The Three Heavens. _W. Hunt and Co._,
1876.
CHAMBERS’ Encyclopædia. _W. and B. Chambers_, 1875.
DREW, JOHN. Practical Meteorology. _Van Voorst_, 1870.
FITZROY, THE LATE ADMIRAL. Weather Book and Barometer Manual.
FLAMMARION, CAMILLE. L’Atmosphere.
GUILLEMIN AMEDÉE. Les Forces de la Nature.
GLAISHER, J., F.R.S. Hygrometrical Tables. _Taylor and Francis_,
1869.
HARTLEY, W. N. Air and its Relations to Life. _Longmans_, 1875.
HERSCHEL, SIR JOHN F. W. Meteorology, from Ency. Brit. _A. and C.
Black_, 1860.
KAEMTZ, L. F. Complete Course of Meteorology. _Baillière London._
MARTIN’S Natural Philosophy. _Simpkin, Marshall and Co._, 1868.
TYNDALL, JOHN, D.C.L., &c. Heat, a Mode of Motion. Fifth Edition.
_Longmans_, 1875.
RODWELL. Dictionary of Science. _E. Moxon and Co._, 1871.
PROCTOR. Science Byways. _Smith, Elder and Co._, 1875.
SCOTT, R. H., M.A., F.R.S. Instructions in the Use of
Meteorological Instruments, 1875.
WARINGTON, GEORGE. Phenomena of Radiation.
CONTENTS.
PAGE
Actinometer 10
Æthrioscope 16
Altitude tables 37
Anemograph 84
Anemometers, velocity 80
Aneroid barometer 35
Atmidometer 25
Atmospheric electricity 89
Barograph 38
Barometer precautions 40
„ description of 29
„ construction of 26
„ self-recording 36
„ warnings 43
„ syphon 30
„ wheel 33
„ corrections of 27
Beaufort’s scale of wind force 76
„ weather notation 82
Black bulb in vacuo 12
Boiling-point thermometer 36
Calorification 8
Condensation 45
Capacity, correction 27
Capillarity, correction 28
Centigrade thermometer 20
Cirro-cumulus cloud 56
Cirro-stratus cloud 56
Cirrus cloud 52
Clouds, forms of 52
„ amount of 57
Compass bearings 71
Conversion of thermometer scales 23
Cumulo-stratus cloud 56
Cumulus cloud 53
Dew-point 48
Electrification 86
Electrometers, forms of 90
Electroscope 89
Evaporation, measurement of 24
Fahrenheit’s thermometer 20
Fortin’s barometer 27
Freezing-point 20
Frost, management of hygrometer in 50
Gold-leaf electroscope 89
Glass, storm 41
Heights, measurement of 37
Hours of observation 39
Howard’s cloud nomenclature 52
Hygrometer, Daniell’s 47
Hygrometer, Mason’s 48
Hygrometer precautions 50
Kew verification 42
Lightning 90
„ conductors 91
Mean sea-level 28
Maximum thermometers 16
Meteorology, list of works on 4
Minimum thermometers 17
Mountain barometers 35
Motion 67
Nimbus clouds, form of 57
Ozone, determination of 91
Ozonometer 92
Packing barometers 32
Position of barometers 33
Pyrheliometer 9
Pressure anemometer 79
Psychrometer 49
Radiation, solar 9
Radio-solar thermometer 13
Rain, measurement of 60
Rain gauges 62 to 67
Rarefaction 26
Réaumur’s scale 20
Regnault’s hygrometer 47
Robinson’s anemometer 81
Solar radiation 9
Six’s thermometer 18
Standard barometer 28
Stevenson’s thermo-screen 51
Stratus cloud 55
Suspension of barometers 40
Sympiesometer 41
Temperature, correction for 27
Terrestrial radiation 13
Thermographs 23
Thermometer scales 20
„ screens 50
„ radiation 11, 14
„ standard 21
True bearings of wind direction 71
Vernier, principle of 30
„ setting the 31
Weather warnings 40, 44, 53, 54, 55, 56, 57,
58, 59, 60, 73, 77, 94
Wet and dry bulb hygrometer 50
Wind, registration of 85
„ gauges 79
„ scales 76, 83
„ vane 78
PREFACE.
The late Admiral Fitzroy entertained the opinion that the various
phenomena which go to form what we call “weather” are “measurable at
any place, and that having these measurements at _various_ places over
a given area, such as the British Isles, we ought to be able to foresee
the peculiar results as regards the direction and force of air currents
which have their distinctive weather characteristics in relation to
temperature, rainfall, and electrical manifestations.”
A conviction of the soundness of this opinion has induced the writer to
make the present compilation, in the hope that many who have hitherto
avoided the subject of meteorology and the weather may find interesting
matter, where before all seemed dull and technical.
Any attempt at rigid mathematical accuracy is disclaimed at the outset;
the leading principles involved in weather forecasting and storm
prevision will, however, be stated in a sufficiently definite manner to
divest the subject of the mystery in which it has hitherto seemed to be
enshrined, and thus enable the unscientific reader to become
weather-wise, and casual observers to note weather phenomena with some
degree of method and precision.
On page 4 will be found a list of works which have proved useful aids
in making the present compilation. The writer desires to acknowledge
his indebtedness to the various authors and publishers, and especially
to Mr. Strachan, for permission to quote from his able pamphlet on
“Weather Forecasts, and Storm Prevision,” and to reproduce the valuable
table on page 37, for Calculating Heights of Mountains, from the fourth
edition of his handy “Pocket Meteorological Register.”
The publication of Weather Reports in the daily journals must have
convinced the most indifferent that much greater importance is now
attached to weather phenomena than formerly; and this conviction will
be deepened when it is remembered that a Parliamentary grant of £10,000
is annually expended in support of the Meteorological Office and its
seven fully organized observatories in this country, while America
expends no less a sum than £80,000 annually in the pursuit of weather
wisdom; and the leading nations of Europe have also established
meteorological observatories in suitable localities.
The balloon ascents of Messrs. Glaisher and Coxwell attracted much
attention to the instruments used in estimating atmospheric phenomena,
and awakened a desire to know something of the functions of a
barometer, thermometer, hygrometer, &c., and especially of the
classification of those important weather-warners, clouds. These
subjects will be found duly noted in their order, and every phenomenon
being traced to its source, Solar Radiation, it is hoped that these
pages may prove generally acceptable, and be deemed not altogether
unworthy of
“THE CLERK OF THE WEATHER.”
WEATHER WARNINGS.
The two great Forces of Nature are Gravitation and Heat, which always
act in opposition to each other.
WEATHER is the result of the action of these forces on matter, and
where one form of force is in excess of another, changes are produced
which become apparent to our senses, or are indicated by suitable
instruments.
THE MATTER composing the earth on which we live is of three
kinds—solid, liquid, and gaseous.
THE FORCE incessantly acting on these is the radiant heat of the sun.
THE RESULTS of this incessant action are:—
1. CALORIFICATION, or Heating, which, besides being appreciable by
our senses, is indicated by the THERMOMETER.
2. EVAPORATION, which alters the weight of the air indirectly, by
the diffusion of aqueous vapour through it. This alteration of
_weight_ is indicated by the BAROMETER, the accompanying
increase of moisture being indicated by the HYGROMETER.
3. RAREFACTION, which alters the _weight_ of the air directly.
4. CONDENSATION, producing fog, dew, rain, hail, and snow; all
sufficiently apparent when they occur, but estimated accurately
only by the Rain Gauge, or PLUVIOMETER.
5. MOTION, producing winds, which we are able to appreciate in the
gentle breeze and the awful cyclone, the force and velocity of
which are indicated by the ANEMOMETER.
6. ELECTRIFICATION, producing lightning, thunder, magnetic
phenomena, and chemical change, respectively indicated by the
ELECTROMETER, MAGNETOMETER, and OZONOMETER.
I.—CALORIFICATION.
Before considering in detail these results of the action of solar
radiation on our globe, an attempt to realize the immensity of this
stupendous force will materially aid in the general comprehension of
the subject.
The earth is a sphere somewhat less than 8,000 miles in diameter; and
if we assume, with the gifted author[1] of “The Phenomena of
Radiation,”—“that it is about 91,300,000 miles from the sun, and moves
around it in a slightly elliptical orbit, occupying rather more than
365 days; that its shape is globular, somewhat flattened at its two
extremities; that it rotates upon its own axis in the space of 24
hours, that axis being inclined to the annual orbit at an angle of
23-1/2—if we further assume that solar radiation is of such kind and
quantity as it is, we are enabled to account for the total amount of
light and heat the earth receives, for the superior temperature and
illumination of equatorial regions, as compared with polar, with the
gradations of intermediate zones, for the alternation of day and night,
and the annual progression of the seasons.
[Footnote 1: George Warington, F.C.S.]
“The actuating force of every wind that blows; of every mighty current
that streams through ocean depths; the motive cause of every particle
of vapour in the air of every mist and cloud and raindrop, is SOLAR
RADIATION.
“The delicate tremor of the sun’s surface particles, shot hither
through thirty million leagues of fine intangible æther, has power to
raise whole oceans from their beds, and pour them down again upon the
earth. We are apt to measure solar heat merely by the sensation it
produces on our skin, and think it small and weak accordingly; a good
coal fire will heat us more. But its true measure is the work it does.
Judged by this standard, its immensity is overpowering. To take a
single instance: the average fall of dew in England is about five
inches annually; for the evaporation of the vapour necessary to produce
this trifling depth of moisture, there is expended _daily_ an amount of
heat equal to the combustion of sixty-eight tons of coal for _every
square mile_ of surface, or, for the whole of England, 4,000,000 tons.
Compare now the size of England with that of the whole earth—only
1/3388th part; extend the calculation to _rain_, as well as dew, the
average fall of which on the whole earth is estimated at five feet
annually, or _twelve_ times greater; and then estimate the sum of
4,000,000 × 3,388 × 12 = 162,624,000,000 tons, or about 3,000 times as
much as is annually raised in the whole world; and we have the number
of tons of coal required to produce the heat expended by the sun merely
in raising vapour from the sea to give us rain during a single day.”
[Illustration: 1.
Pouillet’s Pyrheliometer. Scale about 1/8.]
SOLAR RADIATION.
Seeing, then, that solar radiation plays so important a part in the
production of the natural phenomena classed under the head of
Meteorology, a description of the mode of estimating its amount will
prove interesting, and enable the reader to realize the existence of
this mighty power. M. Pouillet devised for this purpose the apparatus
known as the PYRHELIOMETER, which registers the power of parallel solar
rays by the amount of heat imparted to a disc of a given diameter in a
given time. It consists of a flat circular vessel of steel A having its
outside coated with lamp-black B. A short steel tube is attached to the
side opposite to that covered with lamp-black, and the vessel is filled
with mercury. A registering thermometer C, protected by a brass tube D,
is then attached, and the whole is inverted and exposed to the sun, as
shown at Fig. 1. The purpose of the second disc, E, is to aid in so
placing the apparatus that it shall receive direct parallel rays. It is
obvious that if the shadow of the upper disc completely covers the
lower one, the sun’s rays must be perpendicular to its blackened
surface.
“The surface on which the sun’s rays here fall is known; the quantity
of mercury within the cylinder is also known; hence we can express the
effect of the sun’s heat upon a given area by stating that it is
competent, in five minutes, to raise so much mercury so many degrees in
temperature.”[2]
[Footnote 2: Tyndall, “Heat a Mode of Motion.”]
Sir John Herschel also designed an instrument for observing the heating
power of the sun’s rays in a given time, to which the title Actinometer
is given. It consists of a Thermometer with a long open scale and a
large cylindrical bulb, thus combining the best conditions for extreme
sensibility. An observation is made by exposing the instrument in the
shade for one minute and noting the temperature. It is then exposed to
the sun’s rays for one minute, and a record of the temperature made. It
is again placed in the shade for one minute, and the mean of the two
shade readings being deducted from the solar reading shows the heating
power of the sun’s rays for one minute of time.
[Illustration: 2.
Herschel’s Actinometer. Scale about 1/8.]
The stimulus imparted to the study of this class of phenomena by the
publications of Professor Tyndall’s researches on Radiant Heat has
induced a demand among Meteorologists for instruments capable of
yielding more available indications than those just described. This
demand has been most efficiently supplied by the ingenuity of
scientists and instrument makers.
[Illustration: 3.
Improved Solar Radiation Thermometer in Vacuo.
Scale about 1/3.]
The early form of Solar Radiation Thermometer was a self-registering
maximum thermometer, with blackened bulb, having its graduated _stem_,
only, enclosed in an outer tube. Errors arising from terrestrial
radiation and the _variable_ cooling influences of aërial currents are
all obviated in the improved and patented Solar Radiation Thermometer
shown at Fig. 3, which consists of a self-registering maximum
thermometer, having its _bulb and stem_ dull-blackened, in accordance
with the suggestion of the Rev. F. W. Stow, and the _whole_ enclosed in
an outer chamber of glass, from which the air has been completely
exhausted. The perfection of the vacuum in the enclosing chamber is
proved by the production of a pale white phosphorescent light, with
faint stratification and transverse bands when tested by the spark from
a Ruhmkorff coil. Due provision is made for this by the attachment of
platinum wires to the lower side of the tube, and when tested by a
syphon pressure gauge, the vacua have been proved to exist to within
1/50th of an inch of pressure. It will thus be seen that the
indications are preserved from errors arising from atmospheric
currents, and from the absorption of heat by aqueous or other vapours,
the whole of the solar heat passing through the vacuum direct to the
blackened bulb. The contained mercury expanding, carries the recording
index to the highest point, and thus is obtained a registration of the
maximum amount of solar radiation during the twenty-four hours. The
great advantage accruing from the high degree of perfection to which
this instrument has been brought is, _uniformity_ of construction,
which renders the observations made at different stations
_intercomparable_. An enlarged view of the thermometer is given at Fig.
3, showing the platinum wire terminations, whereby the vacuum is
tested. The Rev. Fenwick W. Stow thus directs the manner in which the
solar radiation thermometer should be used:—
1. Place the instrument four feet above the ground, in an open space,
Fig. 4, with its bulb directed towards the S.E. It is necessary that
the globular part of the external glass should not be placed in contact
with or very near to any substance, but that the air should circulate
round it freely. Thus placed, its readings will be affected only by
direct sunshine and by the temperature of the air.
2. One of the most convenient ways of fixing the instrument will be to
allow its stem to fit into and rest upon two wooden collars fastened
across the ends of a narrow slip of board, which is nailed in its
centre upon a post steadied by lateral supports (Fig. 4).
3. The maximum temperature of the air in shade should be taken by a
thermometer placed on a stand in an open situation. Any stand which
thoroughly screens it from the sun, and exposes it to a free
circulation of air, will do for the purpose.
4. The difference between the maxima in sun and shade, thus taken, is a
measure of the amount of solar radiation.
[Illustration: 4.
Solar Radiation Thermometer, black bulb and
stem in vacuo, on 4 feet stand.
Scale about 1/20.]
The remarkable phenomenon recently discovered by Mr. Crookes, in which
light is apparently converted into motion, has, at the suggestion of
Mr. Strachan, received an interesting application to meteorology. The
arrangement is shown at Fig. 5, where a Solar Radiation Thermometer has
a Crookes’ Radiometer attached to it, which, in addition to forming an
efficient test as to the perfection of the vacuum, will, it is hoped,
aid in eventually establishing a relation between intensity of
radiation, as shown by the thermometer, and the number of revolutions
of the radiometer. The instrument has so recently been devised that any
positive statement as to its usefulness would be premature; it may,
however, prove a valuable auxiliary to the solar thermometer, and
eventually be so far improved as to become a more definite exponent of
solar radiation than the thermometer.
[Illustration: 5.
Radio-Solar Thermometer. Scale about 1/4.]
TERRESTRIAL RADIATION.
It is an established fact, confirmed by careful experiments, that a
mutual interchange of heat is constantly going on between all bodies
freely exposed to view of each other, thus tending to establish a state
of equilibrium. It has further been ascertained that, as the mean
temperature of the earth remains unchanged, “it necessarily follows
that it emits by radiation from and through the surface of its
atmosphere, on an average, the exact amount of heat it receives from
the sun.” This process commences _slowly_ at sunset, and proceeds with
great rapidity at and after midnight, attaining its maximum effect in a
long night, in perfect calm, under a cloudless sky, resulting in the
condensation of vapour in the form of dew, or hoar-frost, when the
temperature of the surface-air is reduced to the dew-point.[3]
[Footnote 3: See page 47.]
The extent to which heat thus escapes by radiation under varying
conditions of sky is measured by a Self-registering Terrestrial Minimum
Thermometer, the bulb of which is placed over short grass, and “a
thermometer so exposed under a clear sky always marks several degrees
below the temperature of the air, and its depression affords a rude
measure of the facility for the escape of heat afforded under the
circumstances of exposure.”[4]
[Footnote 4: Herschel.]
[Illustration: 6.
Terrestrial Radiation Thermometer.
Scale about 1/6.]
[Illustration: 7.
Improved Cylinder Jacket Terrestrial Minimum Thermometer.
Scale about 1/12.]
Fig. 6 shows the ordinary spherical bulb thermometer employed for this
purpose, and Fig. 7 the improved Cylinder Jacket Thermometer, which, by
exposing a larger surface of spirit to the air, gives an instrument
possessing an amount of sensibility in no way inferior to that of
mercury.
There is a drawback to the use of these thermometers enclosed in outer
tubes, arising from moisture getting into the outer cylinder or jacket,
and frequently preventing the observer from reading the thermometer.
This has recently been removed by making a perfectly ground joint of
glass (analogous to a glass stopper in a bottle) as a substitute for
the old form of packing at the open end of the tube, the other end
being fused into contact with the outer cylinder to keep it in its
place. The intrusion and condensation of moisture thus becomes
impossible, while the scale is protected from corrosion or abrasion.
This “ground socket” arrangement is shown at Fig. 8.
[Illustration: 8.
Ground Socket Minimum Thermometer. Scale about 1/4.]
Radiation from the earth upwards proceeds with great rapidity under a
cloudless sky, but a passing cloud, or the presence even of invisible
aqueous vapour in the air, is sufficient to effect a marked
retardation, as is beautifully illustrated by Sir John Leslie’s
Æthrioscope, shown at Fig. 9, which consists of a vertical glass tube,
having a bore so fine that a little coloured liquid is supported in it
by the mere force of cohesion. Each end of the tube terminates in a
glass bulb containing air. A scale, having its zero in the middle, is
attached to the tube, and the bulb A is enclosed in a highly polished
sphere of brass. The upper bulb B is blackened, and placed in the
centre of a highly-gilt and polished metallic cup, having a movable
cover F. These outer metallic coverings protect the bulbs from
extraneous sources of heat. So long as the upper bulb is covered, the
liquid in the tube stands at zero on the scale, but immediately on its
removal radiation commences, the air contained in B contracts, while
the elasticity of that contained in A forces the liquid up the tube to
a height directly proportionate to the rapidity of the radiation.
[Illustration: 9.
Æthrioscope.
Scale about 1/7.]
SHADE TEMPERATURE.
Self-registering Maximum Thermometers are made in two ways. In the
first, the index is a small portion of the mercurial column separated
from it by a minute air bubble. The noontide heat expands the mercury,
and the subsequent contraction as the temperature decreases affects
only that portion of the mercury in connection with the bulb, leaving
the disconnected portion to register the maximum temperature. In the
second form the tube is ingeniously contracted just outside the bulb,
so that the mercury extruded from the bulb by expansion cannot return
by the mere force of cohesion, but remains to register the highest
temperature.
[Illustration: 10.
Self-registering Maximum Thermometer. Scale about 1/5.]
There is a modification of this latter form produced by the addition of
a supplementary chamber just outside the bulb and _over_ the column,
from which, as expansion proceeds, the mercury flows by gravitation,
but into which it cannot return until, as in the other forms, the
instrument is readjusted for a new observation, by unhooking the bulb
end and lowering it until the mercury flows into its place.
[Illustration: 11.
Self-registering Minimum Thermometer. Scale about 1/5.]
Self-registering Minimum Thermometers are of two kinds,—spirit and
mercurial. Fig. 12 shows one of Rutherford’s Alcohol Minimum
Thermometers, which will be seen to consist of a bulb and tube attached
to a scale, which latter may be either of wood, glass, or metal. The
tube contains an index of black glass.
[Illustration: 12.
Self-registering Minimum Thermometer.
Scale about 1/5.]
The Thermometer is “set” for observation by slightly raising the bulb
end until the index slides to the extreme end of the column of spirit.
It is then suspended in the shade with the bulb end a little lower than
the other. The contraction of the spirit consequent on a fall of
temperature draws the index back, but a subsequent expansion does not
carry it forward, it remains at the lowest point to which the spirit
has contracted to register the minimum temperature. A very useful
modification of this instrument is made for gardeners and general
horticultural purposes, in which the scale is of cast zinc with raised
figures, which being filed off flush after the whole has been painted
of a dark colour are easily legible at a little distance.
The advantage of alcohol for the indication of _very_ low temperatures
is that it has never been frozen.[5]
[Footnote 5: Mercury freezes at -39° F.]
Fig. 13 shows a set of Maximum and Minimum and Wet and Dry Bulb
Thermometers, with incorrodible porcelain scales, suspended on a
mahogany screen. Instruments of this quality are generally
engine-divided on the stem, and if, in addition to this, they are
verified by comparison with standard instruments at the Kew
Observatory, they may be regarded as standards, and employed for
accurate scientific observations.
[Illustration: 13.
Standard Set of Instruments on Screen. Scale about 1/6.]
Six’s Self-registering Thermometer consists of a long tubular bulb,
united to a smaller tube more than twice its length, and bent twice,
like a syphon, so that the larger tube is in the centre, while the
smaller one terminates at the top, on the right hand, in a pear-shaped
bulb, as shown in the cut (Fig. 14). This bulb, and the tube in
connection with it, are partly filled with spirit; the long central
bulb and its connecting tube are completely filled, while the lower
portion of the syphon is filled with mercury. A steel index, prevented
from falling by a hair tied round it, to act as a spring, moves in the
spirit in each of the side tubes. The scale on the left hand has the
zero at the top, and that on the right at the bottom. When setting the
instrument, the indices are brought into contact with the mercury by
passing a small magnet down the outside of each tube. Then, should a
rise of temperature take place, the spirit in the central bulb expands,
forcing down the mercury in the left hand tube and causing it to rise
in the right, and _vice versa_ for a diminution of temperature.
It should be always used and carried upright, and the indices should be
drawn _gently_ down by the magnet into contact with the mercury; and,
when a reading is taken, the ends of the indices nearest the mercury
indicate the maximum and minimum temperatures which have been attained
during the stated hours of observation.
[Illustration: 14.
Six’s Thermometer.
Scale about 1/7.]
Six’s form of thermometer has been extensively used for ascertaining
deep sea temperatures.
[Illustration: 15.
Deep Sea Maximum
and Minimum Registering
Thermometer.
Scale about 1/5.]
Evaporation and the mechanical action of winds keep up a constant
circulating motion of the ocean, the currents of which tend to equalize
temperature. The most important of these is known as the Gulf Stream,
taking its name from the Gulf of Mexico, out of which it flows at a
velocity sometimes of five miles an hour, and in a width of not less
than fifty miles. It has an important effect on the climate of Great
Britain, and of all lands subject to its influence, its temperature as
it leaves the Gulf of Mexico being 85° F., diminishing to 75° off the
coast of Labrador, and still further as it nears northern latitudes.
Observations on the temperature of the ocean are therefore included in
the scope of meteorology, and are ascertained by the use of
thermometers of special construction (Fig. 15). In the earlier
experiments made for ascertaining the temperature of the ocean at a
depth of 15,000 feet, where the pressure is equal to three tons on the
square inch, it was found that a considerable error occurred in the
indications in consequence of this enormous pressure; accordingly the
central elongated bulb of the ordinary Six’s Thermometer (see page 19)
is shortened and enclosed in an outer bulb nearly filled with spirit,
which, while effectually relieving the thermometer bulb from undue
pressure, allows any change to be at once transmitted to it, and thus
secures the registration of the exact temperature. The arrangement
possesses the further advantage of making the instrument stronger, more
compact, and more capable of resisting such comparatively rough
treatment as it would receive on board ship.
The honour of constructing the first thermometer, which was an Air and
Spirit Thermometer, is ascribed to Galileo; it assumed a practical
shape in 1620, at the hands of Drebel, a Dutch physician. Hailey
substituted mercury for spirit in 1697; Réaumur improved the instrument
in 1730, and Fahrenheit in 1749. More recently the instrument has been
perfected by the scales being graduated on the actual stem of the
instrument. For many years it was exclusively used by chemists and men
of science; it afterwards received numerous applications in the arts
and manufactures; and is now considered an essential in every household.
Thermometers are instruments for measuring temperature by the
contraction or expansion of fluids in enclosed tubes. The tubes, which
are of glass, have spherical, cylindrical, or spiral bulbs blown on to
one end; they have also an exceedingly fine bore, and when mercury or
spirit is enclosed in them these fluids, in contracting and expanding
with variations of temperature, indicate degrees of heat in relation to
two fixed points—viz., the freezing and boiling points of water. Care
is taken to exclude all air before sealing, so that the upper portion
of the tube inside shall be a perfect vacuum, and thus offer no
resistance to the free expansion of the mercury. In graduating, or
dividing the scales, the points at which the mercury remains stationary
in melting ice and boiling water are first marked on the stem, and the
intervening space divided into as many equal parts as are necessary to
constitute the scales of Fahrenheit, Réaumur, or Celsius, the last
being known as the Centigrade (_hundred steps_) scale, from the
circumstance of the space between the freezing and boiling points of
water being divided into one hundred equal parts (Fig. 16).
[Illustration: 16.
Comparison of Thermometer
Scales.
Scale about 1/5.]
[Illustration: 17.
“Legible”
Scale Thermometer.
Scale about 1/5.]
GRADUATION OF THERMOMETERS.—When the fluid (either mercury or spirit)
has been enclosed in the hermetically sealed tube, it becomes
necessary, in order that its indications may be comparable with those
of other instruments, that a scale having at least two fixed points
should be attached to it. As it has been found that the temperature of
melting ice or freezing water is always constant, the height at which
the fluid _rests_ in a mixture of ice and water has been chosen as one
point from which to graduate the scale. It has been also found that
with the barometer at 29·905 the boiling-point of water is also
constant, and when a thermometer is immersed in pure distilled water
heated to ebullition, the point at which the mercury remains immovable
is, like the freezing-point, carefully marked, the tube is then
calibrated and divided as shown in Fig. 16.
The zero of the scales of Réaumur and Centigrade is the freezing-point
of water, marked, in each case, 0°, while the intervening space, up to
the boiling-point of water, is divided, in the former case, into 80
parts, and in the latter to 100°.
In the Fahrenheit scale, the freezing-point is represented at 32°, and
the boiling-point at 212°, the intervening space being divided into
180°, which admits of extension above and below the points named, a
good thermometer being available for temperature up to 620° Fahr.
The use of the Réaumur scale is confined almost exclusively to Russia
and the north of Germany, while the Centigrade scale is used throughout
the rest of Europe. The Fahrenheit scale is confined to England and her
colonies, and to the United States of America.
[Illustration: 18.
Gridiron-bulb
Thermometer.
Scale
about 1/5.]
Circumstances sometimes arise in which it becomes necessary to convert
readings from one scale into those of the others, according to the
following rules:—
1. To convert Centigrade degrees into degrees of Fahrenheit,
multiply by 9, divide the product by 5, and add 32.
2. To convert Fahrenheit degrees into degrees of Centigrade,
subtract 32, multiply by 5, and divide by 9.
3. To convert Réaumur degrees into degrees of Fahrenheit, multiply
by 9, divide by 4, and add 32.[6]
4. To convert Réaumur degrees into degrees of Centigrade, multiply
by 5 and divide by 4.[7]
[Footnote 6: 8 R = 50 F.]
[Footnote 7: 8 R = 10 C.]
For the production of _continuous_ records, the Meteorological
Committee of the Royal Society have adopted an instrument called a
Thermograph, or self-recording wet and dry bulb thermometer, which is
largely aided by photography. The bulbs of the thermometers are
necessarily placed in the open air, and at a suitable distance from any
wall or other radiating surface; the tubes are of sufficient length to
admit of their being brought inside the building, in due proximity to
the recording apparatus placed in a chamber from which daylight is
rigidly excluded.
[Illustration: 19.
Thermograph and Self-recording Hygrometer.
Scale about 1/18.]
The essential conditions in such an apparatus are:—1. A means of
denoting the height of the mercurial column in the stem of a
thermometer in relation to a fixed horizontal line. 2. A time scale
denoting the exact moment at which the atmosphere reached the
temperature indicated by the mark. 3. As the marks are produced
chemically, and not mechanically (as in the Anemograph), a _dark_ room.
A description of the drawing on page 23 will best show how very
efficiently, through the ingenuity of Mr. BECKLEY, these conditions
have been obtained:—S, wet bulb thermometer; T, atmospheric
thermometer; B, screw for adjusting thermometers; C C, paraffin lamps
or gaslights; D D, condensers, concentrating the light on the mirrors R
R; R R, mirrors reflecting light through air-speck in thermometers V V;
E E, slits through which light passes from mirrors R R; F F,
photographic lenses, producing image of air-speck from both
thermometers on cylinder G; G, revolving cylinder or drum carrying
photographic paper; H, clock, turning cylinder G round once in 48
hours; I, shutter to intercept light four minutes every two hours;
leaving white time-line on developing latent image.
II.—EVAPORATION.
Solar heat rarefies the air by driving its particles asunder; it also
vaporises water from the surface of river, lake, and ocean, diffusing
the vapour through the atmosphere.
Great interest attaches to the subject of Evaporation, on account of
its connection with rainfall and water supply. It is to be regretted,
therefore, that the results hitherto obtained in the endeavour to
measure its rate and quantity do not merit much confidence as regards
their applicability to the evaporation occurring in nature, owing to
the exceptional manner in which the observations have been made.
There is this uncertainty about evaporation, that all the experiments
relate to that taking place from an exposed water surface of a,
comparatively speaking, infinitesimally small area, and can therefore
have but a very partial applicability to the conditions occurring in
nature. There are two main reasons for this statement. Firstly, the
proportion of the surface of the land on the earth which is covered
with lakes and rivers is very limited, and the experiments above
indicated throw no light on the evaporation from the soil. Secondly,
the evaporation from the surface of a small atmometer erected on the
ground, with comparatively dry air all around it, is certainly very
different from that which would take place from an equal area in the
centre of a large water surface, such as a lake.
It is of course easy to make experiments on the evaporation from the
soil by means of a balance atmometer, but in order that these should
possess a practical value, the investigation must be extended so as to
include a wide variety of soils, &c., &c. As regards the second point
which has been raised, it is recommended by the Vienna Congress to
erect atmometers in the centre of water surfaces; but it is not a very
easy matter to conduct such experiments with accuracy, owing to the
risk of in-splashing from waves.
[Illustration: 20.
Atmidometer.
Scale about 1/5.]
BABINGTON’S ATMIDOMETER measures evaporation from water, ice, or snow,
and in form resembles a hydrometer, with the difference that the stem
bears a scale graduated to grains and half grains, and is surmounted by
a light, shallow copper pan. When in use, the hydrometer-like
instrument is immersed in a glass vessel having a hole in the cover,
through which the stem protrudes. The copper pan is then placed on the
top, and sufficient water, ice, or snow placed therein to sink the stem
to the zero of the scale. As the evaporation proceeds, the stem rises;
and, if the _time_ of commencing the experiment is noted, the rate as
well as the amount of evaporation is indicated in grains.
III.—RAREFACTION.
The diffusion of aqueous vapour through the air and the rarefying
influence of heat jointly effect an alteration in the weight of the
atmosphere. This alteration of _weight_ is determined by the Barometer,
an instrument invented by TORRICELLI, in 1643, and in so perfect a form
that in its essential features it has not been superseded.
[Illustration: 21. and 22.
Construction of Barometer.
Scale about 1/18.]
The mode of construction is illustrated by Figs. 21 and 22. It consists
in hermetically sealing a glass tube about three feet long and filling
it with mercury. The finger is placed over the open end of the tube,
which is then inverted and placed in a cistern of mercury and the
finger withdrawn. The left-hand figure shows the result; the mercury is
seen to fall some three or four inches, leaving an empty space at the
top of the tube, which is called the “Torricellian vacuum.”
The mercury is prevented from falling lower than is shown, by the
external pressure of the atmosphere on the cistern. The _weight_ of
this column, therefore, represents the _weight_ or pressure of a
corresponding column of air many miles in height; and so close is the
relation between the column of mercury and the external air that the
_height_ of the former changes with the slightest variation in the
_weight_ of the latter, and the instrument thus becomes a measure of
the weight of the air, from which property its name is derived, the
Greek words _baros_ and _metron_ signifying respectively “weight” and
“measure.”
When the mercury in the barometer tube falls, that in the cistern rises
in corresponding proportion, and _vice versa_, so that there is an
ever-varying relation between the _level_ of the mercury in the tube
and the mercury in the cistern, which affects the accuracy of the
readings. In M. Fortin’s cistern this difficulty is obviated by the use
of a glass, with flexible leather bottom and a brass adjusting screw,
as shown in the cut. Through the top of the cistern is inserted a small
ivory point, the lower end of which corresponds with the zero of the
scale; and, to secure uniformity, the level of the mercury in the
cistern should be adjusted by the screw at each observation, until the
ivory point _appears_ to touch its own reflection on the surface. The
reading is then taken.
[Illustration: 23.
Fortin’s
Cistern.
Scale about
1/6.]
In making barometric observations for comparison with others, it is
necessary that all should be reduced to the common temperature of 32°
F., and for this purpose tables have been calculated which will be
found to save much time.
Tables also for reducing observations of the barometer to sea level, an
operation equally indispensable with the other corrections to make the
readings intercomparable, have been published by direction of the
Meteorological Committee.
For the British Isles the mean sea-level at Liverpool has been selected
by the Ordnance Survey as their datum, and the height of any station
may be ascertained by first noting the nearest Ordnance Bench Mark thus
↑, and purchasing that portion of the Ordnance map which includes the
station, near to which the Bench Mark will be found with the height
above sea-level duly entered. The levellings made for railways will
also furnish the desired information. Failing both these, the observer
should select two or more of the stations nearest his locality for
which official Meteorological Reports are published daily in the
_Times_ and other journals; and taking observations of his barometer at
8 a.m., for a few weeks, should compare them with the mean of the
observations at those stations. The comparison should be omitted when
the barometer pressure is not steady.
[Illustration: 24.
Error of
Capillarity.
Scale about 1/2.]
[Illustration: 25. Standard Barometer. Scale about 1/7.]
A Standard Barometer is constructed on FORTIN’S principle, and should
have its tube about half an inch bore, enclosed in a brass body having
at its upper end two vertical openings, in which the vernier works. The
mercury is seen through these openings, aided by light reflected from a
white opaque glass reflector let into the mahogany board behind. The
scale is divided on one side into English inches and 20ths, and may
have on the other French millimetres, the vernier enabling a reading to
be taken, in each case respectively, of 1/500th of an inch and 1/10th
of a millimetre. In making the instrument, the mercury is boiled in the
tube, to ensure the complete exclusion of air and moisture; while
FORTIN’S principle of cistern ensures a constant level from whence to
take the readings. A sensitive thermometer with scale, engine-divided
on stem, is attached to the brass mount, which is perforated to admit
the attenuated bulb of the thermometer into absolute contact with the
glass tube of the barometer, to ensure its indicating the same
temperature as the contained mercury. The instrument is suspended by a
ring from a brass bracket attached to a mahogany board, and the lower
end passes through a larger ring having three screws for adjusting it
vertically.
A “reading” is taken in the following manner:—1. Note the temperature
by the attached thermometer. 2. Raise or lower the mercury in the
cistern by turning the screw underneath until the reflected image of
the ivory point on the mercury _seems_ to be in contact with the ivory
itself. By the milled head at the side, the vernier is adjusted until
its lower edge just touches the top of the mercurial column, the scale
and vernier then indicate the height of the barometer in inches, 10ths,
100ths, and 1000ths.
High-class instruments, such as that here described, yield _exact_
readings; but, in order to note them accurately, it is important that
the eye, the zero edge of the vernier, the top of the mercurial column,
and the back of the vernier should be in the same horizontal plane;
conditions which may be obtained after some practice.
The accompanying illustration shows a form of barometer which, though
not much used in this country, is deservedly popular on the Continent
as a standard station barometer. It is called a Syphon Barometer, and
was designed by Gay-Lussac. The open end of the tube is bent up in the
form of a syphon, the short limb being from six to eight inches long;
it is furnished with metal scales and verniers, and is mounted on a
mahogany board with attached thermometer.
These barometers require no correction for capillarity or capacity,
each surface of mercury being equally depressed by capillary
attraction, and the quantity of mercury falling from the long limb
occupies the same space in the short limb. The usual correction for
temperature must, however, be applied. A scale of inches, measured from
a zero point taken near the bend of the tube, furnishes the means of
measuring the long and short columns. The difference of readings is the
height of the barometer.
The VERNIER is a movable scale for subdividing parts of a fixed scale,
and was first applied to that purpose by its inventor, M. PIERRE
VERNIER, in 1630. In the barometer the parts to be divided are inches,
which by the aid of this invention are subdivided into 10ths, 100ths,
and 1000ths.
Fig. 27 shows the scale of a standard barometer divided into 1/2-10ths,
or ·05 of an inch. The Vernier C D is made equal to 24 of such
divisions, and is divided into 25 equal parts, from whence it follows
that one division on the scale is 1/25th of ·05 larger than one on the
vernier, so that it shows a difference of ·002 of an inch. The vernier
reads ·0, or zero, upwards; D, therefore, indicates the top of the
mercurial column.
[Illustration: 26.
Syphon
Barometer.
Scale
about 1/12.]
In Fig. 27, zero on the vernier is exactly in line with 29 inches and
5/10ths of the fixed scale; the reading, therefore, is 29·500 inches.
The vernier line _a_ falls short of a division of the scale by
·002-inch; _b_, by ·004; _c_, by ·006; _d_, by ·008; and the succeeding
line by ·010. If the vernier be adjusted to make _a_ coincide with _z_
on the scale, it will have moved through ·002-inch; and if 1 on the
vernier be moved to coincide with _y_ on the scale, the space measured
will be ·010-inch. Consequently, the figures 1, 2, 3, 4, 5, on the
vernier, measure 100ths, and the intermediate lines even 1000ths of an
inch. In Fig. 28 the zero of the vernier is between 29·65 and 29·70 on
the scale. Glancing up the vernier and scale, the second line above 3
will be found in a direct line with one on the scale; this gives ·03
and ·004 to add to 29·65, so that the actual reading is 29·684. In
those instances where no line on the vernier is found _precisely_ to
coincide with a line on the scale, and doubt arises as to which to
select from two equally coincident lines, the rule is to take the
intermediate 1000th of an inch.
[Illustration: 27. and 28.
The Vernier.]
For household and marine barometers such minute subdivisions of the
scale are unnecessary, and the scales of such instruments are therefore
divided only to 10ths, and the verniers made only to read to 100ths of
an inch, which is effected by making the vernier 9/10ths or 11/10ths of
an inch long, and dividing it into 10 equal parts.
In “taking a reading” it is important that it should be done as quickly
as possible, as the heat from the body and the hand is sufficient to
interfere with that accuracy which is necessary where the intention is
to compare the readings with those made by other observers. This
facility is soon acquired by a little practice.
[Illustration: 29. Farmer’s Barometer. Scale about 1/7.]
Pediment Household Barometers, though not so imposing in appearance as
the Wheel Barometer, yield direct readings without the intervention of
the mechanical appliances necessary for moving a needle over an
extended dial. Their mountings are for the most part in oak, walnut,
and other woods, the scales are of ivory, porcelain, or enamelled
glass, and in their graduation due regard is paid to the relative
proportions of cistern and tube, so that the conditions essential to
the production of a Standard Barometer are very closely attained. In
common with other barometers, it should hang in the shade in a vertical
position, so that light may be seen through the tube. As a purchaser
would receive it in what is called a “portable” state, it will be
necessary on first suspending it to take the pinion key, fit it on the
square-headed pin at the bottom of the instrument, and turn gently to
the left till the screw stops. The effect of this is to lower the base
of the cistern, and allow the mercury in the tube to fall to its proper
level. The key should then be replaced for use in moving the vernier.
To make this kind of Barometer portable for travelling it should be
unhung, _very_ gradually sloped until the mercury is at the top of the
tube, when, the instrument being upside down, the base of the cistern
is screwed up by turning the pinion key gently to the right until it
stops. Care should be taken to avoid concussion, and to have the
cistern end always uppermost, or the instrument lying flat.
Fig. 29 shows a useful form of barometer for the farmer, combining as
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_.
In our climate the air is never completely dry, nor completely
saturated with moisture, and the amount of aqueous vapour held in
suspension is very variable. This fact has important bearings on many
branches of industry, as also on the hygienic qualities of the
atmosphere. The consideration that a certain amount of moisture in the
air is necessary to the continuance of health will suggest the
importance of maintaining that due proportion in the atmosphere of sick
rooms, where the artificial heat so injudiciously used, often disturbs
the healthful hygrometric condition of the air. Mr. GLAISHER is of
opinion that the medical profession should enforce, as far as lies in
their power, the use of this simple and effective instrument, which
gives indications so important to the comfort of the patient.
The _amount_ of moisture in the air is _estimated_ by the use of
instruments called Hygrometers, which may be thus classified:—
1. Hygrometers of Absorption.—Made with hair, oatbeard, catgut,
seaweed, grass, chloride of calcium.
2. Hygrometers of Condensation.—Regnault’s, Daniell’s, Leslie’s,
Dyne’s.
3. Hygrometers of Evaporation.—Mason’s Psychrometer, or Wet and Dry
Bulb Thermometers.
By an ingenious application of the affinity of the oatbeard for
moisture, Damp Detectors are constructed for tourists, commercial
travellers, &c., to test moisture and avoid the consequences of
sleeping in damp beds. They are strongly gilt, and resemble in size and
shape a lady’s watch.
[Illustration: 38.
Damp Detector.
Scale about 2/3.]
In Saussaure’s Hygrometer the frame is of brass, and the scale of the
same metal silvered. It has an attached thermometer, and the
indications are the result of the contraction and expansion of a
prepared human hair, consequent upon its absorbing or yielding
moisture. The scale is divided on the arc of a circle, and an index
needle, working on an enlarged arc, multiples the indications.
Regnault’s Hygrometer (Fig. 39) consists of a thin and highly polished
silver tube or bottle, into the neck of which is inserted a delicate
thermometer. The bottle has a lateral tubular opening, to which is
attached a flexible tube with an ivory mouthpiece.
Ether is poured into the silver tube in sufficient quantity to cover
the bulb of the thermometer. The ether is then agitated by breathing
through the flexible tube until, by the rapid evaporation thus
produced, a condensation of moisture takes place, readily observable on
the bright polished silver surface, and the temperature indicated by
the thermometer at that moment is the dew-point.
[Illustration: 39.
Regnault’s Hygrometer. Scale
about 1/10.]
Daniell’s Hygrometer, or Dew-point Thermometer (Fig. 40), consists of a
glass tube, bent twice at right angles, each extremity terminating in a
bulb about 1-1/2 inch in diameter, supported on a brass stand, to which
a thermometer is attached to indicate the temperature of the
surrounding air. The lower bulb is of blackened glass, to facilitate
the observation of the dew-point; it is about three parts filled with
pure ether, and contains a very delicate thermometer. The upper bulb at
the extremity of the short stem is transparent, but covered with thin
muslin, upon which, when an observation is made, pure ether is slowly
dropped. The evaporation rapidly lowers the temperature, until a moment
arrives at which dew condenses on the black bulb. A quick eye is
necessary to note _this_ and the temperature shown by the thermometer
_simultaneously_, the latter showing the degree at which the atmosphere
is saturated with moisture _at the time of observation_. To avoid
error, it is usual to note the temperature at which the dew disappears,
and take the mean of the two temperatures.
[Illustration: 40.
Daniell’s Hygrometer.
Scale about 1/5.]
Dyne’s Hygrometer, for showing the dew-point by direct observation, by
means of iced water and black glass, enables the observer to dispense
with the use of ether, and shows the dew-point with great distinctness.
[Illustration: 41.
Mason’s Hygrometer.
Scale about 1/6.]
The hygrometer in most general use is the wet and dry bulb thermometer,
and for which Mr. Glaisher has calculated an elaborate set of tables, a
brief abstract of which sufficient for general purposes is subjoined.
For finding the Degree of Humidity of the Air from Observations of
a Dry Bulb and a Wet Bulb Thermometer, sometimes called Mason’s
Psychrometer.
+-------------+-----------------------------------------+
| |DIFFERENCE BETWEEN DRY BULB AND WET BULB |
| | READINGS. |
|TEMPERATURE +------+------+------+------+------+------+
| BY THE | | | | | | |
| DRY BULB | 2° | 4° | 6° | 8° | 10° | 12° |
|THERMOMETER. | | | | | | |
| +------+------+------+------+------+------+
| | DEGREE OF HUMIDITY. |
+-------------+------+------+------+------+------+------+
| 34° | 79 | 63 | 50 | ... | ... | ... |
| 36 | 82 | 66 | 53 | ... | ... | ... |
| 38 | 83 | 68 | 56 | 45 | ... | ... |
| 40 | 84 | 70 | 58 | 47 | ... | ... |
| 42 | 84 | 71 | 59 | 49 | ... | ... |
| 44 | 85 | 72 | 60 | 50 | ... | ... |
| 46 | 86 | 73 | 61 | 51 | ... | ... |
| 48 | 86 | 73 | 62 | 52 | 44 | ... |
| 50 | 86 | 74 | 63 | 53 | 45 | ... |
| 52 | 86 | 74 | 64 | 54 | 46 | ... |
| 54 | 86 | 74 | 64 | 55 | 47 | ... |
| 56 | 87 | 75 | 65 | 56 | 48 | ... |
| 58 | 87 | 76 | 66 | 57 | 49 | ... |
| 60 | 88 | 76 | 66 | 58 | 50 | 43 |
| 62 | 88 | 77 | 67 | 58 | 50 | 44 |
| 64 | 88 | 77 | 67 | 59 | 51 | 45 |
| 66 | 88 | 78 | 68 | 60 | 52 | 45 |
| 68 | 88 | 78 | 68 | 60 | 52 | 46 |
| 70 | 88 | 78 | 69 | 61 | 53 | 47 |
| 72 | 89 | 79 | 69 | 61 | 54 | 48 |
| 74 | 89 | 79 | 70 | 62 | 55 | 48 |
| 76 | 89 | 79 | 71 | 63 | 55 | 49 |
| 78 | 89 | 79 | 71 | 63 | 56 | 50 |
| 80 | 90 | 80 | 71 | 63 | 56 | 50 |
| 82 | 90 | 80 | 72 | 64 | 57 | 51 |
| 84 | 90 | 80 | 72 | 64 | 57 | 51 |
| 86 | 90 | 80 | 72 | 64 | 58 | 52 |
+-------------+------+------+------+------+------+------+
The total quantity of aqueous vapour which at any temperature can be
diffused in the air being represented by 100, the percentage of vapour
actually present will be found in the table _opposite_ the temperature
of the dry thermometer, and _under_ the difference between the dry bulb
and wet bulb temperatures. The degree of humidity for intermediate
temperatures and differences to those given in the table can be easily
estimated. Thus dry bulb 51°, wet bulb 46°, give 69 for the degree of
humidity.
The instrument, as shown at page 48, consists of two thermometers
attached to a support, which may be either slate or wood. The bulb of
one of the thermometers has some thin muslin tied over it, and is kept
moist by the capillary action of a thread dipping into a cistern of
water placed underneath. It will be obvious that the amount of
evaporation will be in proportion to the dryness of the air, and that
the differences of temperature indicated by the two thermometers will
be greatest when the atmosphere is dry, and least when the air is damp.
[Illustration: 42.
Board of Trade Thermometer
Screen. Scale about 1/9.]
HYGROMETER PRECAUTIONS.
Hygrometers should be exposed in the shade free from air-currents.
The covering of the wet bulb must be very thin.
The supply of water must be carefully regulated.
The bulb must be constantly _moist_, yet not _too wet_.
The supply of water must be ample in dry weather.
In damp weather water must not drip from the wet bulb.
Water reservoir should be as far as possible from the dry bulb.
Dry bulb must never receive moisture from any source.
Use distilled, rain, or softest water procurable, for wet bulb.
When lime deposits from use of hard water change muslin and worsted.
Replenish reservoir after, or long before, taking an observation.
Well wash muslin and worsted before using.
Also wash occasionally while in use.
Change muslin twice a month or according to condition.
Dust and blacks must not be allowed to accumulate on muslin.
⎧ When wet bulb is frozen, wet with ice-cold water by brush.
⎪ The water will first freeze, then cool to air-temperature.
⎨ After which wet bulb falls a trifle lower than dry one.
⎩ Then temperature of evaporation may be noted.
⎧ In thick fog wet bulb reads _above_ dry bulb.
⎪ In cold calm weather, wet bulb reads _above_ dry bulb.
⎨ This is owing to the air being perfectly saturated.
⎪ Covered bulb cannot therefore show temperature as well as uncovered.
⎩ In such cases both readings are assumed to be identical.
It is important that the instrument should be protected not only from
the sun’s direct rays, from rain and snow, but also from wind, the
currents of which would, by increasing evaporation, cause the wet bulb
thermometer to indicate a temperature not strictly due to the
hygrometric condition of the atmosphere. For this purpose Thermometer
Screens are employed. Illustrations of two forms are shown at Figs. 42
and 43; they should be placed facing the north at a distance of four
feet from the ground. Fig. 42 shows the form adopted by the Board of
Trade, for marine service, while Fig. 43 shows Mr. Stevenson’s
double-louvred screen with perforated bottom, which ensures free
ingress and egress of air, the exclusion of snow and rain, and the
direct rays of the sun. Professor Wild recommends overlapping segments
of sheet zinc for the construction of these screens, as possessing the
advantage over wood of becoming sooner in _thermic equilibrium_ with
the surrounding air, and thus preventing radiation. Stevenson’s Screen
should be erected on legs four feet high, and should stand over grass
on open ground. It should not be under the shadow of trees, nor within
twenty feet of any wall.
[Illustration: 43.
Stevenson’s Thermometer Screen.
Scale about 1/10.]
CLOUDS.
The important office performed by clouds in the economy of nature
entitles them to extended consideration. A cloud may be defined as
“water-dust,” since aqueous vapour diffused through the air is
invisible until the temperature is sufficiently lowered to produce
condensation; no satisfactory explanation, however, has yet been given
of the mode of suspension of this water-dust, nor why it remains
suspended in opposition to gravitation. It is tolerably certain that
electricity is not without its influence, though the apparently
_stationary_ character of some clouds is deceptive, for while there may
be no apparent motion in the mass the particles constituting the mass
are undergoing continuous renewal, which justifies the assertion of
Espy that every cloud is either a forming or dissolving cloud.
Aeronauts in ascending from the earth pass through many successive
alternations of cloud-strata and clear air which owe their existence to
the varying temperature and degrees of humidity of the atmospheric
currents so superposed.
Luke Howard in his Askesian Lectures, 1802, divides clouds into three
primary modifications: cumulus, stratus, and cirrus, with intermediate
forms resulting from combinations of the primaries, viz.,
cirro-cumulus, cirro-stratus, cumulo-stratus, and cumulo-cirro-stratus
or nimbus. This nomenclature is now universally adopted.
[Illustration: 44.
Cirrus.]
CIRRUS, or mare’s tail cloud, appears as parallel, flexuous, or
diverging streaks or fibres, partly straight. It is the lightest and
the highest of all clouds, being seldom less than three miles, and
often ten miles, above the earth, and shows the greatest variety of
form. On account of its great height it is assumed to consist of minute
snowflakes or crystals of ice, the refractions and reflections from
which produce the halos, coronæ, and mock suns and moons which occur
chiefly in this cloud and its derivatives. It retains its varied
outlines longer than any other cloud; at sunrise it is the first to
welcome the sun’s rays, and at sunset the last to part with them. It is
the most useful of all clouds for weather warnings.
1. _Serene, settled weather may be expected_ when groups and
threads of cirri are seen during a gentle wind after severe
weather.
2. _A change to wet may be expected_ when, after continued fair
weather, filaments, or bands of cirri (_apparently_
stationary), with converging ends, travel across the sky.
3. _Rain or snow, and windy, variable weather may be expected_ when
cirri with fine tails vary much in a few hours.
4. _Continued wet weather may be undoubtedly expected_ when
horizontal sheets of cirri fall quickly and pass into the
cirro-stratus.
5. _A storm of wind and rain may be expected within forty-eight
hours_ when fine threads of cirri seem brushed backward from
the south-west.
[Illustration: 45.
Cumulus.]
CUMULUS.—This modification of cloud is most frequently seen on bright
summer days, and is appropriately called “the day cloud” and “the
summer cloud.” It is formed only in the daytime, in summer calms, and
results from the rise of vapours from rivers, lakes, and marshes into
the colder regions of the air, the lower portions of which are readily
saturable. They are characterized by a horizontal base, from which they
rise in dense conical and hemispherical masses rivalling mountains in
their magnitude.
Their formation is due to the convection of heat from the earth’s
surface, which renders the lower atmospheric strata capable of holding
a larger amount of aqueous vapour and simultaneously establishes an
upward current, which reaching the colder regions of the air brings
about the condensation of the aqueous vapour into the elegant and
ever-beautiful forms admired alike “by saint, by savage, and by sage.”
These begin as mere specks, which enlarge until the sky is nearly
covered in the afternoon, and towards sunset they generally disappear,
their tops becoming cirri when the air is dry.
1. _Fine, calm, warm weather may be expected_ when cumuli are of
moderate size and of pleasing form and colour.
2. _Cold, tempestuous, rainy weather may be expected_ when cumuli
cover the sky, rolling over each other in dense, dark, and
abrupt masses.
3. _Thunder may be expected_ when cumuli of hemispherical form are
characterized by an extreme silvery whiteness.
4. _Rain may be expected_ when cumuli increase in number towards
evening, sinking at the same time into the lower portions of
the air.
[Illustration: 46.
Stratus.]
STRATUS.—As its name implies, this is a horizontal sheet of cloud
formed near the earth at night (whence it has been called “the night
cloud”) by the condensation of moist air from rivers, lakes, and
marshes, or damp ground which has lost its day-heat by radiation,
especially in calm clear evenings, after warm days. It appears as a
white mist near, and sometimes touching, the earth. It attains its
maximum density about midnight, but is dissipated by the rays of the
morning sun. Its formation, watched from a height over a large city, is
highly interesting, and is attributed by Sir John Herschel to the soot
suspended over such localities, each particle of which acts as “an
insulated radiant, collects dew on itself, and sinks down rapidly as a
heavy body.” Still more interesting is it to observe from a similar
elevation the dissipation of this cloud when the sun has attained such
an altitude that its rays fall on the upper surface of the stratus
cloud, which then heaves like the billows of the ocean, while the whole
mass seems to rise spontaneously from the earth, and speedily vanishes
“into air, into thin air.”
1. _The finest and most serene weather may be expected_ when
stratus clouds present the appearances just described.
CIRRO-CUMULUS, or “mackerel sky,” is a well-known form of cloud
occurring in small roundish masses, looking like flocks of sheep at
rest, and often at great heights. It is seldom seen in winter.
1. _Increased heat may be expected_ when cirro-cumuli appear.
2. _A storm or thunder may be expected_ when cirro-cumuli occur
mingled with cumulo-stratus in very dense, round, and close
masses.
3. _Warm wet weather, and a thaw, may be expected_ when
cirro-cumuli occur in winter.
CIRRO-STRATUS “appears to result from the subsidence of the fibres of
cirrus to a horizontal position, at the same time approaching
laterally. The form and relative position when seen in the distance
frequently give the idea of shoals of fish.” It is called “the vane
cloud” and “mackerel-backed sky.”
1. _Rain, snow, and storm may be expected_ when _cirro-stratus_ is
seen alone or mingled with cirro-cumulus, especially if the
cirro-cumulus passes away.
2. _Fair weather may be expected_ when from a mixture of
cirro-stratus and cirro-cumulus the former disappears, leaving
the latter in possession of the sky.
3. _Thunder and heat_ are generally attended by waved cirro-stratus.
CUMULO-STRATUS.—This form of cloud results from the mingling of the
cumulus and cirro-stratus; it appears sometimes as a thick bank of
cloud with overhanging masses. The cloud known as “_distinct_”
cumulo-stratus appears as a cumulus surrounded by small fleecy clouds.
1. _Thunder may be expected_ when “distinct” cumulo-stratus appear.
2. _Sudden atmospheric changes may be expected_ when cumulo-stratus
appear.
[Illustration: 47.
Nimbus.]
NIMBUS, OR CUMULO-CIRRO-STRATUS.—The name of this cloud at once
suggests that it is produced by a combination of the three primary
forms of cloud. The _nimbus_ is popularly known as “the rain cloud.” It
is really a system of clouds, having its origin chiefly in the tendency
of the _cumulo-stratus_ to spread, overcast the sky, and settle down to
a dense horizontal black or grey sheet, above which spreads the
_cirrus_, and from below which rain begins to fall.
1. _A cessation of rain may be expected_ when the grey lower
portion of _nimbus_ begins to break up.
2. _A thunderstorm may be expected_ when the _nimbus_ character of
the cloud is very perfect.
3. _Very copious showers may be expected_ when the _cirri_
projected from the top of the rain-cloud are very numerous.
AMOUNT OF CLOUDS.—Any record of the proportion of sky covered by cloud
should be made on a scale of 0 to 10. A clear sky is registered 0, and
a sky wholly obscured as 10, any intermediate condition being
represented by 5—7, or other figures deemed appropriate by the
observer. The _kind_ of cloud should be noted, as also the direction in
which it is driven by the wind, whether in the upper or lower strata of
the air. This operation may be assisted by an ingenious arrangement,
exhibited by Mr. Goddard in 1862, and called a “cloud reflector,”
obtainable at any optician’s. Observations at the Greenwich Observatory
establish the facts that the least amount of cloud exists during the
night, especially in May and June, and the greatest amount at midday,
and in winter; also that from November to February three-fourths of the
heavens are obscured by sun-repelling clouds.
HEIGHT OF CLOUDS.—Great diversity of opinion exists on this point. It
is asserted, on the one hand, that the region of clouds does not extend
beyond five miles above sea-level, but Glaisher has attained a height
of 36,960 feet, and from thence saw clouds floating at a great height
above him; and it is considered probable that cirri are often ten miles
above the earth.
VELOCITY OF CLOUDS.—This is of two kinds: 1st. Velocity of
Propagation; and 2nd. Velocity of Motion. The first occurs when at a
given altitude the dew-point is suddenly attained, when the sky on one
occasion was covered from the eastern to the western horizon at the
rate of 300 miles per hour. The second is dependent on the force of
atmospheric currents, which is much greater in the upper regions of the
air than in those nearer the earth. Accurate observations of the
shadows of clouds, borne across the fields on a summer’s day, warrant
the assertion that an apparently slow motion of clouds is equal to
eighty miles an hour, while a velocity of 120 miles is attained without
impressing the observer with the idea of rapidity.
On the subject of clouds Admiral Fitzroy says:—
May be Expected
-----
Fine weather When clouds are “soft-looking or delicate.”
Wind When clouds are hard-edged or oily-looking.
Less wind In proportion as the clouds look _softer_.
More wind The harder, more “greasy,” rolled, tufted, or
ragged the clouds look.
Rain When small-inky-looking clouds appear.
Wind _and_ rain When light scud clouds are seen driving across
heavy masses.
Wind only When light scud clouds are seen alone.
Change of wind When high upper clouds cross the sun, moon, or
stars in a direction different from that of the
lower clouds, or the wind then felt below.
Wind With tawny or copper-coloured clouds.
The following “Weather Warnings” may be gathered from the COLOUR OF THE
SKY:—
Whether clear or cloudy, a rosy sky at sunset presages fine weather; a
sickly greenish hue, wind and rain; a red sky in the morning, bad
weather, or much wind or rain; a grey sky in the morning, fine weather;
a high dawn (_i. e._, when the first indications of daylight are seen
above a bank of clouds), wind; a low dawn (_i. e._, when the day breaks
on or near the horizon), fair weather. Light, delicate, quiet tints or
colours, 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. Also a bright yellow
sky at sunset presages wind; a pale yellow, wet; orange or
copper-coloured, wind and rain: and thus, by the prevalence of red,
yellow, green, grey, or other tints, the coming weather may be told
very nearly; indeed, if aided by instruments, almost exactly.
After fine, clear weather the first signs in a 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 vapour that grows into cloudiness. This appearance, more or less
oily or watery as wind or rain will prevail, is an infallible sign.
Usually, the higher and more distant such clouds seem to be, the more
gradual, but general, the coming change of weather will prove.
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.
May be Expected
-----
Fine weather When the sky is grey in the morning.
Wind With a high dawn.
Fair weather With a low dawn.
Wind When the sky at sunset is of a _bright_ yellow.
Rain When the sky at sunset is of a _pale_ yellow.
Wind and rain When the sky is orange or copper colour.
Fine weather When the sky has light, delicate, quiet tints and
soft, indefinite forms of clouds.
Rain and wind When the sky has gaudy, unusual hues, with hard,
definite outlined clouds.
Fair weather When sea-birds fly out early and far to seaward.
Stormy weather When sea-birds hang about the land, or fly inland.
Fair weather When dew is deposited. Its formation never
_begins_ under an overcast sky, or when there is
much wind.
Rain On what is called a good _hearing_ day.
Rain When remarkable clearness of atmosphere,
especially near the horizon, exists, distant
objects, objects, such as hills, being unusually
visible or well defined.
RAIN.
The atmosphere at a given temperature is capable of retaining only a
given quantity of aqueous vapour, invisibly diffused through it, at
which temperature it is said to be _saturated_. Should the temperature
from any cause be lowered, the aqueous vapour at once becomes visible
in the form of either cloud, dew, rain, snow, or hail. It has already
been shown that, although marshes and rivers, inland seas and lakes,
yield by evaporation watery vapours to the air, the ocean is the great
source of rain, whence it is lifted in vast quantities by the sun’s
radiant heat, to be subsequently condensed by passing into cooler
regions, or by contact with cold mountain peaks, falling to earth as a
fertilizing shower or a devastating flood.
Sir John Herschel accounts for the formation of raindrops by
saying:—“In whatever part of a cloud the original ascensional movement
of the vapour ceases, the elementary globules of which it consists
being abandoned to the action of gravity, begin to fall. The larger
globules fall fastest, and if (as must happen) they overtake the slower
ones, they incorporate, and the diameter being thereby increased, the
descent grows more rapid, and the encounters more frequent, till at
length the globule emerges from the lower surface of the cloud at the
‘vapour plane’ as a drop of rain, the size of the drops depending on
the thickness of the cloud stratum and its density.”
Rain is very unequally distributed, there being portions of the torrid
zone where it _never_ falls, one locality in Norway where it falls
three days out of four, and another on the western side of Patagonia,
at the base of the Andes, where it falls every day. The quantities
recorded as having fallen at one time in some localities are simply
appalling. A fall of one inch is considered a very heavy rain in Great
Britain, and this fact will enable the reader partially to realize the
following stupendous recorded falls:—Loch Awe, Scotland, 7 inches in
30 hours; Joyeuse, France, 31 inches in 22 hours; Gibraltar, 33 inches
in 26 hours; hills above Bombay, 24 inches in one night; and on the
Khasia Hills, where the annual rainfall is 600 inches, 30 inches have
been known to fall on each of five successive days. Mr. G. J. Symons,
the able editor of the “Meteorological Magazine,” and indefatigable
superintendent of 2,000 Rain Gauges throughout the United Kingdom, has
compiled a table, showing the equivalents of rain in inches, its weight
per acre, and bulk in gallons, the following portion of which, while
very useful to the farmer, will enable the curious reader to make some
interesting calculations, based on the figures quoted above:—
TABLE SHOWING EQUIVALENT OF INCHES OF RAIN IN GALLONS,
AND WEIGHT PER ACRE.
Inches of Rain Tons per Acre Gallons per Acre
0·1 10 2262
0·2 20 4525
0·3 30 6787
0·4 40 9049
0·5 50 11312
0·6 61 13574
0·7 71 15836
0·8 81 18098
0·9 91 20361
1·in. 101 22623
The instruments called Rain Gauges or Pluviometers are, as their name
implies, constructed to measure the amount of rain falling in any given
locality, and those in most general use have this principle in common:
that the graduated glass always bears a definite relation to the area
of the receiving surface. A very extraordinary and hitherto unexplained
fact in connection with the fall of rain, and which justifies the
opinion that its formation is not limited to the region of visible
cloud, is that a series of rain gauges placed at different elevations
above the soil are found to collect very different quantities of rain,
the amount being _greater_ at the _lower_ level. Thus, twelve months’
observations by Dr. Heberden determined that the amount of rain on the
top of Westminster Abbey was only twelve inches, that on a house close
by but much lower eighteen inches, and on the ground during the same
interval of time twenty-two inches. Accordingly, ten inches is the
height at which meteorologists have agreed the edge of the rain gauge
should be placed from the ground. The spot chosen should be perfectly
level, and at least as far distant from any building or tree as the
building or tree is high, and, if the gauge cannot be equally exposed
to all points, a south-west aspect is preferable. It is also important
that the rain gauge should be well supported, in order to avoid its
being blown over by the wind; and, should frost follow a fall of rain,
the instrument should be conveyed to a warm room to thaw before
measuring the collected contents. The graduated glass furnished with
each instrument should stand quite level when measuring the rain, and
the reading be taken midway between the two apparent surfaces of the
water.
The best form of rain gauge is that in use in the Meteorological Office.
[Illustration: 48.
Howard’s Rain Gauge.
Scale about 1/5.]
Howard’s Rain Gauge consists of a vertical glass receiver, or bottle,
through the neck of which the long terminal tube of a circular funnel,
five inches in diameter, is inserted. A metal collar or tube fits over
the outside of the neck of the receiver, and aids in keeping the funnel
level, while the tube extends to within half an inch of the bottom,
thus ensuring the retention of every drop of rain which falls within
the area of the funnel. The glass vessel furnished with the instrument
is graduated to 100ths of an inch. A modification of this instrument is
made with a glass tube at the side graduated to inches, 10ths, and
100ths, showing the amount of rainfall by direct observation, thus
dispensing with the use of a supplementary graduated measure.
In Glashier’s Rain Gauge special provision is made, in two ways, to
prevent possible loss by evaporation, even in the warmest months of the
year. 1. The receiving vessel is partly sunk beneath the soil, thus
keeping the contents cool. 2. The receiving surface of the funnel,
accurately turned to a diameter of eight inches, terminates at its
lower extremity in a curved tube, which, by always retaining the last
few drops of rain, prevents evaporation. The graduated vessel, in this
instance also, is divided to 100ths of an inch, having due regard to
the larger area, 8 in. of the funnel. For use in tropical climates,
where, as has been shown, the rainfall is excessive, a modification of
this instrument is supplied by the instrument makers, having an extra
large receiver and tap for drawing off the collected rain.
Luke Howard, in his “Climate of London,” says: “It must be a subject of
great satisfaction and confidence to the husbandman to know at the
beginning of a summer, by the certain evidence of meteorological
results on record, that the season, in the ordinary course of things,
may be expected to be a dry and warm one, or to find, in a certain
period of it, that the average quantity of rain to be expected for the
month has fallen. On the other hand, when there is reason, from the
same source of information, to expect much rain, the man who has
courage to begin his operations under an unfavourable sky, but with
good ground to conclude, from the state of his instruments and his
collateral knowledge, that a fair interval is approaching, may often be
profiting by his observations, while his cautious neighbour, who
‘waited for the weather to settle,’ may find that he has let the
opportunity go by.” This superiority, however, is attainable by a very
moderate share of application to the subject, and by the keeping of a
plain diary of the barometer and rain gauge, with the hygrometer and
vane under his daily notice.
[Illustration: 49.
Symons’s Rain Gauge.
Scale about 1/7.]
Symons’s Rain Gauge resembles Howard’s, but has the advantage of having
the glass receiver enclosed in a black or white japanned metal or
copper jacket with openings permitting an approximate observation of
the collected rain. The metal jacket is also furnished with strong iron
spikes, which are firmly pressed into the soil, as shown at Fig. 49,
thus ensuring perfect steadiness by its power to resist the wind. The
graduated measure contains half an inch of rain (for a 5 inch circle)
divided into 100ths.
[Illustration: 50.
Symons’s Storm Rain Gauge.
Scale about 1/12.]
Mr. Symons has devised another rain gauge of so ingenious and
interesting a character that it needs only to become generally known
among amateur meteorologists to be in universal demand. By its means an
observer at a distant window may read off the rain as it falls. It is
shown at Fig. 50, where the usual 5-inch funnel surmounts a long glass
tube attached to a black board bearing a very open scale marking tenths
of an inch in _white_ lines; a white float inside the tube constitutes
the index, which rises as the rain increases in quantity. If, as
sometimes happens during a thunderstorm, the rainfall is excessive, a
second tube on the left permits the measurement of a second inch of
rain. It will be obvious that if the _time_ at which the rain begins to
fall be noted the _rate_ at which it falls, as well as the quantity, is
indicated at sight by this instrument.
[Illustration: 51.
Beckley’s Pluviograph. Scale about 1/7.]
Crossley’s Registering Rain Gauge has a receiving surface of 100 square
inches. The rain falling within this area passes through a tube to a
vibrating bucket, which sets in motion a train of wheels, and these
move the indices on three dials, recording the amount of rain in
inches, 10ths, and 100ths. Printed directions are furnished with each
instrument, and the simplicity of the mechanism ensures due accuracy. A
test measure, holding exactly five cubic inches of water, sent with
each gauge, affords the means of checking its readings from time to
time.
Beckley’s Pluviograph possesses the exceptional merit of recording with
equal precision all rainfalls, from a slight summer shower to a heavy
storm of rain. It may be placed in a hole in the ground, with the
receiving surface raised the standard height of ten inches above its
level.
Fig. 51 illustrates the construction of the instrument.
[Illustration: 52. 53.
Stutter’s Self-recording Rain Gauge. Scale about 1/7.]
The funnel has a receiving surface of 100 square inches, protected by a
lip 1-1/4 inch deep, to retain the splashes. The rain flows into a
copper receiving vessel on the right, which, floating in a cistern of
mercury, sinks and draws down with it a pencil, which records the event
on a white porcelain cylinder moved by a clock. When the receiving
vessel is full the syphon comes into action, rapidly drawing off _the
whole_ of the water, the vessel rising almost at a bound, the action
being recorded by a vertical line on the porcelain cylinder. Two or
more cylinders are supplied with each instrument; and, as the pencil
marks are readily removed by a little soap and water, a clean one may
be always kept at hand for exchange once in every twenty-four hours.
The Rev. E. Stutter’s Self-recording Rain Gauge is ingenious, and for a
self-recording instrument is very moderate in price, while it
efficiently shows the rainfall for every hour in the twenty-four (Figs.
52, 53).
An eight-day clock with its upright spindle revolves a small funnel
with a sloping tube, the end of which passes successively over the
mouth of the twelve or twenty-four compartments in the rim of the
instrument; beneath each compartment is placed a tube, as shown in the
sectional figure. All rain received by the outer funnel drips into the
smaller revolving funnel, and flows down the sloping tube, the end of
which is timed to take an hour in passing over each compartment, so
that the rain, for example, which falls between twelve and one o’clock
will be found in the tube marked 1. Each tube can contain half an inch
of rain, and any overflow falls into a vessel beneath, and can be
measured; the tube which has overflown shows the hour.
V.—MOTION.
Wind is air in motion. The motion of the air is caused by inequality of
temperature. The earth becomes warmed by the sun, and radiates the heat
thus acquired back upon the air, which, expanding and becoming lighter,
ascends to higher regions, while colder and denser currents rush in to
occupy the vacated space. Two points are to be noted in connection with
this rush of air which we call wind, viz., its _direction_ and
_velocity_ or _force_. Both are estimated scientifically by instruments
called Anemometers,[13] while mariners and the dwellers on our coasts
have a nomenclature of their own by which to indicate variation in the
_force_ of the wind, founded on the amount of sail a vessel can carry
with safety at the time. In the matter of _direction_ winds are classed
as constant, periodical, and variable.
[Footnote 13: _Anemos_, the wind; _metron_, measure.]
CONSTANT WINDS.—_The Trade Winds._—The violent contrast between the
temperature of the equator and the poles is well known, and from the
vast area included within the tropics ascending currents of rarefied
air are incessantly rising and being as incessantly replaced by a rush
of cold air from the poles to the equator. Were the earth stationary,
this interchange would be of the simplest kind; on arriving within the
influence of the ascending equatorial current the air from the poles
would be carried to the higher regions and turning over would proceed
to the poles, and, becoming cold and dense in traversing the higher
stratum, would descend and resume its course _ad infinitum_. The
revolution of the earth on its axis changes all this: the first effect
is that the air at the equator is borne along with the earth at the
rate of seventeen miles a minute from west to east, a rate which
diminishes at 60° of latitude to one-half that velocity, until at the
poles it is nothing; consequently a _slow_ north wind flowing to the
equator is continually passing over places possessing a higher velocity
than itself, and not immediately acquiring that velocity, there is
according to the law of the composition of forces a compromise effected
resulting in a north-east wind. In a similar manner the same process in
the southern hemisphere results in a south-east wind. These winds have
acquired the name of _Trade Winds_ on account of the facilities
afforded to navigation by their constancy. The _North Trades_ occur in
the Atlantic between 9° and 30° and in the Pacific between 9° and 26°.
The _South Trades_ occur in the Atlantic between lat. 4° N. and 22° S.
and in the Pacific between latitude 4° N. and 23-1/2° S. These limits
extend northward with the sun from January to June, and southward from
July to December.
Parallel to the equator and extending between 2° and 3° on each side is
a broad belt, where the north and south trades neutralize each other,
producing what is called the “_Region or Belt of Calms_.” Though wind
is absent, thunderstorms and heavy rains are of daily occurrence.
When Humboldt ascended Teneriffe the trade wind was blowing at its base
in the usual direction, but on arriving at the summit he found a strong
wind blowing in the opposite direction. Observation has shown that this
upper current prevails north and south of the equator, and that, after
passing the limit of the trade winds, it descends to form the
south-_west_ winds of the north temperate zone and the north-_west_
winds of the south temperate zone; the _westing_ being due to the same
cause as the _easting_ in the regular trades, viz., the rotation of the
earth on its axis. These winds are called the Return Trades, but are
not equal in constancy to the regular trade winds.
PERIODICAL WINDS.—_Land and Sea Breezes_ occur on the coasts, chiefly
in tropical countries, but sometimes in Great Britain during the summer
months when the land during the day becomes very hot, causing an
ascending column of air, which is replaced by a comparatively colder
stream flowing inwards from the sea. At sunset the conditions are
reversed, the earth becomes rapidly cooled by radiation, the sea
continuing comparatively warm, the air over it ascends, and its place
is supplied by a cold breeze, which “blows off the shore,” as
illustrated by the diagrams and the following experiment—In the centre
of a large tub of water float a water plate containing hot water,
imagine the former to be the ocean and the latter the heated land,
rarefying the air over it. Light a candle and blow it out and hold it
while still smoking over the cold water, when the smoke will be seen to
move towards the plate. The reverse of this takes place if the tub be
filled with hot water and the water plate with cold. When this
phenomenon takes place on a large scale, as in the case of the north
trade winds being drawn from their course by the heated shores of
Southern Asia, the gigantic sea breeze thus produced is called the
south-west monsoon. This occurs from April to October, when the sun is
north of the equator. When the sun is south of the equator—that is,
from October to April—the analogue of the land breeze is produced, and
is called the north-east monsoon.
VARIABLE WINDS.—The character of this class of winds is determined by
the physical configuration of the country in which they occur. Some
tracts are marked by luxuriant vegetation, others are bare. Here
mountains lift their awful fronts and “midway leave the storm,” there
an arid plain extends itself to the seashore, or inland, towards a
chain of lakes. Within the tropics these purely local conditions are
insufficient to overcome the force of the prevalent atmospheric
currents: such, however, is not the case beyond the tropical zone.
There the variable winds prevail, for which space permits only the
mention of their names:—The _Simoom_ (from the Arabic _samma_, hot),
peculiar to the hot sandy deserts of Africa and Western Asia. The
_Sirocco_ blows over the two Sicilies as a hot wind from the south. It
extends sometimes to the shores of the Black and Caspian seas,
spreading death among animals and plants. The _Solano_ prevails at
certain seasons in the south of Spain: its direction is south-east. The
_Harmattan_ is another wind of the same class, peculiar to Senegambia
and Guinea. The _Puna Winds_ blow for four months over a barren
tableland called the Puna, in Peru. They are a portion of the
south-east trade winds, which, having crossed the Pampas, are thereby
deprived of moisture, and become the most parching wind in the world.
The _East Winds_, peculiar to the spring in Britain, blowing as they do
through Russia, over Europe, are a portion of the great polar current,
distinctive of that season of the year. They are dry and parching,
every one being familiar with the unpleasant bodily sensations
attendant on this much-abused and yet most beneficent wind.
The _Etesian Winds_ are drawn from the north across the Mediterranean
by the great heat of the African desert. The _Mistral_ is a strong
north-west wind peculiar to the south-east of France. The _Pampero_ is
a north-west wind, blowing in summer from the Pampas of Buenos Ayres.
As long ago as the year 1600 Lord Bacon remarked that the
preponderating tendency of the wind was decidedly to veer _with_ the
sun’s motion, thus passing from N. through N.E., E., S.E., to south,
thence through S.W., W.N.W., to N.; also, that it often makes a
complete circuit in that direction, or more than one in succession
(occupying sometimes many days in so doing), but that it rarely backs,
and very rarely or never makes a complete circuit in the contrary
direction. The merit of having first demonstrated that this tendency is
a direct consequence of the earth’s rotation is due to Professor Dove,
of Berlin, who has also shown that the three systems of atmospheric
currents just treated of, viz., the constant, periodical, and variable
winds, are all amenable to the same influence.
As to the _mode_ of observing the wind, Admiral Fitzroy recommends that
a true east and west line should be marked _about the time of the
equinox_, and the north, south, and other points of the compass being
added, to take the bearings of the wind in relation to a dial so
prepared, the indications of the _lower_ stratum of clouds in
conjunction with vanes and smoke being preferred to any other.
The direction of the wind should always be given according to _true_,
and not to _compass bearings_. Two points to the westward nearly
represents the amount of “Variation of the Compass” for the British
Isles, which yields the following table for the conversion of
directions observed by the compass in Great Britain and Ireland to
approximate true bearings.
+---------------------+-----+-----+-----+-----+-----+-----+-----+
|Compass bearings. N | NNE | NE | ENE | E | ESE | SE | SSE |
|True bearings. NNW| N | NNE | NE | ENE | E | ESE | SE |
+---------------------+-----+-----+-----+-----+-----+-----+-----+
|Compass bearings. S | SSW | SW | WSW | W | WNW | NW | NNW |
|True bearings. SSE| S | SSW | SW | WSW | W | WNW | NW |
+---------------------+-----+-----+-----+-----+-----+-----+-----+
“One may call a very simple diagram, a circle divided by a diameter
from north-east to south-west, the _thermometer compass_. While the
wind is shifting from south-west, by west, north-west, and _north to
north-east_, the thermometer is falling, but while shifting from
north-east, by east, south-east and south, towards _south and
south-west_, the thermometer is rising. Now the barometric column does
just the reverse. From north-east the barometer falls as the wind
shifts through the east to south-east, south, and south-west, and from
the south-west, as the wind shifts round northward to north-east, the
barometer rises—it rises to west, north-west, north, and north-east.
“The effect of the wind thus shifting round when traced upon paper by a
curve, seems certainly wave-like to the eye; but I believe it to be
simply consequent on the wind shifting round the compass, and
indicating alteration in the barometric column.
“If the wind remained north-east, say three weeks, there would be no
wave at all—there would be almost a straight line along a diagram
(varying only a little for _strength_). The atmospheric line, in such a
case, remains at the same height, and the barometer remains at 30
inches and (say) some three or four-tenths, for weeks together. So
likewise when the wind is south-westerly a long time, or near that
point, the atmospheric line remains _low_, towards 29 inches. Thus,
such ‘atmospheric waves’ may be an optical delusion.
“The diagram alluded to above shows how the barometer and thermometer
may be used in connection with each other in foretelling wind, and
consequently weather, that is coming on, because _as the one rises, the
other_ generally _falls_, and if you take the two together and confront
with their indications the amount of moisture in the air at any time,
you will scarcely be mistaken in knowing what kind of weather you are
likely to have for the _next two or three days_, which for the
gardener, the farmer, soldier, sailor, and traveller must be frequently
of considerable importance.”[14]
[Footnote 14: The late Admiral Fitzroy.]
We are indebted to M. Buys Ballot, a Dutch meteorologist, for an
invaluable generalization, the importance of which it is almost
impossible to over-estimate. This distinguished _savant_ says:—“It is
a fact above all doubt that the wind that comes is nearly at right
angles to the line between the places of highest and lowest barometer
readings. The wind has the place of lowest barometer at its left hand,
and is stronger in proportion as the difference of barometer readings
is greater.” These facts have been variously stated by other writers;
for example: “Stand with your back to the wind, and the barometer will
be lower on your left hand than on your right;” “Facing the wind the
centre of depression bears in the right-hand direction,” statements
which can be verified at any time by a brief study of the “Weather
Charts” now published in the daily journals. The value of the law
consists in its connecting the surface winds of our planet with the
actual pressure of the air itself, and it admits of the following
tabulation:—
+-------------+-------------+-------------+-------------+
| The wind is | The wind is | The wind is | The wind is |
| NORTHERLY | SOUTHERLY | EASTERLY | WESTERLY |
| when the | when the | when the | when the |
| BAROMETER | BAROMETER | BAROMETER | BAROMETER |
| is, in the | is, in the | is, in the | is, in the |
| N. ⎫ | N. ⎫ | N. High. | N. Low. |
| & ⎬ about | & ⎬ about | S. Low. | S. High. |
| S. ⎭ equal. | S. ⎭ equal. | E. ⎫ | E. ⎫ |
| E. Low. | E. High. | & ⎬ about | & ⎬ about |
| W. High. | W. Low. | W. ⎭ equal. | W. ⎭ equal. |
+-------------+-------------+-------------+-------------+
which can be verified by the reader from the daily Weather Charts in
the newspapers.
The above are deductions from Buys Ballot’s Law, still further
impressed on the memory by taking four outline maps of the British
Isles, inserting the names of Thurso, Penzance, Yarmouth, and Valentia,
with barometer readings of the kind above named at each place, and then
drawing a large arrow in red ink across the centre of each map in the
direction appropriate to the readings.
Mr. Strachan, in his able pamphlet on “Weather Forecasts,” puts the
matter thus: “It follows from Ballot’s Law that in the northern
temperate zone the winds will circulate around an area of low
atmospherical pressure in the _reverse direction_ to the movement of
the hands of a watch, and that the air will flow away from a region of
high pressure, and cause an apparent circulation of the winds around
it, _in the direction_ of watch hands.” And as the result of a careful
digest of data contained in the eleventh number of meteorological
papers, published by the Board of Trade, he has established the
following valuable propositions. As introductory to the propositions,
it should be stated that the positions of observations were the
following:—
Places. Latitude. Longitude.
Nairn 57° 29´ N. 4° 13´ W.
Brest 48 „ 28 4 „ 29 W.
Valentia 51 „ 56 10 „ 19 W.
Yarmouth 52 „ 37 1 „ 44 E.
Portrush (or Greencastle) 55 „ 12 6 „ 40 W.
Shields 55 „ 0 1 „ 27 W.
Nairn and Brest are situated nearly on the same meridian, about 540
geographical miles apart. Valentia and Yarmouth are nearly on the same
parallel of latitude, about 450 miles apart. Portrush and Shields,
distant 180 miles, are on a parallel which is nearly as remote from the
parallel of Nairn as that of Valentia and Yarmouth is from the one
passing through Brest; and Shields is about as much to the westward of
Yarmouth as Portrush is to the eastward of Valentia. When observations
have not been obtainable for Brest, those made at Penzance have been
used instead.
_Proposition 1._—Whenever the atmospherical pressure is greater at
Brest than at Nairn, while it is of the same or nearly the same value
at Valentia and Yarmouth, being gradually less from south to north, the
winds over the British Isles are _westerly_.
_Proposition 2._—Whenever the pressure at Nairn is greater than at
Brest, while its values at Valentia and Yarmouth are equal, or nearly
so, the winds over the British Isles are _easterly_.
_Proposition 3._—Whenever the pressure at Valentia is greater than at
Yarmouth, while its values at Brest and Nairn are nearly equal, the
winds over the British Isles are _northerly_.
_Proposition 4._—Whenever the pressure at Yarmouth exceeds that at
Valentia, while there is equality of pressure at Nairn and Brest, the
winds of the British Isles are _southerly_.
_Proposition 5._—Whenever the pressure of the atmosphere is equal, or
nearly so, at Brest, Valentia, Nairn, and Yarmouth, and generally
uniform, the winds over the British Isles are variable in direction and
light in force.
The data from which the foregoing propositions were deduced, and indeed
all other cases calculated by Mr. Strachan, show in every well marked
instance that when the atmospherical pressure was
(1) greater in the south than in the north, the wind had westing;
(2) greater in the north than in the south, the wind had easting;
(3) greater in the east than in the west, the wind had southing;
(4) greater in the west than in the east, the wind had northing;
(5) uniformly high, or uniformly low, variable light winds (with
fine weather in the former case, and vapoury or wet weather in
the latter).
Conditions (1) and (3) give winds from the S.W. quarter.
Conditions (1) and (4) give winds from the N.W. quarter.
Conditions (2) and (4) give winds from the N.E. quarter.
Conditions (2) and (3) give winds from the S.E. quarter.
These principles may be employed to set forth the mode of foretelling
the impending change of wind as regards its direction and force; for
the atmospherical pressure may change—
(_a_) uniformly over the whole area of observation;
(_b_) by increasing in the south, or (which causes a similar
statical force) by decreasing in the north;
(_c_) by increasing in the north, or (which has the same effect) by
decreasing in the south;
(_d_) by increasing in the west, or (which has the same effect) by
decreasing in the east;
(_e_) by increasing in the east, or (which has the same effect) by
decreasing in the west;
------------------------------------------------------------------------
Scale, 0 to 6.
|
|Pressure in pounds
| per square foot.
| |
| |Miles per hour.
| | |
| | |Seaman’s Nomenclature.
| | | |
| | | |Scale, 0 to 12.
| | | | |
| | | | | Beaufort Scale.
---+-----+---+---------+----+-------------------------------------------
0·0| 0·00| 2 |Calm | 0 |
| | | | |
0·5| 0·25| 5 |Light Air| 1 | Just sufficient to make steerage way.
| | | | |
| | | Breeze | |
1·0| 1·00|10 | Light | 2 | ⎧With which a ship with ⎫ 1 to 2 knots.
1·5| 2·25|15 | Gentle | 3 | ⎨ all sail set would go ⎬ 3 to 4 „
2·0| 4·00|20 | Moderate| 4 | ⎩ in smooth water. ⎭ 5 to 6 „
2·5| 6·25|27 | Fresh | 5 | ⎧ ⎫ Royals, &c.
3·0| 9·00|35 | Strong | 6 | ⎪In which ⎮ Single Reefs and T.G. Sails.
-- | -- |42 | -- -- | | ⎪ ⎮ Double Reefs and Jib, &c.
| | | | | ⎪ ⎮
| | | Gale | | ⎨ she could ⎬
3·5| -- |50 |Moderate | 7 | ⎪ ⎮
4·0|16·00|60 |Fresh | 8 | ⎪ ⎮ Triple Reefs, &c.
4·5|20·25|-- |Strong | 9 | ⎩ just carry⎭ Close Reefs and Courses.
5·0|25·00|70 |Whole | 10 | ⎧In which she could just bear close-reefed
| | | | | ⎩ Maintopsail and reefed Foresail.
5·5|30·25|80 |Storm | 11 | Under Storm Staysails or Trysails.
6·0|36·00|90 |Hurricane| 12 | Bare Poles.
---+-----+---+---------+----+-------------------------------------------
With (_a_) similar wind and weather will continue.
„ (_b_) winds will veer towards west.
„ (_c_) „ „ east.
„ (_d_) „ „ north.
„ (_e_) „ „ south.
“The probable strength of wind will be in proportion to the rate of
increase of statical force, or differences of barometrical readings.
The position of least pressure must be carefully considered; as, in
accordance with the law, the wind will blow around that locality. The
same remark applies to areas of high pressure, which, however, very
rarely occur in a well-defined manner over the British Isles.”
Referring to the table on page 76, the scale 0 to 6 was formerly used
by meteorological observers at land stations, and it was intended to
express, when the square of the grade was obtained, the pressure of the
wind as given in the second column.
“The velocity is an approximation as near as can be obtained, from the
values assigned by Neumayer, Stow, Laughton, Scott, Harris, James,
&c.”[15]
[Footnote 15: Strachan’s “Portable Meteorological Register,”
4th edition.]
Few meteorological axioms are better established than that which
embodies the fact that “every wind brings its weather,” and the primary
cause of wind being the motion of the air induced by rarefaction, it is
obvious that there is a constant tendency for the equatorial and polar
currents in any locality to establish an equilibrium, and this
consideration is found to facilitate weather predictions for extended
periods. Thus, in consequence of the unusual prevalence of _east_ winds
in the spring of 1862, a wet summer was predicted. The prediction was
fully borne out by an incessant continuance of _south-west winds_, with
clouded skies and the usual accompaniment of deluges of rain. These
winds continuing, with slight intermissions only, till the spring of
the following year, less than the usual number of south-west winds was
looked for during the summer; the result fully justified the
anticipation, the summer of 1863 being fine and warm, especially during
the earlier portion. Similarly, without committing the inaccuracies of
Murphy in 1838, the summer of 1877 may be reasonably expected to be a
dry and cool one from the long continuance of warm and wet months in
the winter of 1876-7.
The scientific research and mechanical ingenuity directed of late years
to producing trustworthy estimates of the direction, pressure, and
velocity of the wind, have resulted in the production of a series of
instruments, possessing great precision and accuracy.
[Illustration: 54.
Wind Vane. Scale
about 1/20.]
The _direction_ of the wind is indicated by vanes, a very efficient
form of which is shown at Fig. 54, the _velocity_ by revolving cups,
and the _pressure_ by the pressure plate and by calculation from the
known velocity.
The Pendulum Anemometer (Fig. 56) shows in a simple manner the
direction and pressure of the wind. The peculiarly shaped vane ensures
the surface of the swinging pressure plate B being always kept towards
the wind. The pendulum plate hangs, during a calm, quite vertically,
indicating zero, and as the pressure increases it will be raised
through all degrees of elevation from 1 to 12. The vane is perforated
with holes large enough to be visible at some distance from the ground,
the 5 and 10 being specially larger, so that the angle to which the
pressure plate is raised can be quickly noted.
[Illustration: 55.
Compass Bearings.
Scale about 1/20.]
There is a simple contrivance (for the convenience of travellers)
called a Portable Wind Vane, or Anemometer, It is furnished with a
compass and bar needle, &c., and will tell the true direction of the
wind to within a half point.
[Illustration: 56.
Prestel’s Pendulum Anemometer.
Scale about 1/12.]
[Illustration: 57.
Lind’s Anemometer. Scale about 1/5.]
Lind’s Anemometer or Wind Gauge ranks among the earliest forms of
instruments designed to estimate the force of the wind. It consists of
a glass syphon, the limbs of which are parallel to each other, mounted
on a vertical rod, on which it freely oscillates by the action of the
vane which surmounts it. The upper end of one limb of the syphon is
bent outward at right angles to the main direction, and the action of
the vane keeps this open end of the tube always towards the quarter
from whence the wind blows. Between the limbs of the syphon is placed a
scale graduated from 0 to 3 in inches and 10ths, the zero being in the
centre of the scale. When the instrument is used, it is only necessary
to fill the tube with water to the zero of the scale, and then expose
it to the wind. The natural consequence of wind acting on the surface
of the water is to depress it in one limb and raise it in the other,
and the sum of the depression and elevation is the height of a column
of water which the wind is capable of sustaining at the time of
observation. Sudden gusts of wind are apt to produce a jumping effect
on the water in the tube, and to diminish this the bend of the syphon
is contracted. A brass plate is attached to the foot of the instrument,
bearing the letters indicating the cardinal points of the compass, to
show the direction of the wind.
Dr. Robinson, of Armagh, introduced an instrument, in 1850, which
consists of four hemispherical copper cups attached to the arms of a
metal cross. The vertical axis upon which these are secured has at its
lower extremity an endless screw placed in gear with a train of wheels
and pinions. Each wheel is graduated respectively to 1/10th, 1 mile, 10
miles, 100 miles, 1,000 miles, and these revolve behind a fixed index,
the readings of which are taken according to the indications on the
dials.
Dr. Robinson entertained the theory that the cups (measuring from their
centres) revolved with one-third of the wind’s velocity; and this
theory having been fully supported by experiment, due allowance has
been made in graduating the wheels so that the true velocity is
obtained by direct observation.
In an improved form of this anemometer the hemispherical cups are
retained, but the index portion of the instrument consists of two
graduated concentric circles, the inner one representing five miles
divided into 10ths, and the outer one bearing 100 divisions, each of
which is equivalent to five miles. At the top of the dial is a fixed
index, which, as the toothed wheel revolves, marks on the inner circle
the miles (up to five) and 10ths of miles the wind has travelled, while
a movable index, which revolves with the wheel, indicates on the outer
circle the passage of every five miles.
[Illustration: 58.
Improved Anemometer. Scale about 1/5.]
This instrument can be made very portable by removing the arms bearing
the cups, when the whole may be packed with iron shaft in a case 15 ×
13 × 4 inches. It may be placed in any desired position by screwing the
iron shaft supplied with it into the hole provided for the purpose, and
fixing the apparatus on a pole or on an elevated stand, if possible, in
an open space exposed to the _direct_ action of the wind.
If, when placing the instrument, the hands stand at 0, the next reading
will, of course, show the number of miles the wind has traversed; but,
should they stand otherwise, the reading may be noted and deducted from
the second reading, thus: Suppose the fixed index points to 2·5 and the
movable index to 125, the reading after 12 hours may be 200 on the
outer circle and 3·0 on the inner circle: these added together yield
203. By deducting the previous reading 127·5, we have the true
reading—viz., 75·5 miles as the distance travelled by the wind.
Having obtained the velocity of the wind in this manner in miles per
hour, the table on page 83, from Col. Sir Henry James’s “Instructions
for Taking Meteorological Observations,” will enable the observer to
calculate the pressure in pounds per square foot.
WEATHER NOTATION.
The following letters are used to denote the state of the weather:—
_b_ denotes blue sky, whether with clear or slightly hazy atmosphere.
_c_ „ cloudy, that is detached opening clouds.
_d_ „ drizzling rain.
_f_ „ fog.
_h_ „ hail.
_l_ „ lightning.
_m_ „ misty, or hazy so as to interrupt the view.
_o_ „ overcast, gloomy, dull.
_p_ „ passing showers.
_q_ „ squally.
_r_ „ rain.
_s_ „ snow.
_t_ „ thunder.
_u_ „ ugly, threatening appearance of sky.
_v_ „ unusual visibility of distant objects.
_w_ „ wet, that is dew.
A letter repeated denotes much, as _rr_, heavy rain; _ff_, dense fog;
and a figure attached denotes duration in hours, as 14_r_, 14 hours’
rain.
By the combination of these letters all the ordinary phenomena of the
weather may be recorded with certainty and brevity.
_Examples._—_bc_, blue sky with less proportion of cloud; _cb_, more
cloudy than clear; 2_rrllt_, heavy rain for two hours, with much
lightning, and some thunder.
VELOCITY AND PRESSURE OF THE WIND.
The Pressure varies as the Square of the Velocity, or _P_ ∝
_V_^2. The Square of the Velocity in Miles per Hour multiplied
by ·500 gives the Pressure in lbs. per square Foot, or _V_^2 ×
·005 = _P_. The Square Root of 200 times the Pressure equals
the Velocity, or √(200 × _P_) = _V_.
The subjoined Table is calculated from this data, by COL. SIR HENRY
JAMES, of the Ordnance Survey Office.
+-------------------------------------------------------------------+
|Pressure in |
|lbs. per |
|Square Foot. |
| |Velocity in |
| |Miles |
| |per Hour. |
| | |Pressure in |
| | |lbs. per |
| | |Square Foot. |
| | | |Velocity in |
| | | |Miles |
| | | |per Hour. |
| | | | |Pressure in |
| | | | |lbs. per |
| | | | |Square Foot. |
| | | | | |Velocity in |
| | | | | |Miles |
| | | | | |per Hour. |
| | | | | | |Pressure in |
| | | | | | |lbs. per |
| | | | | | |Square Foot. |
| | | | | | | |Velocity in |
| | | | | | | |Miles |
| | | | | | | |per Hour. |
| | | | | | | | |Pressure in |
| | | | | | | | |lbs. per |
| | | | | | | | |Square Foot. |
| | | | | | | | | |Velocity |
| | | | | | | | | |in Miles |
| | | | | | | | | |per Hour.|
+-----+------+-----+------+-----+------+-----+------+-----+---------+
| oz. | | lbs.| | lbs.| | lbs.| | lbs.| |
| 0·08| 1·000| 6·75|36·742|17·75|59·581|28·75|75·828|39·75| 89·162 |
| 0·25| 1·767| 7·00|37·416|18·00|60·000|29·00|76·157|40·00| 89·442 |
| 0·50| 2·500| 7·25|38·078|18·25|60·415|29·25|76·485|40·25| 89·721 |
| 0·75| 3·061| 7·50|38·729|18·50|60·827|29·50|76·811|40·50| 90·000 |
| 1·00| 3·535| 7·75|39·370|18·75|61·237|29·75|77·136|40·75| 90·277 |
| 2·00| 5·000| 8·00|40·000|19·00|61·644|30·00|77·459|41·00| 90·553 |
| 3·00| 6·123| 8·25|40·620|19·25|62·048|30·25|77·781|41·25| 90·829 |
| 4·00| 7·071| 8·50|41·231|19·50|62·449|30·50|78·102|41·50| 91·104 |
| 5·00| 7·905| 8·75|41·833|19·75|62·819|30·75|78·421|41·75| 91·378 |
| 6·00| 8·660| 9·00|42·426|20·00|63·245|31·00|78·740|42·00| 91·651 |
| 7·00| 9·354| 9·25|43·011|20·25|63·639|31·25|79·056|42·25| 91·923 |
| 8·00|10·000| 9·50|43·588|20·50|64·031|31·50|79·372|42·50| 92·195 |
| 9·00|10·606| 9·75|44·158|20·75|64·420|31·75|79·686|42·75| 92·466 |
|10·00|11·180|10·00|44·721|21·00|64·807|32·00|80·000|43·00| 92·736 |
|11·00|11·726|10·25|45·276|21·25|65·192|32·25|80·311|43·25| 93·005 |
|12·00|12·247|10·50|45·825|21·50|65·574|32·50|80·622|43·50| 93·273 |
|13·00|12·747|10·75|46·368|21·75|65·954|32·75|80·932|43·75| 93·541 |
|14·00|13·228|11·00|46·904|22·00|66·332|33·00|81·240|44·00| 93·808 |
|15·00|13·693|11·25|47·434|22·25|66·708|33·25|81·547|44·25| 94·074 |
| | |11·50|47·958|22·50|67·082|33·50|81·853|44·50| 94·339 |
| lbs.| |11·75|48·476|22·75|67·453|33·75|82·158|44·75| 94·604 |
| 1·00|14·142|12·00|48·989|23·00|67·823|34·00|82·462|45·00| 94·868 |
| 1·25|15·811|12·25|49·497|23·25|68·190|34·25|82·764|45·26| 95·393 |
| 1·50|17·320|12·50|50·000|23·50|68·556|34·50|83·066|45·50| 95·131 |
| 1·75|18·708|12·75|50·497|23·75|68·920|34·75|83·366|45·75| 95·655 |
| 2·00|20·000|13·00|50·990|24·00|69·282|35·00|83·666|46·00| 95·916 |
| 2·25|21·213|13·25|51·478|24·25|69·641|35·25|83·964|46·25| 96·176 |
| 2·50|22·360|13·50|51·961|24·50|70·000|35·50|84·261|46·50| 96·436 |
| 2·75|23·452|13·75|52·440|24·75|70·356|35·75|84·567|46·75| 96·695 |
| 3·00|24·494|14·00|52·915|25·00|70·710|36·00|84·852|47·00| 96·953 |
| 3·25|25·495|14·25|53·385|25·25|71·063|36·25|85 146|47·25| 97·211 |
| 3·50|26·457|14·50|53·851|25·50|71·414|36·50|85·440|47·50| 97·467 |
| 3·75|27·386|14·75|54·313|25·75|71·763|36·75|85·732|47·75| 97·724 |
| 4·00|28·284|15·00|54·772|26·00|72·111|37·00|86·023|48·00| 97·979 |
| 4·25|29·154|15·25|55·226|26·25|72·456|37·25|86·313|48·25| 98·234 |
| 4·50|30·000|15·50|55·677|26·50|72·801|37·50|86·602|48·50| 98·488 |
| 4·75|30·822|15·75|56·124|26·75|73 143|37·75|86·890|48·75| 98·742 |
| 5·00|31·622|16·00|56·568|27·00|73·484|38·00|87·177|49·00| 98·994 |
| 5·25|32·403|16·25|57·008|27·25|73·824|38·25|87·464|49·25| 99·247 |
| 5·50|33·166|16·50|57·415|27·50|74·161|38·50|87·749|49·50| 99·498 |
| 5·75|33·911|16·75|57·879|27·75|74·498|38·75|88·034|49·75| 99·749 |
| 6·00|34·641|17·00|58·309|28·00|74·833|39·00|88·317|50·00| 100·000 |
| 6·25|35·355|17·25|58·736|28·25|75·166|39·25|88·600| | |
| 6·50|36·055|17·50|59·160|28·50|75·498|39·50|88·881| | |
+-----+------+-----+------+-----+------+-----+------+-----+---------+
This is the only table hitherto much in use for converting velocity
into pressure, and was prepared by Smeaton and others. It does not,
however, express the true relation, which has yet to be determined.
The Anemograph, or Self-Recording Wind Gauge, has for its object the
registration of the velocity and direction of the wind from day to day.
Figs. 59 and 60 show the form designed and arranged by Mr. Beckley, of
the Kew Observatory, which has been adopted by the Meteorological
Office.
[Illustration: 59.
Anemograph. Scale about 1/20.
Portion for exterior of observatory.]
It consists of a set of hemispherical cups and vanes, which are exposed
on the roof of the house, and of the recording apparatus, which is
placed inside the house.
The motion imparted to the hemispherical cups by the wind is
communicated to the steel shaft B, which, passing through the hollow
shaft C, and having at its lower end an endless screw, works into a
series of wheels in the iron box D, which reduces the angular velocity
7,000 times. At the required distance the motion, having emerged at E,
is connected with F, where, by means of bevelled wheels, it moves the
spiral brass registering pencil C, which is arranged so that each
revolution records 50 miles of velocity on the prepared paper H.
The direction of the wind is indicated by the arrow L, which is kept in
position by the fans M. These communicate, by an endless screw and
train of wheels, through the shaft C and the box D to the recording
apparatus, consisting of a spiral brass pencil, which in one revolution
records variations through the cardinal points of the compass, on the
same prepared paper as that which receives the record of velocity.
[Illustration: 60.
Anemograph. Scale about 1/20.
Portion for interior of observatory.]
The paper is held on the drum by two small clips, and may be readily
changed, by unclamping the cross V, without disturbing the drum or any
other part of the instrument.
[Illustration: 61.
Self-recording Magnetometer, Kew Observatory.]
VI.—ELECTRIFICATION.
William Gilbert, a physician of Colchester, first showed in 1600 that
the earth as a whole has the properties of a magnet, and consequently
that the directive action exerted by it upon a compass needle
represents only a special case of the mutual action of two magnets. In
1845, Faraday established the fact that susceptibility to magnetic
force is not, as was generally believed, confined to iron, nickel, and
a few other substances, but is a property of all substances. According
to Balfour Stewart, auroræ and earth currents may be regarded as
secondary currents resulting from changes in the earth’s magnetism.
Magnetic phenomena are included under the general term terrestrial
magnetic elements, and consist of magnetic declination, inclination,
and intensity.
These are for convenience determined separately; the first by an
instrument called a _Declinometer_, and the second by an _Inclinometer_
or _Dipping Needle_. The Declinometer is also made to serve the
additional purpose of measuring the _intensity_ of the earth’s magnetic
force, which it effects on a principle similar to that by which the
force of gravity is determined by the oscillations of a pendulum of
known length on any given portion of the earth’s surface. The
declinometer needle is made to oscillate, and the number of
oscillations in a given time counted; due allowance being made for the
strength of the needle, it is obvious that the force which restores the
needle to rest can be estimated. To ascertain the angle of
_declination_, the zero line of the compass card is made to coincide
with the geographical north and south line; and the angle which the
direction of the needle makes with this line is then read off on a
graduated circle over which the needle turns. The magnetic
_inclination_ or _dip of the needle_ is estimated by observing the
inclination to a horizontal plane of a needle turning on the vertical
plane which passes through the magnetic north and south points.
[Illustration: 62.]
Fig. 62 shows a simple form of magnetic needle suspended on a fine
steel point, which is supported by a brass stand; the addition of a
graduated circle would constitute such an arrangement a Declinometer.
[Illustration: 63.]
[Illustration: 64.]
Fig. 63 gives the appearance of the dipping needle, or Inclinometer,
and Fig. 64 an arrangement by which both kinds of terrestrial as well
as local attraction may be shown.
These components of the earth’s magnetism undergo not only an annual
but a daily and even hourly variation, apparently connected in some
occult manner with the frequency of the sun’s spots. The needle
sometimes suffers such exceptional perturbations as to suggest the idea
of a magnetic storm. These disturbances are usually accompanied (in
polar regions) by luminous phenomena called auroræ. Continuous
automatic records of them, therefore, is of great value, as
facilitating inductive research which may lead to valuable practical
results.
Accordingly the Royal Society have adopted for the Kew and other
observatories the form of Magnetograph, or Self-recording Magnetometer,
shown at Fig. 61, by means of which the variations just referred to are
registered by the oscillations of three magnets on photographically
prepared paper, stretched on a drum revolved by clockwork.
One magnet is suspended in the magnetic meridian by a silk thread, and,
by the aid of a mirror attached, it describes on the cylinder, moved by
clockwork in the centre pier, all the variations in the magnetic
_declination_.
The other two components of the magnetic force of the earth are given
by the other magnets. That recording the vertical variations rests on
two agate edges under a glass shade, while the horizontal component
magnet is suspended by a double silk thread, under the shade to the
right of the picture, being retained by the tension of the thread in a
position nearly at right angles to the magnetic meridian.
The clock box in the centre covers the three revolving cylinders
bearing the sensitive photographic paper, and to each magnet is
attached a semicircular mirror, which reflects the rays from a gas jet
to one of the cylinders, and thus describes by a curved line the
oscillations of the magnet. A second semicircular mirror is _fixed_ to
the pier on which the instrument stands, and consequently describes a
straight line, or zero, from whence the curves are measured.
To avoid errors attending sudden changes of temperature, underground
vaults are always chosen for magnetic observations, and also on account
of light being more easily and perfectly excluded.
ATMOSPHERIC ELECTRICITY.
Since the performance of Franklin’s famous kite experiment, by which he
determined the identity of lightning with the electrical discharge from
a machine, much attention has been devoted, not only to that form of
atmospheric electricity which displays itself in the thunder-cloud, but
to the electric condition of the air in all states of the weather.
These researches have established the fact that the air is always in an
electrical condition, even when the sky is clear and free from
thunder-clouds. The instruments employed for ascertaining the kind and
intensity of atmospheric electricity are called Electroscopes. Fig. 65
shows a modification of Saussure’s Electroscope, the basis of which is
a narrow-mouthed flint glass bottle with a divided scale to indicate
the degree of divergence of the gold leaves or straws. To protect the
lower part from rain, it is covered by a metallic shield about five
inches in diameter. Bohnenberger’s Electroscope indicates the presence
and quality of _feeble_ electric currents. Peltier’s Electrometer
yields the same result by the deflection of a magnetic needle. This
latter has been in use at Brussels for thirty years, and at Utrecht for
twenty years, and is highly recommended.
[Illustration: 65.
Electroscope.
Scale about 1/7.]
Singer’s Atmospheric Electroscope is an efficient form of the
instrument in which an ordinary gold-leaf electrometer has attached to
its circular brass plate a brass rod two feet in length, with a clip at
its upper extremity to receive a lighted paper or cigar fusee. The
electricity of the air in immediate contact with the flame, causes, by
induction, electricity of the opposite nature to accumulate at the
upper extremity, where it is constantly carried off by the convection
currents in the flame, leaving the conductor charged with the same kind
and power of electricity as that contained in the air at the time of
the experiment. The principle of this method was initiated by Volta,
and has been extended and applied by Sir William Thomson in his
Water-dropping Collector, which consists of an insulated cistern from
which water escapes through a jet so fine that it breaks into drops
immediately after leaving the nozzle of the tube. The result of this is
that in half a minute from the starting of the stream the can is found
to be electrified to the same extent as the air at the point of the
tube. The scale value of each instrument has to be separately
determined by repeated comparative experiments, and involves much
delicacy of manipulation.
It is chiefly important for the ordinary observer to know that the
occurrence of thunder and lightning should be always noted in the
column headed “Remarks.”
[Illustration: 66.
Lightning
Conductor.
Scale about 1/10.]
The destructive effects of lightning are too well known to need
description here; the means, however, by which these may be averted
demand a brief notice. Lightning when discharged from a cloud will
always choose the better of any two conductors which may present
themselves. The _stone_ of a church steeple and the _wood_ of a ship’s
mast are bad conductors, but a galvanized iron wire rope is the best
possible conductor, and accordingly this material is now generally
employed for the purpose. A lightning conductor consists of three
parts: 1, the rod, which extends beyond the summit of the building, 2,
the conductor, which connects the rod with the underground portion, and
3, the part underground. The connection between each of these must be
absolutely perfect, or the conductor will be faulty. The top is usually
of solid copper tipped with platinum (Fig. 66), the body of galvanized
iron rope, so as to adapt itself to the inequalities of the building
and yet have no sharp turns in it, while the part underground is of
solid iron rod. This latter portion should extend straight underground
for two feet, and being bent at right angles away from the wall, should
rest in a horizontal drain 10 to 15 feet long filled with charcoal, and
be again bent downwards into a well of water. Should water not be
available, it should rest in the centre of a hole 15 feet deep and 10
inches in diameter, tightly packed with charcoal, which, while
conducting the electricity from the rod into the earth, serves also to
preserve the iron from rusting.
OZONE.
The atmosphere, besides holding the vapour of water diffused throughout
its mass, contains also minute traces of carbonic acid and ammonia, and
a very remarkable substance called Ozone. Oxygen, one of the component
gases of the atmosphere, is capable of existing in two conditions; one
in which it is comparatively passive, and another in which it possesses
exceptional chemical activity, dependent apparently upon its electrical
condition, and in which state it possesses a peculiar smell which has
caused it to be named ozone.[16] The characteristic odour is always
observable near a powerful electric machine when it is being worked,
near a battery used for the decomposition of water, and in the air
after the passage of a flash of lightning. Its presence is most marked
near the sea-coast, and in localities remarkable for their salubrity;
and on account of its influence on health, it has been proposed by
Schonbein and others to include ozonometrical observations with the
ordinary meteorological observations.
[Footnote 16: Greek _ozo_, I smell.]
Although in minute quantities it is favourable to health, when existing
in undue proportion it irritates the mucous membrane of the nose and
throat, producing painful sores. It attacks india-rubber, bleaches
indigo, and oxidizes silver and mercury, differing in all these points
from ordinary atmospheric oxygen.
The chemical energy it possesses (which exceeds that of ordinary oxygen
as much as the latter exceeds atmospheric air as an oxidizing agent)
affords the means of ascertaining its presence and quantity. It
liberates iodine from its combination with potassium, and free iodine
colours starch a deep blue.
Schonbein, the discoverer of ozone, found that when strips of paper
previously saturated with starch and iodide of potassium and dried were
exposed freely to the air but protected from rain and the direct action
of the sun, they underwent a peculiar discoloration (when immersed in
water) after an exposure of 24 hours. A scale of tints numbered from
one to ten afforded the means of comparative observation, and thus the
Ozonometer was constructed, and a means established of registering the
amount of ozone in the air of various localities from day to day.
Schonbein also observed that the proportion of ozone was largely
augmented after heavy falls of snow. For the exposure of the ozone
papers, an ozone cage is employed, as shown at Fig. 67.
[Illustration: 67.
Ozone Cage.
Scale about 1/6.]
Ozone may be prepared artificially as a disinfectant by cautiously
mixing without friction or concussion equal parts of peroxide of
manganese, permanganate of potash, and oxalic acid. For a room
containing 1,000 cubic feet, two teaspoonfuls of the powder, placed in
a dish and moistened with water occasionally, will develop the ozone
and disinfect the surrounding air without producing cough.
The most important and interesting series of facts, however, connected
with ozone are those established by the researches of M. Houzeau, who
states:—
1. That country air contains an odorous oxidizing substance,
with the power of bleaching blue litmus, without previously
reddening it, of destroying bad smells, and of bluing iodized
red litmus.
2. That this substance is ozone.
3. That the amount of ozone in the air at different times and
places is variable, but this is at most 1/700,000 of its
volume, or 1 volume of ozone in 700,000 of air.
4. That ozone is found much more frequently in the country than
in towns.
5. That ozone is in greatest quantity in spring, less in
summer, diminishes in autumn, and is least in winter.
6. It is most frequently detected on rainy days, and during
great atmospheric disturbances.
7. That atmospheric electricity is apparently the great
generator of ozone.
The subject is one of great interest in its bearings on health, and
opens a wide field of scientific research, as may be inferred from the
opinion expressed by the Vienna Congress, which is that “the existing
methods of determining the amount of ozone in the atmosphere are
insufficient, and the Congress therefore recommends investigations for
the discovery of better methods.”
Mr. Lowe has published the valuable weather warnings tabulated on page
94, which are interesting as showing from a given number of
observations the value of each phenomenon:—
+------------------------------------------+--------------+-------------+
| | No. of | Followed in |
| | observations.| 24 hours by |
| +--------------+------+------+
| DEW. | | Fine.| Rain.|
| Dew profuse | 241 | 196 | 43 |
| Dew from 1st April to 30th Sept. | 185 | 161 | 24 |
| Dew from 1st Oct. to 30th March | 56 | 37 | 19 |
| CLOUDS. | | | |
| White stratus in the valley | 229 | 201 | 28 |
| Coloured clouds at sunset | 35 | 26 | 9 |
| SUN. | | | |
| Solar halos | 204 | 133 | 71 |
| Sun red and shorn of rays | 34 | 31 | 3 |
| Mock suns | 35 | 19 | 6 |
| Sun shone through thin cirro-stratus | 13 | 6 | 7 |
| Sun pale and sparkling | 51 | 27 | 24 |
| FROST. | | | |
| White frost | 73 | 59 | 14 |
| MOON. | | | |
| Lunar halos | 102 | 51 | 51 |
| Mock moons | 9 | 7 | 2 |
| Lunar burr | 64 | 47 | 17 |
| Moon shining dimly | 18 | 12 | 6 |
| Moon rose of a red colour | 8 | 7 | 1 |
| STARS. | | | |
| Falling stars abundant | 85 | 65 | 20 |
| Stars bright | 83 | 64 | 19 |
| Stars dim | 54 | 32 | 22 |
| Stars scintillated | 14 | 12 | 2 |
| AURORA. | | | |
| Aurora borealis | 76 | 49 | 27 |
| ANIMALS. | | | |
| Bats flying about in the evening | 61 | 45 | 16 |
| Toads in the evening | 17 | 12 | 5 |
| Landrails clamorous | 14 | 13 | 1 |
| Ducks and geese noisy | 10 | 7 | 3 |
| Spiders hanging on webs in the evening | 8 | 5 | 3 |
| Fish rise in the lake | 15 | 9 | 6 |
| SMOKE. | | | |
| Smoke rising perpendicularly | 6 | 5 | 1 |
+------------------------------------------+--------------+------+------+
Among the animals whose movements give weather warnings few are more
trustworthy than the leech. The reader may verify this by placing one
in a broad glass bottle, tied over with perforated leather, or bladder.
If placed in a northern aspect, the leech will be found to behave in
the following manner:—
1. On the approach of fine or frosty weather, according to the season,
it will be found curled up at the bottom. 2. On the approach of rain,
snow, or wind, it will rise excitedly to the surface. 3. Thunder will
cause it to be much agitated, and to leave the water entirely.
PERIODS.—M. Köppen states, as the result of his examination into the
chances of a change of weather, that _the weather has a decided
tendency to preserve its character_. Thus, at Brussels, if it has
rained for nine or ten days successively, the _next_ day will be wet
also in four cases out of five; and the chance of a change decreases
with the length of time for which the weather _from_ which the change
is to take place has lasted.
In the case of temperature for five-day periods, the same principle
holds good;[17] for if a cold five-day period sets in after warm
weather, we can bet two to one that the next such period will be cold
too; but if the cold has lasted for two months, we can bet nearly eight
to one that the first five days of the next month will be cold too. The
chance of change is, however, greater for the five-day periods than for
single days. Similar results follow for the months, but here again the
chance of change shows an increase.
[Footnote 17: “Recent Progress in Weather Knowledge,”
by R. H. Scott, F.R.S.]
“If we revert to the instance first cited, that of rain, the result is,
_not_ that if it once begins to rain the chances are in favour of its
never ceasing; all that is implied is, that the chances are against its
ceasing on a definite day, and that they increase with the length of
time the rain has lasted. The problem is similar to that of human life:
the chance of a baby one year old living another year is less than that
of a man of thirty.
“The practical meaning of all this is, that although we know that a
compensating anomaly for all extraordinary weather exists somewhere on
the earth’s surface, _e.g._, the very common case of intense cold in
America, while we have a mild winter in Britain, there is no reason as
yet ascertained to anticipate that this compensation will occur at any
given place during the year. In other words, when definite conditions
of weather have thoroughly established themselves, it is only with
great difficulty that the courses of the atmospheric currents are
changed.”
To bring within the limits of a popular pamphlet a notice of the
various phenomena classed under the head of Meteorology, it has been
necessary to exercise the utmost brevity. Brief, however, as the
treatment has been, reference has been made to the sciences of Heat,
Light, Electricity, Magnetism, Gravitation, Astronomy, Chemistry,
Geography, and Geology, thus corroborating the testimony of Sir John
Herschel, who states that “it can hardly be impressed forcibly enough
on the attention of the student of nature that there is scarcely any
natural phenomenon which can be fully and completely explained in all
its circumstances without a union of several—perhaps of all—the
sciences; and it cannot be doubted that whatever walk of science he may
determine to pursue, impossible as it is for a finite capacity to
explore all with any chance of success, he will find it illuminated in
proportion to the light which he is enabled to throw upon it from
surrounding regions. But, independently of this advantage, the glimpse
which may thus be obtained of the harmony of Creation, of the unity of
its plan, of the theory of the material universe, is one of the most
exalted objects of contemplation which can be presented to the
faculties of a rational being. In such a general survey he perceives
that science is a whole whose source is lost in infinity, and which
nothing but the imperfection of our nature obliges us to divide. He
feels his nothingness in his attempts to grasp it, and he bows with
humility and adoration before that Supreme Intelligence who alone can
comprehend it, and who ‘in the beginning saw everything that He had
made, and behold it was very good.’”
J. AND W. RIDER, PRINTERS, LONDON.
------------------------------------------------------------------------
Transcriber’s note:
Footnotes moved to end of paragraph.
All scale values in illustration captions retained. The value may
not be visually correct.
All fractions regularised to numerator/denominator.
Page 3, ‘Reaumur’s’ changed to ‘Réaumur’s,’ “Réaumur’s scale”
Page 9, closing single quote changed to double quote.
Page 12, full stop appended to illustration caption, “Scale about
1/20.”
Page 16, full stop appended to illustration caption, “9.”
Page 17, semicolon changed to full stop, “...it has never been
frozen.”
Page 23, first footnote changed from “8 R = 18 F.” to “8 R = 50 F.”
Page 27, ‘vice versâ’ changed to ‘vice versa,’ “...proportion, and
vice versa,...”
Page 30, dash changed to space, “29·500 inches.”
Page 34, ‘Hook’ changed to ‘Hooke,’ “...invented by Dr. Hooke.”
Page 37, ‘Aneriod’ changed to ‘Aneroid,’ “...by means of Barometer
or Aneroid,...”
Page 39, space changed to stop, “...between nine and ten p.m.”
Page 42, space inserted between ‘no’ and ‘less,’ “...at no less
than 212 miles.”
Page 46, illustration number added to caption, “38. Damp Detector.”
Page 51, comma moved to after ‘weather,’ “In cold calm weather,...”
Page 57, ‘!’ changed to ‘,’ “cloud reflector,”
Page 58, ‘2.’ changed to ‘2nd.,’ “...and 2nd. Velocity of Motion.,”
Page 74, degrees changed to minutes, “57° 29´ N.”
Page 79, ‘Guage’ changed to ‘Gauge,’ “Lind’s Anemometer or Wind
Gauge...”
Page 83, powers changed from subscripts to superscripts, “V^2.”
Page 87, reference in text to Fig. 65 changed to Fig. 64, “...and
Fig. 64 an arrangement...”
Page 94, full stops added after ‘STARS’ and ‘ANIMALS.’
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