[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.