273 



Rotary Action of the Providence Tornado. 



be considered as whirling in the manner here represented, but 

 without any change of location^ its action may be supposed as 

 concentrically equal on all sides; the motion, however, becoming 

 quickened towards the center in the inverse ratio of the succes- 

 sive concentric areas : that is, each particle of air as it revolves 

 about the axis, continuing to describe nearly equal areas in equal 

 times, in its progress towards the center, where it rises spirally in 

 the direction of discharge ; this direction being vertically at the 

 center, the point or area of least atmospheric resistance or pres- 

 sure. Thus, the course of a single particle, horizontally, may 

 he ah c d efg hi k ] — and so on or between each of the four 

 involuted lines which constitute the figure. 



Fig. IV. 



For further reference, we may divide this figure by the cross 

 lines of arrow heads, into the four quadrants 1, 2, 3, 4. 



We will now consider this whirl as having a constant progres- 

 sive motion on the line of the long arrow c c, at a rate equal to 

 one fourth or fifth of its average rotative velocity. It will then 

 follow, that as the force of the whirl on the trees and other ob- 



