In this order the movements take place daily, except when the currents are disturbed by accidental circumstances or by the influence of currents which proceed from the pole to the equator in order to supply the place of the heated air which ascends from the surface of the earth between the tropics..
The sun is not the sole cause of the currents which are observed in the atmosphere, for they often arise front the condensation of the aqueous vapours which are constantly rising from the surfaces of seas and rivers. Such vapours, being lighter than air, ascend in the atmo sphere, carrying with them a quantity of heat, which escapes on arriving in a region where there is less heat than at the surface of the earth; and the vapour being then reduced to a state of water, a partial vacuum is produced, into which the neighbouring air rushes. The heat is at the same time conveyed by the wind with the drops of water, and thus the region in Which the rain is falling is sometimes warmer than those which surround it. Rain-winds are produced by the air which descends to the ground with the globules of water ; the particles of air being disengaged from the globules on the latter striking.the ground, are then driven off, with considerable force, in every direction from the place where the rain is falling.
All mountain districts are subject to sudden and violent gusts of wind from the interruptions which the ridges of high land create to the general currents of the air ; but that which is called the Helm wind, at Crossfell, in Cumberland, is one of the most remarkable of these phenomena. It occurs at uncertain times between the end of September and the month of May, and occasionally, though rarely, in summer. It is stated that, when not a breath of wind is stirring, and scarcely a cloud is to be seen, there is suddenly formed a line of clouds, called the " Helm," extending nearly north and south along the top ridge of the mountains; and nearly parallel to this, another line of clouds, called the " Bar," forms itself : the first of these lines of clouds is well defined at its western and the other at its eastern edge; and the lines unite together at their northern and southern extremities so as to contain between them an elliptical space whose length, in the north and south direction, varies from 8 to 30 miles, and its breadth, in an east and west direction, from half a mile to 4 or 5 miles; the highest point of the ridge of mountains being about the middle of the first line of clouds. In a few minutes after the formation of the Helm a violent
wind begins, within the space between the clouds, to blow from some eastern point of the compass, but generally frdm due east to due west : its force is such as to break trees, disperse the grain in stacks, and overturn a cart with its horse. It continues frequently for nine successive days, and its noise is said to resemble that of the sea in a violent storm ; but it is seldom accompanied by rain. No satis factory hypothesis has yet been offered to account for the phenome non ; but that which seems most probable is, that the air from the coast of Northumberland, being cooled as it rises to the summit of the mountain, and there condensed, descends from thence with great force, by its gravity, into the district at the foot of the western escarpment. (Rev. J. Watson, in the ' Reports of the British Asso ciation,' vol. vii) If we contemplate the influence . of the winds in the economy of human life, we shall find them highly beneficial. Though storms are often destructive to life and property, both st sea and land, yet they contribute greatly to preserve the health of animated beings by the dissipation of noxious exhalations : the winds impel the clouds from place to place, and thus diffuse over great tracts of country the rains which contribute so much to fertilise the ground. Wind is extensively employed in giving motion to machinery ; and, till the recent appli cation of steam, it was the only power by which ships were transported across the ocean between different regions of the earth.
Whirlwind is a violent movement of the atmosphere in a circular or spiral direction apparently about a mathematical axis, the latter having at the same time a progressive motion, rectilinear or curvilinear, on the surface of the land or sea.