228 



the more rapidly, on account of the centrifugal force causing a colli- 

 sion with inert portions of the atmosphere beyond the zone, at which 

 the generating forces may have been applied. 



The centripetal theory assumes that an ascending column about the 

 centre of a storm, causes an afflux of wind from opposite quarters. 

 . Professor Dove alleges that such winds must neutralize each other, 

 their intensity lessening more and more, as they approach their place 

 of meeting. This shows that he does not understand the theory which 

 he has undertaken to refute; since it must be quite evident, that, in 

 rushing towards an ascending column, the velocity must become 

 greater as the space within which the affluent air has to move be- 

 comes less. 



It is also inferred by this negligent reasoner, that the sudden change 

 of wind from one direction to another directly opposite, for which hur- 

 ricanes are notorious, and in proof of which he adduces additional 

 evidence, is inconsistent with the centripetal theory, upon the erro- 

 neous inference above noticed; when this suddenness of change has 

 been shown to be irreconcilable with the idea that hurricanes are 

 whirlwinds. In that case the wind would blow comparatively with 

 little force within a large central area, which, at the rate of travelling 

 admitted by him, would require several hours to pass over an observer, 

 so situated as to be successively exposed to opposite winds. 



That an upward current, having a base of 10,000 feet, or two miles 

 nearly, would require more wind to supply it (if of equal velocity) 

 than 400 of the largest tornadoes ever known to take place in the Uni- 

 ted States. It was believed that no tornado had been observed to have 

 a focal area over 500 feet,* which would be only one-twentieth of the 

 diameter above assumed, and of course the areas would be as 400 

 to 1. 



According to Professor Dove, storms travel at the rate of thirty 

 miles an hour. At this rate a focal area of 10,000 feet would require 

 less than two minutes to pass over an observer, so as to subject him 

 to winds blowing directly opposite to each other. 



* The space within which the water rose, during the passage of a tornado 

 over the river near Providence, in August 1838, was estimated by Mr. Allen 

 at 300 feet. Of course, a focal area of 500 feet would have been productive 

 of a tornado nearly three times as large ; or as the square of 5 to that of 3, or as 

 25 to 9. See Silliman's Journal for 1 840, Vol. XXXVIII. p. 77, or Transactions, 

 A. P. S. Vol. VI. 



It is taken for granted that the space, within which the water rose, was the 

 focal area, or base of the ascending column. 



