318 PHILOSOPHY OF STORMS. 



beneath, by its sudden rarefaction, had thrown up small 

 portions of the soil, which was rather dry and porous ; and 

 it is, perhaps, worth consideration, whether this cause may 

 not, in this and similar occurrences, have facilitated the 

 overturning of trees themselves. 



It was a subject of regret at the moment, that want of 

 time, and of a suitable instrument to measure the exact 

 course of the tornado, and the precise position of trees in 

 the different parts of the track, prevented carrying out a 

 plan, which suggested itself on the spot, as the most satis- 

 factory method of arriving at precision on those points. 



In conclusion it may be remarked, that the directions 

 and intensities of the forces in this occurrence, together 

 with the hygrornetric states of the air, preceding and fol- 

 lowing the meteor, and the inverted conical form of the 

 moving column, as confirmed by several witnesses, not less 

 than the fall of hail, and the distribution of fragments of 

 materials beyond the path of the ground current seem 

 most satisfactorily accounted for, on the supposition that a 

 disturbance of atmospheric equilibrium, results from a dep- 

 osition of moisture in the higher regions of the atmosphere 

 giving out a great amount of latent heat; the expansion of 

 pure air by an addition of heat, being in such cases much 

 greater than the contraction of the atmospheric mixture by 

 a condensation of its moisture. In this effect, is, of course, 

 involved the well known principle that the capacity of air 

 for heat is augmented as its volume expands, but the in- 

 crease of capacity for heat being less rapid than the supply 

 of heat from aqueous depositions, an ascending current is 

 maintained with a force due to the difference of these two 

 causes. 1 



The above is the testimony of Professors Henry and John- 



1 The origin of this view of the subject with which the writer had been 

 made acquainted previously to the examination above detailed, is due to Mr. 

 J. P. Espy. 



