WHIRLWINDS AND WATERSPOUTS. Whirlwinds differ in many respects from the storms described in the articles STORMS and TyrnooNs. They seldom continue longer than a minute at any place, and sometimes only a few seconds; their breadth varies from a few yards to nearly a quarter of a mile; during their short continuance, the changes of the are sudden and violent; and the barometer is not observed to fall. The di rection of the eddy of the whirlwinds, especially when the diameter is very small, differs from the rotation of winds in a storm, in that it may take place either way—right to left, or left to right—according to the direction of the stronger of the two winds which give rise to the whirlwind. Thus, suppose it to arise from a n. wind blowing side by side with a s. wind, and to the w. of it, then, if the n. wind be stronger, the whir] will be n. w., s., and e.; but it will be in a contrary direction if the s. wind be the stronger. Whirlwinds often originate within the tropics during the hot season, especially in flat sandy deserts: these becoming unequally heated by the sun, give rise to ascending columns of heated air. In their contact with each other, the ascending currents result in eddies, which draw up with them large clouds of dust, and the whole is borne for ward by the wind that may, happen to be blowingat the time. This is the origin of the dust whirlwinds of India, which have been admirably described and illustrated by P. P. H. Baddeley. These dust-storms are frequent in dry warm regions; and in the case of the 811)100211 (q.v.), which may be regarded as a succession of such whirlwinds, they appear on a scale of the most appalling grandeur.
Extensive fires, such as the burning of the prairie in America, and volcanic erup tions, also cause whirlwinds, by the conflicting currents of heated air they occasion; and these, as well as the whirlwinds already mentioned, are generally accompanied with heavy rains, hail, and electrical displays. Whirlwinds are also of frequent occurrence
in France, doing great damage to the vineyards and other crops; but in Great Britain they seldom occur.
Waterspouts arc whirlwinds occurring on the sea or on lakes. When fully formed they appear as tall pillars of cloud stretching from the sea to the sky, whirling round their axes, and exhibiting the progressive movement of the whole mass precisely as in the case of the dust-whirlwind. The sea at the base of the whirling vortices is thrown into the most violent commotion, resembling the surface of water in rapid ebullition. It is a popular fallacy that the water of the sea is sucked up in a solid mass by water spouts, it being only the spray from the broken waves which is carried up. Observa tions of the rain-g,uage conclusively prove this.
What are sometimes called waterspouts on land are quite distinct from these phen omena. They are merely heavy falls of rain of a very local character, and may or may not be accompanied with whirling winds. They generally occur during thunder storms, being quite analogous to severe hail-storms, from which they differ only in point of temperature. Also all the moisture that falls is the result of condensation; whereas, in the true waterspout the rain is mixed with spray which has been caught up from the broken waves, and carried aloft by the ascending currents of the whirlwind.