_125 THE niYSICAL GEOGRAPHY OF THE SEA. 



find the physical necessity for spiral motion is demonstrated from 

 the fact that there is circular motion and an "uprising in the centre. 

 This spiral movement and the uprising may be illustrated by fa- 

 miliar examples: The angles and corners of the Observatory, 

 and its wings, are so arranged that at a certain place there is, with 

 westerly winds, always a whirlwind. This whirlwind 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 be- 

 ing 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. So, too, with the little autumnal whirlwinds in the road 

 Dust whirlwinds and ^^^^ ^^ ^^^ Isiwu '. the dust, Icavcs, and trash will 

 water-spouts. -^q swcpt lu toward 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 whirlwinds pass 

 across the Potomac, raising from the river a regular water-spout, 

 and, when it reached the land, it appeared as a common whirl- 

 wind, 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 minia- 

 ture on land may give us an idea of the great original on the 

 ocean. 



798. The unequally heated plain is thought to be the cause of 

 A vei-a causa, the ouc. But there are no unequally heated plains 



at sea ; nevertheless, the ^^rmwrn mobile there is said, and rightly 

 said, to be heat. Electricity, or some other imponderable, may 

 be concerned in the birth of tlic \vhirlwind both ashore and afloat. 



