Chapter I: Part 1

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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.”

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.

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.

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.

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.

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.]

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.

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.

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.

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.

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.

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.

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.

Six’s form of thermometer has been extensively used for ascertaining deep sea temperatures.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.]

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.

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. | 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.

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Weather Warnings for WatchersChapter I: Part 1

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