II.

ON WINDS.

GENERAL CONSIDERATIONS.-So long as the density of the air is the same every where, the atmosphere remains at rest; but, as soon as this equilibrium is broken by any cause whatever, a motion occurs, which is called wind. If, in one part of the atmosphere, the air becomes dense, it passes away to those parts where the density is less, in the same manner as air, compressed in a pair of bellows, escapes by the orifice. This displacement of air is analogous to that of water in rivers; it is a flowing of the aerial ocean from one region towards another.

These currents, the laws of which we are about to study, play a grand part in nature. They favour the fecundation of flowers, by agitating the branches of plants, and transporting the pollen to great distances. They renew the air of cities; and they mitigate the climates of the north by bringing to them the heat of the south. But for them rain would be unknown in the interior of continents, which would be transformed into arid deserts.

DIRECTION OF THE WINDS.-To indicate the direction of the winds, the four cardinal points would be insufficient. The horizon is, therefore, divided into eight equal parts, and the wind is designated by giving it the name of the points of the horizon whence it blows. The eight kinds of winds are north, north-east, east, south-east, south, southwest, west, and north-west.

In meteorological registers, we merely write the initial of these words, that is: N., N.E., E., S.E., S., S.W., W., N.W. Many meteorologists divide the horizon into sixteen equal parts, and designate the points, intermediate between the eight above mentioned, by placing before them the letters N. or S., according as the region whence the wind blows

is placed between the meridian and one of the points N.E., N.W., S.E., S.W.; or the letters E. or W., if the region is intermediate between the said points and the line E.W., which is perpendicular to the meridian. Thus, the region situated between the N. and the N.W. is named the N.N.W.; that situated between the N.E. and the E. will be designated by E.N.E. The sixteen points into which the horizon is divided are, therefore, N., N.N.E., N.E., E.N.E., E., E.S.E., S.E., S.S.E., S., S.S.W., S.W., W.S.W., W., W.N.W., N.W., N.N.W. In some particular cases, it is useful to obtain a still further approximation. The region is then described by means of the ordinary divisions of the circle into 360°, starting from the N. or from the S., and pointing out whether the elevation is east or west from the meridian. Thus S. 83° E. indicates a wind coming from a point situated between the E. and the S. at 83° from the meridian; N. 12° W. is a wind which blows from a point situated between the N. and the W., but at 12° distance from the north.

Vanes indicate the general direction of the wind on the surface of the earth. They are commonly placed on elevated buildings, such as steeples, towers, so that small variations, resulting from accidents of the ground, may not have any action on them. Clouds indicate the direction of the upper aerial currents. As it differs very often from the direction of the wind on the surface of the earth, it is well to note both in meteorological registers.

VELOCITY OF THE WIND.-The unequal force of the wind is a fact of daily observation. We find every conceivable transition between an almost insensible breeze and hurricanes, which overthrow walls, and root up the largest trees. According to their rapidity, they are divided into gentle wind (slight breeze), moderate wind (brisk breeze), strong wind (fresh breeze), violent wind (strong gale), storm and tempest. The name of hurricane is given to those violent and continued tempests which are observed in the bad season. Within the tropics this word is applied to winds which have nothing in common with our European hurricanes. In mean latitudes, violent storms of the finer season are also designated by this name..

For the most part the force of the wind is estimated by the sensation it produces on our body, and is designated by the figures 1, 2, 3, 4, the wind No. 4 being the most violent of all. But, in order to obtain exact measures, we must have recourse to an anemometer. If we suspend a surface vertically, in such a manner that it shall be perpendicular to the direction of the wind, and shall be able to turn around

one of its horizontal edges as upon a hinge, the wind will deviate it from the vertical; and, in order to restore it to its former position, a force must be employed, which will be greater as the wind is stronger. If we, therefore, suspend a weight, capable of restoring it to its vertical position, to a lever constituting a continuation of the surface, the force of the wind may be deduced from the weight employed. At first sight this idea appears exceedingly simple; it is, however, very difficult to put in practice. It is better to oppose to the wind a heavy plate, and to measure its angle of deviation from the vertical. One defect is common to all forms of this apparatus; it is, that they can only indicate the force of the wind at the moment of observation; and, in order to obtain the mean velocity of the wind, it is necessary to make continued observations.

Woltmann's anemometer appears to me the best contrived of all. Imagine an ordinary vane, furnished, on the side which turns toward the wind, with an horizontal axis, carrying two small windmill sails. The aerial current first places the vane in the proper direction; it then sets these sails in motion. The stronger the wind is, the more rapidly do they turn. The axis carries an endless screw, which corresponds with a toothed wheel, in order to estimate the rotations. If its position is noted at the commencement and at the end of an observation, the number of rotations made in a minute is easily calculated. In order to obtain from this the velocity of the wind, nothing more is necessary than to choose a calm day, and to travel in a carriage or on a railway any known distance in a given time. It is evident that the effect will be the same as if the air were in motion, while the instrument remained at rest. A table is then constructed, which informs us of the velocity of the wind which turns the sails 40, 50, or 60 times in a minute. We might also accurately regulate the instrument, by placing it on an exposed plain, and observing the distance per minute to which the wind carries light bodies, such as small pieces of paper, down, or leaves.

It would undoubtedly be very desirable to have a great number of exact measures of the velocity of the wind; but serious difficulties have, at all times, enfettered this class of researches. Indeed, the instrument ought to be erected on a vast plain, in the open air or on a roof, situations which are very inconvenient for the observer. (Vide Note B.)

The velocity of the higher aerial currents is measured by the rapidity with which the shadow of a cloud moves along the surface of the earth.

MEAN DIRECTION OF THE WIND.-We will suppose that, in a given place, the force and the direction of the winds have been noted for a certain time; our first inquiry shall be, as to which wind has blown most frequently. We thus obtain eight or sixteen numbers, which do not teach us what we desire to know; but, if we know how often and with what force each of these winds blew, we might arrive at a positive result. Indeed, each wind drives through the place where the observer dwells a mass of air derived from the region whence it comes. The velocities being equal, this mass of air is greater in proportion as the wind has prevailed longer. If the wind is succeeded by another blowing in a diametrically opposite direction, the same mass of air will be brought back. As the velocity of winds is rarely estimated, we are compelled to take account of their frequency only, and to suppose their force equal. Let us suppose then that, in a given place, the north wind has blown thirty times and the south wind twenty times; the former will have brought with it a mass of air, which we may designate by thirty; but the south wind has taken back twothirds of this mass, or twenty; and the definite result is the same as if the north wind had blown 30-20, or ten times. If the north wind and the east wind had each blown twenty times, the result would have been the same as if the wind had blown from the N.E. By considering the winds in this way, as forces which set the air in motion, we may seek after their resultant according to the laws of mechanics, and we thus obtain the mean direction of the wind. But we have to determine not only the direction, but also the force of this resultant. To arrive at this, let us suppose that the sum of the observed directions amounts to 1000, and let us divide the velocity calculated for the mean direction of the wind by this number. If, then, we find that, at a given place, the mean direction of the wind is S. 63° W., and its force 158, it means that the thousand winds, which have been noted at this spot, have acted in displacing the atmosphere, precisely in the same manner as if the S. 63° W. wind had blown 158 times.*

This mean direction may be easily obtained by a general formula. On a compass card, draw a radius from the centre to the point of the circumference, whence the wind comes, and agree to designate this direction by the angle (azimuth) which this line makes with the meridian, calculating from the north toward the east. The trigonometric tangent of this angle will be given by the formula

E.-W.+ }✓✓√2 (N.E.+S.E.-N.W.-S.W.)

N.-S.+2 (N.E.+N.W.-S.E.-S.W.

This is Lambert's method. M. Schouw seeks the numeric relation between the east (N.E., E., S.E.) and the west winds (N.W., N., S.W.), and that between the north (N.W., N., N.E.) and the south (S.W., S., S.E.) winds. If, at the same place and in a given time, the winds have blown as follows: the N. 84 times; N.E. 98; E. 119; S.E. 87; S. 97; S.W. 185; W. 198; N.W. 131; and we suppose that the total number of winds is 1000; we shall find that the total number of east winds has been

98119

that of west winds,

87=304;

185198 131 = 514;

that of north winds,

131 +98 +84 = 313;

and, lastly, that of south winds,

We find, therefore, for this locality, that the frequency of east winds is to that of west winds as 304: 514, or as 1: 1,69; and the frequency of north to that of south winds is as 313: 369; or as 1: 1,18. So that the frequency of south and of west winds is superior to that of north and of east winds. The same result is obtained by Lambert's method; which, in the preceding example, gives S. 76° W. for the direction, and 177 for the mean force of the wind.

CAUSES OF WINDS.-As currents are always produced by a disturbance of equilibrium in the state of the atmosphere, it would seem, at first sight, that they must needs recognise an infinite number of causes. But a more detailed analysis shews that all these causes are deducible to differences of temperature between neighbouring countries. Suppose that two columns of air have the same temperature throughout their entire height, they would be in equilibrio but if the earth, on which they rest, were unequally heated, the equilibrium would be destroyed.

Throwing out of the account the sphericity of the globe, suppose that the air has the same density throughout its

where we represent the number of times that each corresponding wind has blown in a total of 1000 times, by N., N.E., E., &c.

When this angle is once obtained, the product of the denominator of the above fraction, by the trigonometric secant of the same angle, will give the general resultant of the wind.

The result of these calculations is not very accurate, as, for want of convenient and exact instruments, fitted to measure the velocity of the wind, we are obliged to suppose that each kind of wind has blown with the same mean velocity.-M.