Weather Warnings for Watchers

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.

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.

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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]

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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-point of Water for pressure in next col.	Barometer at lower Station.	BAROMETER AT UPPER STATION.—INCHES.
	In.	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15
213·78	31	859	873	889	905	921	939	957	977	998	1020	1043	1068	1095	1124	1155	1188
212·13	30	....	888	904	920	937	955	974	994	1015	1038	1062	1087	1115	1144	1176	1210
210·43	29	....	....	919	936	953	971	991	1012	1033	1056	1081	1107	1135	1165	1198	1233
208·67	28	....	....	....	952	970	989	1009	1028	1051	1075	1100	1127	1156	1187	1220	1257
206·87	27	Factor A.				988	1007	1028	1050	1073	1097	1122	1150	1180	1211	1246	1283
205·01	26	....	....	....	....	....	1027	1048	1070	1093	1118	1145	1173	1203	1236	1271	1309
203·09	25	....	....	....	....	....	....	1069	1092	1116	1141	1169	1198	1229	1262	1299	1338
201·11	24	....	....	....	....	....	....	....	1115	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 in 1,000 feet.	Factor D. additive.	Latitude.	Factor C.	Mean Temperature.	Factor B.
2	5	0°	†2·7	10°	0·951
4	11	10	†2·5	20	0·973
6	17	20	†2·0	30	0·996
8	23	30	†1·4	40	1·018
10	30	40	†0·6	50	1·040
12	37	45	0·0	60	1·062
14	44	50	-0·5	70	1·084
16	52	60	-1·3	80	1·127
18	60	70	-2·0

Rule I.—If the temperature of boiling water be observed at either or both Stations, find the equivalent pressure in the 2nd column, and calculate the height as for barometer.

Rule II.—The readings of the Barometer being corrected and reduced to 32° F., multiply the difference of pressure between the Stations by factor A, found in line with pressure at lower Station, and under that at upper Station; multiply again by factor B, corresponding to the mean temperature of the air at the Station; apply as many times C as there are thousand feet in the height, corresponding to the latitude; and add D, the correction for gravity.

Example.—At the top of Snowdon, lat. 53° N., an aneroid read 26·48, correction -0·18, the pressure at sea-level was 29·91; the temperature of the intermediate air was 57°; find the height.

Lower Station	29·91 inches.
Upper      „	26·30
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	3·61
Factor A	933
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	1083
	1083
	3249
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	3368	(neglecting decimals.)
Factor B	1.055
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	16840	N.B.—In taking out the quantities, if accuracy is aimed at, it will be necessary to proportion for parts in the usual manner with such Tables.
	16840
	3368
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	3553
Cor. C = 3 × 1 = -3
Cor. D	+ 10
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Height	3560 feet.

The illustration (Fig. 33) shows the instrument with the telescopic tube drawn out for use, and the thermometer surrounded by the vapour of boiling water. The lamp is protected from wind by a perforated japanned tin case covered with wire gauze. When the boiler is charged and the lamp ignited the mercury ascends, and the point at which it becomes stationary shows the temperature, which will give the elevation in feet above the sea-level on reference to the table supplied by the optician from whom the instrument is purchased.

A highly-refined automatic arrangement is adopted at some observatories called a Barograph, which, by the aid of photography, becomes a self-recording mercurial barometer. It is simpler in its arrangement than the thermograph, and includes a clock of superior construction, causing a cylinder bearing photographic paper to make one complete revolution in forty-eight hours. A double combination of achromatic lenses brings to a focus rays passing through a slit placed in front of the mercurial column, behind which is a strong gaslight or paraffin lamp, the rays of which are condensed upon the slit by a combination of two plano-convex lenses.

Although a barometer is an instrument artificially constructed by man, it should not be forgotten that when once made the column of mercury is placed in a passive or quiescent state in direct relation with the great forces of nature, so that its indications become to some extent natural phenomena. This is aptly illustrated by what is called the “daily fluctuation” of the barometer which occurs in all countries, though the hours and extent vary with the latitude, diminishing as the latitude increases, according to a definite law. The phenomena does not admit of a satisfactory explanation, but is doubtless connected with the daily variations of temperature and of vapour in the air. The mercury falls naturally (so to speak) from nine or ten to between three and four p.m.; it then rises till between nine and ten p.m. It falls again about four a.m., and rises again about ten a.m. It is usually highest at nine a.m. and nine p.m., and lowest at three a.m. and three p.m.

These natural elevations and depressions of the mercury should be allowed for in reading the barometer, as any rise or fall in opposition to the natural rise and fall possesses for that reason increased importance. For instance, fine weather may be expected if the mercury rises between nine a.m. and three p.m.; in like manner rain may be expected should a fall take place between three p.m. and nine p.m.

It will be inferred from the preceding facts that there are certain hours better suited for “taking a reading” than others. When one observation only is made daily, noon is the best time, two observations should be made at nine a.m. and nine p.m., and for three the best hours are nine a.m. (maximum), noon (mean), and three p.m. (minimum).

The opinion generally entertained that a high barometer is an indication of fine weather, and a low one a warning of bad weather, is open to exception, and an increased value would attach to the indications of the instrument in proportion as the following points are noted and allowed for:—

1. The actual height of the mercury. 2. Whether it is rising or falling. 3. The rate of rise and fall. 4. Whether the rise or fall has been long continued.

The state of the barometer foretells coming weather, and when the present weather disagrees with the barometer a change will soon take place. A fall of half a tenth, or more, in an hour is a sure warning of a storm, a rapid rise is a warning of unsettled weather.

The barometer is generally lowest with wind from the S.W., and highest with wind N.E., or with a calm. N.E. and S.W. may be called the wind’s poles, and the difference of height due to direction only from one of these bearings to another amounts to about half an inch.

The Storm Glass (Fig. 36) is a glass bottle, ten inches long, containing a mixture of camphor, nitre, sal-ammoniac, alcohol, and water. As “temperature affects the mixture much,” an arrangement has recently been designed in which the stem of a thermometer is immersed in the fluid, as shown at Fig. 37, thus imparting a higher value to its indications. The late Admiral Fitzroy says—

“Since 1825, we have generally had some of these glasses, as curiosities rather than otherwise; for nothing certain could be made of their variations until lately, when it was fairly demonstrated that if fixed undisturbed in free air, not exposed to radiation, fire, or sun, but in the ordinary light of a well-ventilated room, or, preferably, in the outer air, the chemical mixture in a so-called storm glass varies in character with the direction of the wind—not its force.”

The quarter from which the wind or storm is blowing is indicated by the substance adhering more closely to the bottom of the glass opposite to the point whence the wind or tempest arises.

The Sympiesometer is an instrument used chiefly at sea for purposes of comparison with the mercurial and aneroid barometers. Its indications result partly from the pressure and partly from the temperature of the atmosphere; it would, therefore, be more correctly named a Thermo-Barometer.

The height of the atmosphere has been variously estimated:—By Bravais, from the duration of twilight, at 66 to nearly 100 miles; by Dalton, in 1819, from observations of the auroral light, at 102 miles; by Sir John Herschel, from similar observations in 1861, at 83 miles; from observations of meteors, from 100 to 200 miles; by Liais, in 1859, from observations on the polarisation of the sky, at no less than 212 miles.

The density of the atmosphere diminishes with distance from the earth’s surface, in accordance with the following rule:—“At a height of seven miles the density of the atmosphere is reduced to one-fourth the density at the sea-level, and for every additional seven miles, the rarity of the air is similarly quadrupled.”

The Kew Committee of the Royal Society receive, for verification and comparison with the standard instruments of the Kew Observatory, barometers, thermometers, and other instruments intended for meteorological observation or scientific investigations.

Any persons ordering instruments of opticians may direct them to be previously forwarded to the observatory for verification.

A scale of charges is issued by the Committee which is exclusive of packing and carriage, or of rail expenses, when a special messenger is sent out. The Meteorological Office, Victoria Street, London, also receives and forwards instruments for verification to the Kew Observatory.

The Committee wish it to be understood that they cannot undertake the verification of an inferior class of instruments (such as barometers mounted upon wooden frames, and thermometers not graduated on the stem), and that the superintendent of the observatory may at his discretion decline to receive such instruments as he may consider unfit for scientific observation.

	May be Expected
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Increasing storm	If mercury falls during a high wind from S.W., S.S.W., W. or S.
Violent but short	If the fall be rapid.
Less violent but of longer continuance	If the fall be slow.
A violent storm from the N.W. or N.	If the mercury falls suddenly while the wind is due W.
N.W., N., or N.E. winds, or less wind, or less rain, or less snow	If the mercury having been at its usual height, 29·95, is steady or rising, while the thermometer falls and the air becomes drier.
Wind and rain from S.E., S., and S.W.	If the mercury falls, while the thermometer rises and the air becomes damp.
A violent storm from N.W., N., or N.E.	When the mercury falls suddenly with a W. wind.
Snow	If the mercury falls when the thermometer is low.
Less wind, or a change to N., or less wet	When the mercury rises, after having been some time below its average height.
Strong wind or heavy squalls from N.W., N., or N.E.	With the first rise of the mercury after it has been very low (say 29).
Improved weather	When a gradual continuous rise of the mercury occurs with a falling thermometer.
Winds from S. or S.W.	If the mercury suddenly rising, the thermometer also rises.
Heavy gales from N.	Soon after the first rise of the mercury from a very low point.
Unsettled weather	With a rapid rise of the mercury.
Settled weather	With a slow rise of the mercury.
Very fine weather	With a continued steadiness of the mercury with dry air.
Stormy weather with rain (or snow)	With a rapid and considerable fall of the mercury.
Threatening, unsettled weather	With an alternate rising and falling of the mercury.
Much wind, rain, hail, or snow, with or without lightning	When the mercury falls considerably. If the thermometer be low (for the season) the wind will be N., if high, from S.
Lightning only	When the mercury is low, the storm being beyond the horizon.
Fine weather	With a rosy sky at sunset.
Wind and rain	When the sky has a sickly greenish hue.
Rain	When the clouds are of a dark Indian red.
Bad weather or much wind	When the sky is red in the morning.

EXPLANATORY CARD.
BY THE LATE VICE-ADMIRAL FITZROY, F.R.S., ETC.
WEATHER GLASSES.
The Barometer Rises		The Barometer Falls
for Northerly wind		for Southerly wind
(including from North-west, by the North, to the Eastward),		(including from South-east, by the South, to the Westward),
for dry, or less wet weather,—for less wind,—or for more than one of these changes:—		for wet weather,—for stronger wind,—or for more than one of these changes:—
Except on a few occasions when rain, hail, or snow comes from the Northward with strong wind.		Except on a few occasions when moderate wind with rain (or snow) comes from the Northward.
For change of wind toward Northerly directions,—		For change of wind toward Southerly directions,—
a thermometer falls.		a thermometer rises.
Moisture or dampness in the air (shown by a Hygrometer) increases before rain, fog, or dew.
On barometer scales the following contractions may be useful:—		Add one-tenth of an inch to the observed height for each hundred feet the Barometer is above the half-tide level.
RISE	FALL
FOR	FOR	The average height of the Barometer, in England, at the sea-level, is about 29·94 inches; and the average temperature of air is nearly 50 degrees (London latitude).
NORTH	SOUTH
N.W.—N.—E.	S.E.—S.—W.
DRY	WET	The Thermometer falls about one degree for each three hundred feet of elevation from the ground, but varies with wind.
OR	OR
LESS	MORE
WIND.	WIND.	“When the wind shifts against the sun,
		Trust it not, for back it will run.”
EXCEPT	EXCEPT	First rise after very low
		Indicates a stronger blow.
WET FROM	WET FROM
NORTH.	NORTH.	Long foretold—long last,
		Short notice—soon past.
(In South Latitude read South for North.)

Wind is air in motion. The motion of the air is caused by inequality of temperature. The earth becomes warmed by the sun, and radiates the heat thus acquired back upon the air, which, expanding and becoming lighter, ascends to higher regions, while colder and denser currents rush in to occupy the vacated space. Two points are to be noted in connection with this rush of air which we call wind, viz., its direction and velocity or force. Both are estimated scientifically by instruments called Anemometers,^[13] while mariners and the dwellers on our coasts have a nomenclature of their own by which to indicate variation in the force of the wind, founded on the amount of sail a vessel can carry with safety at the time. In the matter of direction winds are classed as constant, periodical, and variable.

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Constant Winds.—The Trade Winds.—The violent contrast between the temperature of the equator and the poles is well known, and from the vast area included within the tropics ascending currents of rarefied air are incessantly rising and being as incessantly replaced by a rush of cold air from the poles to the equator. Were the earth stationary, this interchange would be of the simplest kind; on arriving within the influence of the ascending equatorial current the air from the poles would be carried to the higher regions and turning over would proceed to the poles, and, becoming cold and dense in traversing the higher stratum, would descend and resume its course ad infinitum. The revolution of the earth on its axis changes all this: the first effect is that the air at the equator is borne along with the earth at the rate of seventeen miles a minute from west to east, a rate which diminishes at 60° of latitude to one-half that velocity, until at the poles it is nothing; consequently a slow north wind flowing to the equator is continually passing over places possessing a higher velocity than itself, and not immediately acquiring that velocity, there is according to the law of the composition of forces a compromise effected resulting in a north-east wind. In a similar manner the same process in the southern hemisphere results in a south-east wind. These winds have acquired the name of Trade Winds on account of the facilities afforded to navigation by their constancy. The North Trades occur in the Atlantic between 9° and 30° and in the Pacific between 9° and 26°. The South Trades occur in the Atlantic between lat. 4° N. and 22° S. and in the Pacific between latitude 4° N. and 23-1/2° S. These limits extend northward with the sun from January to June, and southward from July to December.

Parallel to the equator and extending between 2° and 3° on each side is a broad belt, where the north and south trades neutralize each other, producing what is called the “Region or Belt of Calms.” Though wind is absent, thunderstorms and heavy rains are of daily occurrence.

When Humboldt ascended Teneriffe the trade wind was blowing at its base in the usual direction, but on arriving at the summit he found a strong wind blowing in the opposite direction. Observation has shown that this upper current prevails north and south of the equator, and that, after passing the limit of the trade winds, it descends to form the south-west winds of the north temperate zone and the north-west winds of the south temperate zone; the westing being due to the same cause as the easting in the regular trades, viz., the rotation of the earth on its axis. These winds are called the Return Trades, but are not equal in constancy to the regular trade winds.

Periodical Winds.—Land and Sea Breezes occur on the coasts, chiefly in tropical countries, but sometimes in Great Britain during the summer months when the land during the day becomes very hot, causing an ascending column of air, which is replaced by a comparatively colder stream flowing inwards from the sea. At sunset the conditions are reversed, the earth becomes rapidly cooled by radiation, the sea continuing comparatively warm, the air over it ascends, and its place is supplied by a cold breeze, which “blows off the shore,” as illustrated by the diagrams and the following experiment—In the centre of a large tub of water float a water plate containing hot water, imagine the former to be the ocean and the latter the heated land, rarefying the air over it. Light a candle and blow it out and hold it while still smoking over the cold water, when the smoke will be seen to move towards the plate. The reverse of this takes place if the tub be filled with hot water and the water plate with cold. When this phenomenon takes place on a large scale, as in the case of the north trade winds being drawn from their course by the heated shores of Southern Asia, the gigantic sea breeze thus produced is called the south-west monsoon. This occurs from April to October, when the sun is north of the equator. When the sun is south of the equator—that is, from October to April—the analogue of the land breeze is produced, and is called the north-east monsoon.

Variable Winds.—The character of this class of winds is determined by the physical configuration of the country in which they occur. Some tracts are marked by luxuriant vegetation, others are bare. Here mountains lift their awful fronts and “midway leave the storm,” there an arid plain extends itself to the seashore, or inland, towards a chain of lakes. Within the tropics these purely local conditions are insufficient to overcome the force of the prevalent atmospheric currents: such, however, is not the case beyond the tropical zone. There the variable winds prevail, for which space permits only the mention of their names:—The Simoom (from the Arabic samma, hot), peculiar to the hot sandy deserts of Africa and Western Asia. The Sirocco blows over the two Sicilies as a hot wind from the south. It extends sometimes to the shores of the Black and Caspian seas, spreading death among animals and plants. The Solano prevails at certain seasons in the south of Spain: its direction is south-east. The Harmattan is another wind of the same class, peculiar to Senegambia and Guinea. The Puna Winds blow for four months over a barren tableland called the Puna, in Peru. They are a portion of the south-east trade winds, which, having crossed the Pampas, are thereby deprived of moisture, and become the most parching wind in the world. The East Winds, peculiar to the spring in Britain, blowing as they do through Russia, over Europe, are a portion of the great polar current, distinctive of that season of the year. They are dry and parching, every one being familiar with the unpleasant bodily sensations attendant on this much-abused and yet most beneficent wind.

The Etesian Winds are drawn from the north across the Mediterranean by the great heat of the African desert. The Mistral is a strong north-west wind peculiar to the south-east of France. The Pampero is a north-west wind, blowing in summer from the Pampas of Buenos Ayres.

As long ago as the year 1600 Lord Bacon remarked that the preponderating tendency of the wind was decidedly to veer with the sun’s motion, thus passing from N. through N.E., E., S.E., to south, thence through S.W., W.N.W., to N.; also, that it often makes a complete circuit in that direction, or more than one in succession (occupying sometimes many days in so doing), but that it rarely backs, and very rarely or never makes a complete circuit in the contrary direction. The merit of having first demonstrated that this tendency is a direct consequence of the earth’s rotation is due to Professor Dove, of Berlin, who has also shown that the three systems of atmospheric currents just treated of, viz., the constant, periodical, and variable winds, are all amenable to the same influence.

As to the mode of observing the wind, Admiral Fitzroy recommends that a true east and west line should be marked about the time of the equinox, and the north, south, and other points of the compass being added, to take the bearings of the wind in relation to a dial so prepared, the indications of the lower stratum of clouds in conjunction with vanes and smoke being preferred to any other.

The direction of the wind should always be given according to true, and not to compass bearings. Two points to the westward nearly represents the amount of “Variation of the Compass” for the British Isles, which yields the following table for the conversion of directions observed by the compass in Great Britain and Ireland to approximate true bearings.

Compass bearings.	N	NNE	NE	ENE	E	ESE	SE	SSE
True bearings.	NNW	N	NNE	NE	ENE	E	ESE	SE
Compass bearings.	S	SSW	SW	WSW	W	WNW	NW	NNW
True bearings.	SSE	S	SSW	SW	WSW	W	WNW	NW

“One may call a very simple diagram, a circle divided by a diameter from north-east to south-west, the thermometer compass. While the wind is shifting from south-west, by west, north-west, and north to north-east, the thermometer is falling, but while shifting from north-east, by east, south-east and south, towards south and south-west, the thermometer is rising. Now the barometric column does just the reverse. From north-east the barometer falls as the wind shifts through the east to south-east, south, and south-west, and from the south-west, as the wind shifts round northward to north-east, the barometer rises—it rises to west, north-west, north, and north-east.

“The effect of the wind thus shifting round when traced upon paper by a curve, seems certainly wave-like to the eye; but I believe it to be simply consequent on the wind shifting round the compass, and indicating alteration in the barometric column.

“If the wind remained north-east, say three weeks, there would be no wave at all—there would be almost a straight line along a diagram (varying only a little for strength). The atmospheric line, in such a case, remains at the same height, and the barometer remains at 30 inches and (say) some three or four-tenths, for weeks together. So likewise when the wind is south-westerly a long time, or near that point, the atmospheric line remains low, towards 29 inches. Thus, such ‘atmospheric waves’ may be an optical delusion.

“The diagram alluded to above shows how the barometer and thermometer may be used in connection with each other in foretelling wind, and consequently weather, that is coming on, because as the one rises, the other generally falls, and if you take the two together and confront with their indications the amount of moisture in the air at any time, you will scarcely be mistaken in knowing what kind of weather you are likely to have for the next two or three days, which for the gardener, the farmer, soldier, sailor, and traveller must be frequently of considerable importance.”^[14]

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We are indebted to M. Buys Ballot, a Dutch meteorologist, for an invaluable generalization, the importance of which it is almost impossible to over-estimate. This distinguished savant says:—“It is a fact above all doubt that the wind that comes is nearly at right angles to the line between the places of highest and lowest barometer readings. The wind has the place of lowest barometer at its left hand, and is stronger in proportion as the difference of barometer readings is greater.” These facts have been variously stated by other writers; for example: “Stand with your back to the wind, and the barometer will be lower on your left hand than on your right;” “Facing the wind the centre of depression bears in the right-hand direction,” statements which can be verified at any time by a brief study of the “Weather Charts” now published in the daily journals. The value of the law consists in its connecting the surface winds of our planet with the actual pressure of the air itself, and it admits of the following tabulation:—

The wind is	The wind is	The wind is	The wind is
NORTHERLY	SOUTHERLY	EASTERLY	WESTERLY
when the	when the	when the	when the
BAROMETER	BAROMETER	BAROMETER	BAROMETER
is, in the	is, in the	is, in the	is, in the
N. & S. ⎫ ⎬ ⎭ about equal.	N. & S. ⎫ ⎬ ⎭ about equal.	N. High.	N. Low.
		S. Low.	S. High.
		E. & W. ⎫ ⎬ ⎭ about equal.	E. & W. ⎫ ⎬ ⎭ about equal.
E. Low.	E. High.
W. High.	W. Low.

which can be verified by the reader from the daily Weather Charts in the newspapers.

The above are deductions from Buys Ballot’s Law, still further impressed on the memory by taking four outline maps of the British Isles, inserting the names of Thurso, Penzance, Yarmouth, and Valentia, with barometer readings of the kind above named at each place, and then drawing a large arrow in red ink across the centre of each map in the direction appropriate to the readings.

Mr. Strachan, in his able pamphlet on “Weather Forecasts,” puts the matter thus: “It follows from Ballot’s Law that in the northern temperate zone the winds will circulate around an area of low atmospherical pressure in the reverse direction to the movement of the hands of a watch, and that the air will flow away from a region of high pressure, and cause an apparent circulation of the winds around it, in the direction of watch hands.” And as the result of a careful digest of data contained in the eleventh number of meteorological papers, published by the Board of Trade, he has established the following valuable propositions. As introductory to the propositions, it should be stated that the positions of observations were the following:—

Places.	Latitude.	Longitude.
Nairn	57°  29´ N.	4°  13´ W.
Brest	48 „ 28	4 „ 29  W.
Valentia	51 „ 56	10 „ 19  W.
Yarmouth	52 „ 37	1 „ 44  E.
Portrush (or Greencastle)	55 „ 12	6 „ 40  W.
Shields	55 „   0	1 „ 27  W.

Nairn and Brest are situated nearly on the same meridian, about 540 geographical miles apart. Valentia and Yarmouth are nearly on the same parallel of latitude, about 450 miles apart. Portrush and Shields, distant 180 miles, are on a parallel which is nearly as remote from the parallel of Nairn as that of Valentia and Yarmouth is from the one passing through Brest; and Shields is about as much to the westward of Yarmouth as Portrush is to the eastward of Valentia. When observations have not been obtainable for Brest, those made at Penzance have been used instead.

Proposition 1.—Whenever the atmospherical pressure is greater at Brest than at Nairn, while it is of the same or nearly the same value at Valentia and Yarmouth, being gradually less from south to north, the winds over the British Isles are westerly.

Proposition 2.—Whenever the pressure at Nairn is greater than at Brest, while its values at Valentia and Yarmouth are equal, or nearly so, the winds over the British Isles are easterly.

Proposition 3.—Whenever the pressure at Valentia is greater than at Yarmouth, while its values at Brest and Nairn are nearly equal, the winds over the British Isles are northerly.

Proposition 4.—Whenever the pressure at Yarmouth exceeds that at Valentia, while there is equality of pressure at Nairn and Brest, the winds of the British Isles are southerly.

Proposition 5.—Whenever the pressure of the atmosphere is equal, or nearly so, at Brest, Valentia, Nairn, and Yarmouth, and generally uniform, the winds over the British Isles are variable in direction and light in force.

The data from which the foregoing propositions were deduced, and indeed all other cases calculated by Mr. Strachan, show in every well marked instance that when the atmospherical pressure was

(1) greater in the south than in the north, the wind had westing;

(2) greater in the north than in the south, the wind had easting;

(3) greater in the east than in the west, the wind had southing;

(4) greater in the west than in the east, the wind had northing;

(5) uniformly high, or uniformly low, variable light winds (with fine weather in the former case, and vapoury or wet weather in the latter).

Conditions (1) and (3) give winds from the S.W. quarter.

Conditions (1) and (4) give winds from the N.W. quarter.

Conditions (2) and (4) give winds from the N.E. quarter.

Conditions (2) and (3) give winds from the S.E. quarter.

These principles may be employed to set forth the mode of foretelling the impending change of wind as regards its direction and force; for the atmospherical pressure may change—

(a) uniformly over the whole area of observation;

(b) by increasing in the south, or (which causes a similar statical force) by decreasing in the north;

(c) by increasing in the north, or (which has the same effect) by decreasing in the south;

(d) by increasing in the west, or (which has the same effect) by decreasing in the east;

(e) by increasing in the east, or (which has the same effect) by decreasing in the west;

Scale, 0 to 6.	Pressure in pounds per square foot.	Miles per hour.	Seaman’s Nomenclature.	Scale, 0 to 12.	Beaufort Scale.
0·0	0·00	2	Calm	0
0·5	0·25	5	Light Air	1	Just sufficient to make steerage way.
1·0	1·00	10	Light Breeze	2	With which a ship with all sail set would go in smooth water.		1 to 2 knots.
1·5	2·25	15	Gentle Breeze	3			3 to 4 „
2·0	4·00	20	Moderate Breeze	4			5 to 6 „
2·5	6·25	27	Fresh Breeze	5	In which she could just carry	Royals, &c.
3·0	9·00	35	Strong Breeze	6		Single Reefs and T.G. Sails.
—	—	42	—			Double Reefs and Jib, &c.
3·5	—	50	Moderate Gale	7		Triple Reefs, &c.
4·0	16·00	60	Fresh Gale	8
4·5	20·25	—	Strong Gale	9		Close Reefs and Courses.
5·0	25·00	70	Whole Gale	10	In which she could just bear close-reefed Maintopsail and reefed Foresail.
5·5	30·25	80	Storm	11	Under Storm Staysails or Trysails.
6·0	36·00	90	Hurricane	12	Bare Poles.

With	(a)	similar wind and weather	will continue.
„	(b)	winds will veer towards	west.
„	(c)	„         „	east.
„	(d)	„         „	north.
„	(e)	„         „	south.

“The probable strength of wind will be in proportion to the rate of increase of statical force, or differences of barometrical readings. The position of least pressure must be carefully considered; as, in accordance with the law, the wind will blow around that locality. The same remark applies to areas of high pressure, which, however, very rarely occur in a well-defined manner over the British Isles.”

Referring to the table on page 76, the scale 0 to 6 was formerly used by meteorological observers at land stations, and it was intended to express, when the square of the grade was obtained, the pressure of the wind as given in the second column.

“The velocity is an approximation as near as can be obtained, from the values assigned by Neumayer, Stow, Laughton, Scott, Harris, James, &c.”^[15]

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Few meteorological axioms are better established than that which embodies the fact that “every wind brings its weather,” and the primary cause of wind being the motion of the air induced by rarefaction, it is obvious that there is a constant tendency for the equatorial and polar currents in any locality to establish an equilibrium, and this consideration is found to facilitate weather predictions for extended periods. Thus, in consequence of the unusual prevalence of east winds in the spring of 1862, a wet summer was predicted. The prediction was fully borne out by an incessant continuance of south-west winds, with clouded skies and the usual accompaniment of deluges of rain. These winds continuing, with slight intermissions only, till the spring of the following year, less than the usual number of south-west winds was looked for during the summer; the result fully justified the anticipation, the summer of 1863 being fine and warm, especially during the earlier portion. Similarly, without committing the inaccuracies of Murphy in 1838, the summer of 1877 may be reasonably expected to be a dry and cool one from the long continuance of warm and wet months in the winter of 1876-7.

The scientific research and mechanical ingenuity directed of late years to producing trustworthy estimates of the direction, pressure, and velocity of the wind, have resulted in the production of a series of instruments, possessing great precision and accuracy.

The direction of the wind is indicated by vanes, a very efficient form of which is shown at Fig. 54, the velocity by revolving cups, and the pressure by the pressure plate and by calculation from the known velocity.

The Pendulum Anemometer (Fig. 56) shows in a simple manner the direction and pressure of the wind. The peculiarly shaped vane ensures the surface of the swinging pressure plate B being always kept towards the wind. The pendulum plate hangs, during a calm, quite vertically, indicating zero, and as the pressure increases it will be raised through all degrees of elevation from 1 to 12. The vane is perforated with holes large enough to be visible at some distance from the ground, the 5 and 10 being specially larger, so that the angle to which the pressure plate is raised can be quickly noted.

There is a simple contrivance (for the convenience of travellers) called a Portable Wind Vane, or Anemometer, It is furnished with a compass and bar needle, &c., and will tell the true direction of the wind to within a half point.

Lind’s Anemometer or Wind Gauge ranks among the earliest forms of instruments designed to estimate the force of the wind. It consists of a glass syphon, the limbs of which are parallel to each other, mounted on a vertical rod, on which it freely oscillates by the action of the vane which surmounts it. The upper end of one limb of the syphon is bent outward at right angles to the main direction, and the action of the vane keeps this open end of the tube always towards the quarter from whence the wind blows. Between the limbs of the syphon is placed a scale graduated from 0 to 3 in inches and 10ths, the zero being in the centre of the scale. When the instrument is used, it is only necessary to fill the tube with water to the zero of the scale, and then expose it to the wind. The natural consequence of wind acting on the surface of the water is to depress it in one limb and raise it in the other, and the sum of the depression and elevation is the height of a column of water which the wind is capable of sustaining at the time of observation. Sudden gusts of wind are apt to produce a jumping effect on the water in the tube, and to diminish this the bend of the syphon is contracted. A brass plate is attached to the foot of the instrument, bearing the letters indicating the cardinal points of the compass, to show the direction of the wind.

Dr. Robinson, of Armagh, introduced an instrument, in 1850, which consists of four hemispherical copper cups attached to the arms of a metal cross. The vertical axis upon which these are secured has at its lower extremity an endless screw placed in gear with a train of wheels and pinions. Each wheel is graduated respectively to 1/10th, 1 mile, 10 miles, 100 miles, 1,000 miles, and these revolve behind a fixed index, the readings of which are taken according to the indications on the dials.

Dr. Robinson entertained the theory that the cups (measuring from their centres) revolved with one-third of the wind’s velocity; and this theory having been fully supported by experiment, due allowance has been made in graduating the wheels so that the true velocity is obtained by direct observation.

In an improved form of this anemometer the hemispherical cups are retained, but the index portion of the instrument consists of two graduated concentric circles, the inner one representing five miles divided into 10ths, and the outer one bearing 100 divisions, each of which is equivalent to five miles. At the top of the dial is a fixed index, which, as the toothed wheel revolves, marks on the inner circle the miles (up to five) and 10ths of miles the wind has travelled, while a movable index, which revolves with the wheel, indicates on the outer circle the passage of every five miles.

This instrument can be made very portable by removing the arms bearing the cups, when the whole may be packed with iron shaft in a case 15 × 13 × 4 inches. It may be placed in any desired position by screwing the iron shaft supplied with it into the hole provided for the purpose, and fixing the apparatus on a pole or on an elevated stand, if possible, in an open space exposed to the direct action of the wind.

If, when placing the instrument, the hands stand at 0, the next reading will, of course, show the number of miles the wind has traversed; but, should they stand otherwise, the reading may be noted and deducted from the second reading, thus: Suppose the fixed index points to 2·5 and the movable index to 125, the reading after 12 hours may be 200 on the outer circle and 3·0 on the inner circle: these added together yield 203. By deducting the previous reading 127·5, we have the true reading—viz., 75·5 miles as the distance travelled by the wind.

Having obtained the velocity of the wind in this manner in miles per hour, the table on page 83, from Col. Sir Henry James’s “Instructions for Taking Meteorological Observations,” will enable the observer to calculate the pressure in pounds per square foot.

The following letters are used to denote the state of the weather:—

b	denotes	blue sky, whether with clear or slightly hazy atmosphere.
c	„	cloudy, that is detached opening clouds.
d	„	drizzling rain.
f	„	fog.
h	„	hail.
l	„	lightning.
m	„	misty, or hazy so as to interrupt the view.
o	„	overcast, gloomy, dull.
p	„	passing showers.
q	„	squally.
r	„	rain.
s	„	snow.
t	„	thunder.
u	„	ugly, threatening appearance of sky.
v	„	unusual visibility of distant objects.
w	„	wet, that is dew.

A letter repeated denotes much, as rr, heavy rain; ff, dense fog; and a figure attached denotes duration in hours, as 14r, 14 hours’ rain.

By the combination of these letters all the ordinary phenomena of the weather may be recorded with certainty and brevity.

Examples.—bc, blue sky with less proportion of cloud; cb, more cloudy than clear; 2rrllt, heavy rain for two hours, with much lightning, and some thunder.

The Pressure varies as the Square of the Velocity, or P ∝ V². The Square of the Velocity in Miles per Hour multiplied by ·500 gives the Pressure in lbs. per square Foot, or V² × ·005 = P. The Square Root of 200 times the Pressure equals the Velocity, or √(200 × P) = V.

Pressure in	Velocity in	Pressure in	Velocity in	Pressure in	Velocity in	Pressure in	Velocity in	Pressure in	Velocity in
lbs. per	Miles	lbs. per	Miles	lbs. per	Miles	lbs. per	Miles	lbs. per	Miles
Square Foot.	per Hour.	Square Foot.	per Hour.	Square Foot.	per Hour.	Square Foot.	per Hour.	Square Foot.	per Hour.
oz.		lbs.		lbs.		lbs.		lbs.
0·08	1·000	6·75	36·742	17·75	59·581	28·75	75·828	39·75	89·162
0·25	1·767	7·00	37·416	18·00	60·000	29·00	76·157	40·00	89·442
0·50	2·500	7·25	38·078	18·25	60·415	29·25	76·485	40·25	89·721
0·75	3·061	7·50	38·729	18·50	60·827	29·50	76·811	40·50	90·000
1·00	3·535	7·75	39·370	18·75	61·237	29·75	77·136	40·75	90·277
2·00	5·000	8·00	40·000	19·00	61·644	30·00	77·459	41·00	90·553
3·00	6·123	8·25	40·620	19·25	62·048	30·25	77·781	41·25	90·829
4·00	7·071	8·50	41·231	19·50	62·449	30·50	78·102	41·50	91·104
5·00	7·905	8·75	41·833	19·75	62·819	30·75	78·421	41·75	91·378
6·00	8·660	9·00	42·426	20·00	63·245	31·00	78·740	42·00	91·651
7·00	9·354	9·25	43·011	20·25	63·639	31·25	79·056	42·25	91·923
8·00	10·000	9·50	43·588	20·50	64·031	31·50	79·372	42·50	92·195
9·00	10·606	9·75	44·158	20·75	64·420	31·75	79·686	42·75	92·466
10·00	11·180	10·00	44·721	21·00	64·807	32·00	80·000	43·00	92·736
11·00	11·726	10·25	45·276	21·25	65·192	32·25	80·311	43·25	93·005
12·00	12·247	10·50	45·825	21·50	65·574	32·50	80·622	43·50	93·273
13·00	12·747	10·75	46·368	21·75	65·954	32·75	80·932	43·75	93·541
14·00	13·228	11·00	46·904	22·00	66·332	33·00	81·240	44·00	93·808
15·00	13·693	11·25	47·434	22·25	66·708	33·25	81·547	44·25	94·074
		11·50	47·958	22·50	67·082	33·50	81·853	44·50	94·339
lbs.		11·75	48·476	22·75	67·453	33·75	82·158	44·75	94·604
1·00	14·142	12·00	48·989	23·00	67·823	34·00	82·462	45·00	94·868
1·25	15·811	12·25	49·497	23·25	68·190	34·25	82·764	45·26	95·393
1·50	17·320	12·50	50·000	23·50	68·556	34·50	83·066	45·50	95·131
1·75	18·708	12·75	50·497	23·75	68·920	34·75	83·366	45·75	95·655
2·00	20·000	13·00	50·990	24·00	69·282	35·00	83·666	46·00	95·916
2·25	21·213	13·25	51·478	24·25	69·641	35·25	83·964	46·25	96·176
2·50	22·360	13·50	51·961	24·50	70·000	35·50	84·261	46·50	96·436
2·75	23·452	13·75	52·440	24·75	70·356	35·75	84·567	46·75	96·695
3·00	24·494	14·00	52·915	25·00	70·710	36·00	84·852	47·00	96·953
3·25	25·495	14·25	53·385	25·25	71·063	36·25	85 146	47·25	97·211
3·50	26·457	14·50	53·851	25·50	71·414	36·50	85·440	47·50	97·467
3·75	27·386	14·75	54·313	25·75	71·763	36·75	85·732	47·75	97·724
4·00	28·284	15·00	54·772	26·00	72·111	37·00	86·023	48·00	97·979
4·25	29·154	15·25	55·226	26·25	72·456	37·25	86·313	48·25	98·234
4·50	30·000	15·50	55·677	26·50	72·801	37·50	86·602	48·50	98·488
4·75	30·822	15·75	56·124	26·75	73 143	37·75	86·890	48·75	98·742
5·00	31·622	16·00	56·568	27·00	73·484	38·00	87·177	49·00	98·994
5·25	32·403	16·25	57·008	27·25	73·824	38·25	87·464	49·25	99·247
5·50	33·166	16·50	57·415	27·50	74·161	38·50	87·749	49·50	99·498
5·75	33·911	16·75	57·879	27·75	74·498	38·75	88·034	49·75	99·749
6·00	34·641	17·00	58·309	28·00	74·833	39·00	88·317	50·00	100·000
6·25	35·355	17·25	58·736	28·25	75·166	39·25	88·600
6·50	36·055	17·50	59·160	28·50	75·498	39·50	88·881

This is the only table hitherto much in use for converting velocity into pressure, and was prepared by Smeaton and others. It does not, however, express the true relation, which has yet to be determined.

The Anemograph, or Self-Recording Wind Gauge, has for its object the registration of the velocity and direction of the wind from day to day. Figs. 59 and 60 show the form designed and arranged by Mr. Beckley, of the Kew Observatory, which has been adopted by the Meteorological Office.

It consists of a set of hemispherical cups and vanes, which are exposed on the roof of the house, and of the recording apparatus, which is placed inside the house.

The motion imparted to the hemispherical cups by the wind is communicated to the steel shaft B, which, passing through the hollow shaft C, and having at its lower end an endless screw, works into a series of wheels in the iron box D, which reduces the angular velocity 7,000 times. At the required distance the motion, having emerged at E, is connected with F, where, by means of bevelled wheels, it moves the spiral brass registering pencil C, which is arranged so that each revolution records 50 miles of velocity on the prepared paper H.

The direction of the wind is indicated by the arrow L, which is kept in position by the fans M. These communicate, by an endless screw and train of wheels, through the shaft C and the box D to the recording apparatus, consisting of a spiral brass pencil, which in one revolution records variations through the cardinal points of the compass, on the same prepared paper as that which receives the record of velocity.

The paper is held on the drum by two small clips, and may be readily changed, by unclamping the cross V, without disturbing the drum or any other part of the instrument.

William Gilbert, a physician of Colchester, first showed in 1600 that the earth as a whole has the properties of a magnet, and consequently that the directive action exerted by it upon a compass needle represents only a special case of the mutual action of two magnets. In 1845, Faraday established the fact that susceptibility to magnetic force is not, as was generally believed, confined to iron, nickel, and a few other substances, but is a property of all substances. According to Balfour Stewart, auroræ and earth currents may be regarded as secondary currents resulting from changes in the earth’s magnetism. Magnetic phenomena are included under the general term terrestrial magnetic elements, and consist of magnetic declination, inclination, and intensity.

These are for convenience determined separately; the first by an instrument called a Declinometer, and the second by an Inclinometer or Dipping Needle. The Declinometer is also made to serve the additional purpose of measuring the intensity of the earth’s magnetic force, which it effects on a principle similar to that by which the force of gravity is determined by the oscillations of a pendulum of known length on any given portion of the earth’s surface. The declinometer needle is made to oscillate, and the number of oscillations in a given time counted; due allowance being made for the strength of the needle, it is obvious that the force which restores the needle to rest can be estimated. To ascertain the angle of declination, the zero line of the compass card is made to coincide with the geographical north and south line; and the angle which the direction of the needle makes with this line is then read off on a graduated circle over which the needle turns. The magnetic inclination or dip of the needle is estimated by observing the inclination to a horizontal plane of a needle turning on the vertical plane which passes through the magnetic north and south points.

Fig. 62 shows a simple form of magnetic needle suspended on a fine steel point, which is supported by a brass stand; the addition of a graduated circle would constitute such an arrangement a Declinometer.

Fig. 63 gives the appearance of the dipping needle, or Inclinometer, and Fig. 64 an arrangement by which both kinds of terrestrial as well as local attraction may be shown.

These components of the earth’s magnetism undergo not only an annual but a daily and even hourly variation, apparently connected in some occult manner with the frequency of the sun’s spots. The needle sometimes suffers such exceptional perturbations as to suggest the idea of a magnetic storm. These disturbances are usually accompanied (in polar regions) by luminous phenomena called auroræ. Continuous automatic records of them, therefore, is of great value, as facilitating inductive research which may lead to valuable practical results.

Accordingly the Royal Society have adopted for the Kew and other observatories the form of Magnetograph, or Self-recording Magnetometer, shown at Fig. 61, by means of which the variations just referred to are registered by the oscillations of three magnets on photographically prepared paper, stretched on a drum revolved by clockwork.

One magnet is suspended in the magnetic meridian by a silk thread, and, by the aid of a mirror attached, it describes on the cylinder, moved by clockwork in the centre pier, all the variations in the magnetic declination.

The other two components of the magnetic force of the earth are given by the other magnets. That recording the vertical variations rests on two agate edges under a glass shade, while the horizontal component magnet is suspended by a double silk thread, under the shade to the right of the picture, being retained by the tension of the thread in a position nearly at right angles to the magnetic meridian.

The clock box in the centre covers the three revolving cylinders bearing the sensitive photographic paper, and to each magnet is attached a semicircular mirror, which reflects the rays from a gas jet to one of the cylinders, and thus describes by a curved line the oscillations of the magnet. A second semicircular mirror is fixed to the pier on which the instrument stands, and consequently describes a straight line, or zero, from whence the curves are measured.

To avoid errors attending sudden changes of temperature, underground vaults are always chosen for magnetic observations, and also on account of light being more easily and perfectly excluded.

Since the performance of Franklin’s famous kite experiment, by which he determined the identity of lightning with the electrical discharge from a machine, much attention has been devoted, not only to that form of atmospheric electricity which displays itself in the thunder-cloud, but to the electric condition of the air in all states of the weather. These researches have established the fact that the air is always in an electrical condition, even when the sky is clear and free from thunder-clouds. The instruments employed for ascertaining the kind and intensity of atmospheric electricity are called Electroscopes. Fig. 65 shows a modification of Saussure’s Electroscope, the basis of which is a narrow-mouthed flint glass bottle with a divided scale to indicate the degree of divergence of the gold leaves or straws. To protect the lower part from rain, it is covered by a metallic shield about five inches in diameter. Bohnenberger’s Electroscope indicates the presence and quality of feeble electric currents. Peltier’s Electrometer yields the same result by the deflection of a magnetic needle. This latter has been in use at Brussels for thirty years, and at Utrecht for twenty years, and is highly recommended.

Singer’s Atmospheric Electroscope is an efficient form of the instrument in which an ordinary gold-leaf electrometer has attached to its circular brass plate a brass rod two feet in length, with a clip at its upper extremity to receive a lighted paper or cigar fusee. The electricity of the air in immediate contact with the flame, causes, by induction, electricity of the opposite nature to accumulate at the upper extremity, where it is constantly carried off by the convection currents in the flame, leaving the conductor charged with the same kind and power of electricity as that contained in the air at the time of the experiment. The principle of this method was initiated by Volta, and has been extended and applied by Sir William Thomson in his Water-dropping Collector, which consists of an insulated cistern from which water escapes through a jet so fine that it breaks into drops immediately after leaving the nozzle of the tube. The result of this is that in half a minute from the starting of the stream the can is found to be electrified to the same extent as the air at the point of the tube. The scale value of each instrument has to be separately determined by repeated comparative experiments, and involves much delicacy of manipulation.

It is chiefly important for the ordinary observer to know that the occurrence of thunder and lightning should be always noted in the column headed “Remarks.”

The destructive effects of lightning are too well known to need description here; the means, however, by which these may be averted demand a brief notice. Lightning when discharged from a cloud will always choose the better of any two conductors which may present themselves. The stone of a church steeple and the wood of a ship’s mast are bad conductors, but a galvanized iron wire rope is the best possible conductor, and accordingly this material is now generally employed for the purpose. A lightning conductor consists of three parts: 1, the rod, which extends beyond the summit of the building, 2, the conductor, which connects the rod with the underground portion, and 3, the part underground. The connection between each of these must be absolutely perfect, or the conductor will be faulty. The top is usually of solid copper tipped with platinum (Fig. 66), the body of galvanized iron rope, so as to adapt itself to the inequalities of the building and yet have no sharp turns in it, while the part underground is of solid iron rod. This latter portion should extend straight underground for two feet, and being bent at right angles away from the wall, should rest in a horizontal drain 10 to 15 feet long filled with charcoal, and be again bent downwards into a well of water. Should water not be available, it should rest in the centre of a hole 15 feet deep and 10 inches in diameter, tightly packed with charcoal, which, while conducting the electricity from the rod into the earth, serves also to preserve the iron from rusting.

The atmosphere, besides holding the vapour of water diffused throughout its mass, contains also minute traces of carbonic acid and ammonia, and a very remarkable substance called Ozone. Oxygen, one of the component gases of the atmosphere, is capable of existing in two conditions; one in which it is comparatively passive, and another in which it possesses exceptional chemical activity, dependent apparently upon its electrical condition, and in which state it possesses a peculiar smell which has caused it to be named ozone.^[16] The characteristic odour is always observable near a powerful electric machine when it is being worked, near a battery used for the decomposition of water, and in the air after the passage of a flash of lightning. Its presence is most marked near the sea-coast, and in localities remarkable for their salubrity; and on account of its influence on health, it has been proposed by Schonbein and others to include ozonometrical observations with the ordinary meteorological observations.

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Although in minute quantities it is favourable to health, when existing in undue proportion it irritates the mucous membrane of the nose and throat, producing painful sores. It attacks india-rubber, bleaches indigo, and oxidizes silver and mercury, differing in all these points from ordinary atmospheric oxygen.

The chemical energy it possesses (which exceeds that of ordinary oxygen as much as the latter exceeds atmospheric air as an oxidizing agent) affords the means of ascertaining its presence and quantity. It liberates iodine from its combination with potassium, and free iodine colours starch a deep blue.

Schonbein, the discoverer of ozone, found that when strips of paper previously saturated with starch and iodide of potassium and dried were exposed freely to the air but protected from rain and the direct action of the sun, they underwent a peculiar discoloration (when immersed in water) after an exposure of 24 hours. A scale of tints numbered from one to ten afforded the means of comparative observation, and thus the Ozonometer was constructed, and a means established of registering the amount of ozone in the air of various localities from day to day.

Schonbein also observed that the proportion of ozone was largely augmented after heavy falls of snow. For the exposure of the ozone papers, an ozone cage is employed, as shown at Fig. 67.

Ozone may be prepared artificially as a disinfectant by cautiously mixing without friction or concussion equal parts of peroxide of manganese, permanganate of potash, and oxalic acid. For a room containing 1,000 cubic feet, two teaspoonfuls of the powder, placed in a dish and moistened with water occasionally, will develop the ozone and disinfect the surrounding air without producing cough.

The most important and interesting series of facts, however, connected with ozone are those established by the researches of M. Houzeau, who states:—

1. That country air contains an odorous oxidizing substance, with the power of bleaching blue litmus, without previously reddening it, of destroying bad smells, and of bluing iodized red litmus.

2. That this substance is ozone.

3. That the amount of ozone in the air at different times and places is variable, but this is at most 1/700,000 of its volume, or 1 volume of ozone in 700,000 of air.

4. That ozone is found much more frequently in the country than in towns.

5. That ozone is in greatest quantity in spring, less in summer, diminishes in autumn, and is least in winter.

6. It is most frequently detected on rainy days, and during great atmospheric disturbances.

7. That atmospheric electricity is apparently the great generator of ozone.

The subject is one of great interest in its bearings on health, and opens a wide field of scientific research, as may be inferred from the opinion expressed by the Vienna Congress, which is that “the existing methods of determining the amount of ozone in the atmosphere are insufficient, and the Congress therefore recommends investigations for the discovery of better methods.”

Mr. Lowe has published the valuable weather warnings tabulated on page 94, which are interesting as showing from a given number of observations the value of each phenomenon:—

	No. of observations.	Followed in 24 hours by
DEW.		Fine.	Rain.
Dew profuse	241	196	43
Dew from 1st April to 30th Sept.	185	161	24
Dew from 1st Oct. to 30th March	56	37	19
CLOUDS.
White stratus in the valley	229	201	28
Coloured clouds at sunset	35	26	9
SUN.
Solar halos	204	133	71
Sun red and shorn of rays	34	31	3
Mock suns	35	19	6
Sun shone through thin cirro-stratus	13	6	7
Sun pale and sparkling	51	27	24
FROST.
White frost	73	59	14
MOON.
Lunar halos	102	51	51
Mock moons	9	7	2
Lunar burr	64	47	17
Moon shining dimly	18	12	6
Moon rose of a red colour	8	7	1
STARS.
Falling stars abundant	85	65	20
Stars bright	83	64	19
Stars dim	54	32	22
Stars scintillated	14	12	2
AURORA.
Aurora borealis	76	49	27
ANIMALS.
Bats flying about in the evening	61	45	16
Toads in the evening	17	12	5
Landrails clamorous	14	13	1
Ducks and geese noisy	10	7	3
Spiders hanging on webs in the evening	8	5	3
Fish rise in the lake	15	9	6
SMOKE.
Smoke rising perpendicularly	6	5	1

Among the animals whose movements give weather warnings few are more trustworthy than the leech. The reader may verify this by placing one in a broad glass bottle, tied over with perforated leather, or bladder. If placed in a northern aspect, the leech will be found to behave in the following manner:—

1. On the approach of fine or frosty weather, according to the season, it will be found curled up at the bottom. 2. On the approach of rain, snow, or wind, it will rise excitedly to the surface. 3. Thunder will cause it to be much agitated, and to leave the water entirely.

Periods.—M. Köppen states, as the result of his examination into the chances of a change of weather, that the weather has a decided tendency to preserve its character. Thus, at Brussels, if it has rained for nine or ten days successively, the next day will be wet also in four cases out of five; and the chance of a change decreases with the length of time for which the weather from which the change is to take place has lasted.

In the case of temperature for five-day periods, the same principle holds good;^[17] for if a cold five-day period sets in after warm weather, we can bet two to one that the next such period will be cold too; but if the cold has lasted for two months, we can bet nearly eight to one that the first five days of the next month will be cold too. The chance of change is, however, greater for the five-day periods than for single days. Similar results follow for the months, but here again the chance of change shows an increase.

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“If we revert to the instance first cited, that of rain, the result is, not that if it once begins to rain the chances are in favour of its never ceasing; all that is implied is, that the chances are against its ceasing on a definite day, and that they increase with the length of time the rain has lasted. The problem is similar to that of human life: the chance of a baby one year old living another year is less than that of a man of thirty.

“The practical meaning of all this is, that although we know that a compensating anomaly for all extraordinary weather exists somewhere on the earth’s surface, e.g., the very common case of intense cold in America, while we have a mild winter in Britain, there is no reason as yet ascertained to anticipate that this compensation will occur at any given place during the year. In other words, when definite conditions of weather have thoroughly established themselves, it is only with great difficulty that the courses of the atmospheric currents are changed.”

To bring within the limits of a popular pamphlet a notice of the various phenomena classed under the head of Meteorology, it has been necessary to exercise the utmost brevity. Brief, however, as the treatment has been, reference has been made to the sciences of Heat, Light, Electricity, Magnetism, Gravitation, Astronomy, Chemistry, Geography, and Geology, thus corroborating the testimony of Sir John Herschel, who states that “it can hardly be impressed forcibly enough on the attention of the student of nature that there is scarcely any natural phenomenon which can be fully and completely explained in all its circumstances without a union of several—perhaps of all—the sciences; and it cannot be doubted that whatever walk of science he may determine to pursue, impossible as it is for a finite capacity to explore all with any chance of success, he will find it illuminated in proportion to the light which he is enabled to throw upon it from surrounding regions. But, independently of this advantage, the glimpse which may thus be obtained of the harmony of Creation, of the unity of its plan, of the theory of the material universe, is one of the most exalted objects of contemplation which can be presented to the faculties of a rational being. In such a general survey he perceives that science is a whole whose source is lost in infinity, and which nothing but the imperfection of our nature obliges us to divide. He feels his nothingness in his attempts to grasp it, and he bows with humility and adoration before that Supreme Intelligence who alone can comprehend it, and who ‘in the beginning saw everything that He had made, and behold it was very good.’”

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Footnotes moved to end of paragraph.

All scale values in illustration captions retained. The value may not be visually correct.

All fractions regularised to numerator/denominator.

Page 3, ‘Reaumur’s’ changed to ‘Réaumur’s,’ “Réaumur’s scale”

Page 9, closing single quote changed to double quote.

Page 12, full stop appended to illustration caption, “Scale about 1/20.”

Page 16, full stop appended to illustration caption, “9.”

Page 17, semicolon changed to full stop, “...it has never been frozen.”

Page 23, first footnote changed from “8 R = 18 F.” to “8 R = 50 F.”

Page 27, ‘vice versâ’ changed to ‘vice versa,’ “...proportion, and vice versa,...”

Page 30, dash changed to space, “29·500 inches.”

Page 34, ‘Hook’ changed to ‘Hooke,’ “...invented by Dr. Hooke.”

Page 37, ‘Aneriod’ changed to ‘Aneroid,’ “...by means of Barometer or Aneroid,...”

Page 39, space changed to stop, “...between nine and ten p.m.”

Page 42, space inserted between ‘no’ and ‘less,’ “...at no less than 212 miles.”

Page 46, illustration number added to caption, “38. Damp Detector.”

Page 51, comma moved to after ‘weather,’ “In cold calm weather,...”

Page 57, ‘!’ changed to ‘,’ “cloud reflector,”

Page 58, ‘2.’ changed to ‘2nd.,’ “...and 2nd. Velocity of Motion.,”

Page 74, degrees changed to minutes, “57° 29´ N.”

Page 79, ‘Guage’ changed to ‘Gauge,’ “Lind’s Anemometer or Wind Gauge...”

Page 83, powers changed from subscripts to superscripts, “V².”

Page 87, reference in text to Fig. 65 changed to Fig. 64, “...and Fig. 64 an arrangement...”

Page 94, full stops added after ‘STARS’ and ‘ANIMALS.’