Strictures on Prof. Dove's Essay. 387 



takes place in the movements of the air below. A like advance 

 of the storm in the higher region is indicated by the fall of the 

 barometer, which usually commences before any other indica- 

 tion of its approach can be perceived. Often, too, the clouded 

 axis of a whirling tornado or water-spout, has been seen in an in- 

 clined or curved position, while moving onwards. And in so 

 dense a fluid as water, I have myself seen the axis of a vortex 

 which was curved through sixty or seventy degrees from the ver- 

 tical, while its vorticular gyration was so rapid as to sink and 

 carry off a continued series of large bubbles of air, 800 times 

 lighter than the fluid vortex* 



The additional objection, that such forward inclination of the 

 axis of the storm " would lift the base in the rear," as if the ro- 

 tary mass were a solid, seems hardly to require an answer. In a 

 fluid stratum of any considerable thickness, we have just seen it 

 cannot be necessary that the general plane of its rotation should 

 be at right angles with the line of the axis. As aerial particles 

 move freely over each other, the storm may be viewed as a series 

 of plane or spiral strata, of any conceivable thinness, superimpo- 

 sed one above another, but each in advance of the next beneath. 



In order that the " lower and warmer strata" in a storm may 

 gradually rise by their spiral movements, to a higher and colder 

 region, thus producing rain, it cannot be necessary " for the lower 

 stratum to exchange places with the upper f for the former will 

 be continuously followed and replaced by the next contiguous 

 portions of the lower strata of air which surround the storm; 

 thus affording a continued supply of aqueous vapor for condensa- 

 tion in a higher region. But were it otherwise, a movement of 

 " five hundred miles" in a given particle, or piano-series of parti- 

 cles, is but a diminutive item in the sum of the aerial movements, 

 in a great storm or hurricane. 



That the dimensions of a great storm may have "a diameter 

 two hundred times as great as its altitude," is a fact neither new 



° ... 



nor doubtful, — and the retardation of its lowest portion by its 



contact with the " terrestrial surface" is equally undeniable ; but it 

 does not follow, that, because the whirling body is likewise " con- 



incessan 



* But in this case, as elsewhere, we might have looked in vain for greater or 

 even equal velocities at increased distances from the axis of the vortex. See ante 

 °n §101 and §107. 



