STORMS, HURRICANES AND TYTHOONS. 425 



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 tliat 

 as tliey come within the influence of the " suck " they will ap- 

 proach 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 tchirlwinds and loater-spoids. — 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 tlie 

 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 pimum 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 Avhere the upward tendency of the indraught is still 



