STORMS, HURRICANES, AND TYPHOONS. 409 



which the centrifugal tendency will help to make a low barometer 

 at the centre of a cyclone. 



796. The low barometer, the revolving storm, and the ascend- 

 An illustration, ing column roquiro for a postulate the approach by 



spirals of the wind from circumference to centre. The wind 

 (§ 665) blows towards the place of low barometer ; that is ad- 

 mitted by all. It can only reach that place by a direct or by a 

 curvilinear motion. If l}y the former, then there can be no revo- 

 lution ; but if there be revolution, then the air, while as wind it 

 is revolving around the centre in the gyrations of the storm, is 

 approaching the centre also. Hence we derive the elements of a 

 spiral curve ; and 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 familiar 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 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. So, too, with the little autumnal whirlwinds in the road 

 Dust whirlwinds and and ou the lawu : the dust, leaves, and trash will 

 water-spouts. ]^q swopt iu 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 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 



