Chapter III: Part 3

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

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

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

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

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

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

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

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

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

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

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

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

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

[Footnote 14: The late Admiral Fitzroy.]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

(_a_) uniformly over the whole area of observation;

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

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

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

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

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

With (_a_) similar wind and weather will continue.
„ (_b_) winds will veer towards west.
„ (_c_) „ „ east.
„ (_d_) „ „ north.
„ (_e_) „ „ south.

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

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

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

[Footnote 15: Strachan’s “Portable Meteorological Register,”
4th edition.]

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

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

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.

WEATHER NOTATION.

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

_b_ denotes blue sky, whether with clear or slightly hazy atmosphere. _c_ „ cloudy, that is detached opening clouds. _d_ „ drizzling rain. _f_ „ fog. _h_ „ hail. _l_ „ lightning. _m_ „ misty, or hazy so as to interrupt the view. _o_ „ overcast, gloomy, dull. _p_ „ passing showers. _q_ „ squally. _r_ „ rain. _s_ „ snow. _t_ „ thunder. _u_ „ ugly, threatening appearance of sky. _v_ „ unusual visibility of distant objects. _w_ „ wet, that is dew.

A letter repeated denotes much, as _rr_, heavy rain; _ff_, dense fog; and a figure attached denotes duration in hours, as 14_r_, 14 hours’ rain.

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

_Examples._—_bc_, blue sky with less proportion of cloud; _cb_, more cloudy than clear; 2_rrllt_, heavy rain for two hours, with much lightning, and some thunder.

VELOCITY AND PRESSURE OF THE WIND.

The Pressure varies as the Square of the Velocity, or _P_ ∝
_V_^2. The Square of the Velocity in Miles per Hour multiplied
by ·500 gives the Pressure in lbs. per square Foot, or _V_^2 ×
·005 = _P_. The Square Root of 200 times the Pressure equals
the Velocity, or √(200 × _P_) = _V_.

The subjoined Table is calculated from this data, by COL. SIR HENRY
JAMES, of the Ordnance Survey Office.

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

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

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

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.

VI.—ELECTRIFICATION.

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

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

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.

ATMOSPHERIC ELECTRICITY.

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

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.

OZONE.

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

[Footnote 16: Greek _ozo_, I smell.]

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

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

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

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

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

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

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

2. That this substance is ozone.

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

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

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

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

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

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

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

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

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

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

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

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

[Footnote 17: “Recent Progress in Weather Knowledge,”
by R. H. Scott, F.R.S.]

“If we revert to the instance first cited, that of rain, the result is, _not_ that if it once begins to rain the chances are in favour of its never ceasing; all that is implied is, that the chances are against its ceasing on a definite day, and that they increase with the length of time the rain has lasted. The problem is similar to that of human life: the chance of a baby one year old living another year is less than that of a man of thirty.

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

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

J. AND W. RIDER, PRINTERS, LONDON.

------------------------------------------------------------------------

Transcriber’s note:

Footnotes moved to end of paragraph.

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

All fractions regularised to numerator/denominator.

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

Page 9, closing single quote changed to double quote.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Page 83, powers changed from subscripts to superscripts, “V^2.”

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

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

End of Project Gutenberg's Weather Warnings for Watchers, by The Clerk

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