WIND is a motion of the atmosphere independent of that which it has in consequence of the diurnal and annual movements of the earth. The latter motion being performed in a part of space which may be considered as devoid of any resisting medium, the particles of air suffer no partial displacements on that account ; and the friction of the particles against each other, and against the earth which they sur round, must have long since brought the diurnal movements of the atmosphere and earth to a state of equality : thus, the angular velocity of the air on any parallel of terrestrial latitude being the seine as that of an observer on the same parallel, the air would seem to be at rest about him. But if, from any disturbance of the equilibrium of the atmosphere, the particles should move less rapidly than the observer from west to east, or should acquire movements in some other direction, then the sensation of a wind would be experienced. The tides which take place in the atmosphere by the attractions of the sun, moon, and planets on the particles of air giving rise to differences in the heights of tho vertical columns, they must necessarily cause inequalities of pressure in horizontal directions, and thus produce winds or currents of air; but it has been shown by La Place that these currents are scarcely sensible ; and such attractions are by no means adequate to the production of the winds which are observed on the earth's surface. [Aemosruens, col. 692.] The immediate effect of the solar radiation, communicating heat to any region of the earth's surface, is to generate an ascensional move ment in the incumbent atmosphere,a bodily overflowing of its material above, and a relief of barometrical• pressure below. The air of the cooler surrounding region, not being so relieved, will be diivenln by the difference of hydrostatic—in this case of atmospheric—pressure, so arising, and thus originate two distinct winds : an upper one setting outward from the heated region ; a lower inward, or towards it. " If the region heated be a limited one, these currents will radiate from and to it as a centre ; if a linear belt, or a whole zone of the globe intervene, such as the generally heated intertropical region, they will assume the character of two sheets of air setting inwards on both aisles below, uniting and flowing vertically upwards along the medial line, and again separating aloft and taking on a reversed movement." Those effects of heat upon the atmosphere, together with the rotatory motion of the earth, are the primary causes of all the phenomena of the winds, the latter having been found by modern meteorologists to have a much more extensive influence than was supposed when its part in the production of the trade-winds was first duly recognised. On this account, and because the consequences of those united actions impart to us a key to the whole subject, —including even that part of it which relates to revolving storms, or cyelonea,—we shall now pro ceed to consider the trade-winds, both with respect to their theory and to their facts, in 8011.10 adequate detail.
Tradell'inds is the term used by seamen to indicate the perpetual or constant winds, because they promote mere than any other circum stance navigation and trade. These perpetual or trade-winds occur in
all open seas on both sides of the equator, and to the distance of about 30 degrees north and south of it. They were not known to the ancients, and seem to have been unknown even to modern seamen up to the time of Columbus. Though before his time Portuguese navigators had pro ceeded as far sa the Cape of Good Hope, they had not ventured to any great distance from the coast of Africa, and consequently they had not entered the regions where the trade-Winds blow. Columbus, however, who had passed some time at the Canaries, to which the trade-winds extend in summer, seems to have conceived a just idea of their extent. On his first voyage, after leaving the Canaries, his crew were greatly alarmed at finding that the wind always blew from the north-east and east, and feared that they would be prevented by it from returning to their native country. Columbus did not participate in their fears; and ou his return from the newly-discovered islands his track was north of this trade-winds, in the region of the variable winds. After the time of Columbus, European navigation extended rapidly in the Atlantic and Indian oceans, and the trade-winds became generally known. It does not, however, appear that any attempt to explain this phenomenon was made before the time of Galileo, who, in adopting the astronomical system of Copernicus and the revolution of the earth round its axis, thought he found some confirmation of this opinion in the trade-winds, which, as he conjectured, owed their origin to the revolution of the earth and to the circumstance that the atmosphere, though it partici pated in that motion, could not follow with equal speed the motion of the dense parts of our planet, and that a motion in tho air was thus produced which was contrary to that of the earth round its axis, or from east to west. The strongest fact in favour of this hypothesis was the circumstance that the trade-winds occur only in the lower lati tudes, where the surface of the earth, in its revolution round its axis, has to make a large circle in twenty-four hours, and consequently must move with a greater degree of rapidity than in the higher latitudes. Galileo's theory was relinquished about the end of the 17th century, in favour of a not less fallacious one proposed by Dr. Edmund Halley, who, however, had collected extensive information respecting these winds, and had indeed discovered several facts which were incom patible with the opinion of Galileo. The two most decisive were, that there are no trade-winds near the equator, where the diurnal motion of the earth is greatest, and that the trade-winds change their position according to the seasons, which could not take place if they were only the effect of the rotation of the earth. Galileo, however, appears to have had a true, though obscure, perception that the rotatory motion of the earth must be somehow concerned in the production of the trade-winds, though he singularly omitted to consider the :operation of the sun's heat.