WATERSPOUT, a meteorological phenomenon of the same class probably as the whirlwinds which raise pillars of sand in the deserts of Africa: such whirlwinds, in fact, become waterspouts when they reach the sea; and when waterspouts reach the shore they in some cases become or produce whirlwinds. But there is much reason to believe that the name has been properly applied to several very different phenomena.
The following is a general description of the production of a water spout at sea :- Below a thick cloud the sea appears to be greatly disturbed within a circular area, whose diameter varies from 100 to 120 yards, the waves tending rapidly towards the centre of the agitated mass, where there is formed a vast body of water or aqueous vapour : from hence there rises, with a spiral movement, towards the cloud, a column of a conical form, resembling a trumpet. Vertically above this ascending column there is formed in the cloud, but in an inverted position, a correspond ing cone, whose lower extremity (the apex of the cone) gradually approaches the summit of the ascending column ; and at length both are united, the diameter at the place of junction being only two or three feet. The waterspout is said to be accompanied, during its formation, by a rumbling noise ; and, when complete, it assumes a magnificent appearance. The whole column, which extends from the sea to the clouds, is of a light colour near its axis, but dark along the sides, which gives it the appearance of being hollow.
The spout appears to move with the wind, though even when no wind is felt it sometimes varies its position, tending successively in different directions. It frequently happens that the upper and lower parts of a column move with different velocities, and then, after the whole has taken an inclined position, the parts separate from one another, often with a loud report. Previously to the rupture of the column, the dark parts seem to be drawn upwards irregularly, leaving only a slender tube in connection with the water below. The whole of the vapour is at length absorbed in the air, or it descends into the sea in a heavy shower of rain. The duration of the phenomenon is various : some spouts disappear almost as seen as they are formed, and others have been known to continue nearly an hour : occasionally they form themselves, continue for a short time, vanish, and again appear, and so on several times successively.
Waterspouts are occasionally seen above land (of which some re markable examples will be described in the sequel), and consequently there is then no ascending column of water or vapour to meet that which descends from the clouds. In Dr. (Sir David) Brewster's ' Journal of Science' (No. 5) there is an account of one which was seen in France : it is stated to have appeared like a conical mass of vapour, and to have given out a strong sulphureous smell ; flashes of lightning issued from it, and threw off a great quantity of water. It moved forward in one direction over high grounds and valleYs, and it crossed the course of a river, but on coming to hills of a conical form, it passed round them. The alleged aulphureous smell was no doubt that of the electric aura, so called, perceived when lightning has taken effect very near the observer, and probably often that of ozone in reality. Water spouts have occasionally been witnessed in this country. In 1718 one of them burst In Laneaahire, when, at the place where it fell, the ground was torn up to the extent of about half a mile in length, and to the depth of seven feet, so as to lay bare the surface of the rock under Phil. Trans.,' No. 363.) The formation of waterspouts has been ascribed to a whirling motion produced in the sir by torrents coming in opposite directions; it. has been supposed that the particles of vapour in the upper regions thus acquire, by the centrifugal force, a tendency to move towards the exterior parts of the column, leaving the interior void or in a rarefied state. The pressure of the atmosphere being thus removed from the surface of the sea or ground immediately below, that which takes effect on the surrounding water (when the spout Is formed at sea) must impel the Latter towards that part, and cause it to rise into the space where the partial vacuum exists. There is great probability that the elevation of the sea under the cloud is in part caused by the rarefaction of tho air; but as the pressure of the atmosphere could only raise the water in a perfect vacuum to the height of about 30 feet, and as the height of a waterspout is known to be sometimes about half a mile, some other explanation of this part of the phenomenon must be sought for.