Page:Physical Geography of the Sea and its Meteorology.djvu/451

Rh This whirl wind is six to eight feet in diameter; and when there is snow, there is a pile, of it in the centre, with a naked path, in the shape of a ring, three or four feet broad about it. It is the spiral motion which brings the drift-snow to the centre or vortex, and the upward motion not being strong enough to carry the snow up, it is left behind, forming a sort of cone, which serves as a cast for the base of the vortex. If you throw chips or trashy matter into the lock of a canal and watch them, you will see that as they come within the influence of the "suck" they will approach the whirl by a spiral until they reach the centre, when, notwithstanding they may be lighter than the water, they will be "sucked" down. Here we see the effects of centrifugal force upon a fluid revolving within itself. The "suck" is funnel-shaped. As it goes down, the lateral pressure of the water increases; it counteracts more and more the effect of centrifugal force, and diminishes, by its increase, the size of the "suck."

797. Dust whirlwinds and water-spouts.—So, too, with the little autumnal whirlwinds in the road and on the lawn: the dust, leaves, and trash will be swept in towards the centre at the bottom, whirl round and round, go up in the middle, and be scattered or spread out at the top. I recollect seeing one of these whirl-winds pass across the Potomac, raising from the river a regular water-spout, and, when it reached the land, it appeared as a common whirlwind, its course being marked, as usual, by a whirling column of leaves and dust. These little whirlwinds are, I take it, the great storms of the sea in miniature; and a proper study of the miniature on land may give us an idea of the great original on the ocean.

798. A vera causa.—The unequally heated plain is thought to be the cause of the one. But there are no unequally heated plains at sea; nevertheless, the primum mobile there is said, and rightly said, to be heat. Electricity, or some other imponderable, may be concerned in the birth of the whirlwind both ashore and afloat. But that is conjecture; the presence of heat is a fact. In the middle of the cyclone there is generally rain, or hail, or snow; and the amount of heat set free during the process of condensing the vapour for this rain, or hail, or snow, is sufficient to raise from the freezing to the boiling point more than five times the whole amount of water that falls. This vast amount of heat is set free, not at the surface of the sea, it is true, but in the cloud-region, and where the upward tendency of the indraught is still