48 PHILOSOPHY OF STORMS. 



great height, in these circumstances, could be formed, for 

 the upper part would be blown away from the lower. 



79. I have frequently seen those large columnar clouds, 

 which form in mid air during a warm summer's day, have 

 their tops blown off by an upper current, when the lower 

 air was almost still, and thus a vortex of great strength 

 prevented from forming. That these clouds are actually 

 formed by rising vortices, occasioned by the disturbance of 

 the equilibrium of the air during the day, is rendered almost 

 certain by the following facts. First. When the supply is 

 cut off in the evening, by the air near the surface of the 

 earth becoming cold, these clouds cease to form, and not 

 unfrequently disappear, and a day with many clouds is fol- 

 lowed by a cloudless night. On the supposition of upward 

 vortices, this phenomenon is very simple and natural ; but 

 on any other supposition, it is utterly paradoxical, (especially 

 when it is now known that depression of temperature is the 

 only cause of the condensation of aqueous vapor,) how clouds 

 can be formed under a meridian sun, which will be dissi- 

 pated under the refrigerating influences of a nocturnal sky. 



Second. I once saw, during a profound calm, those co- 

 lumnar clouds, in all parts of the heavens, appear to be coming 

 slowly towards me, which I think can only be accounted 

 for by supposing that they were all rising perpendicularly. 

 These clouds, however, were gradually dissipated after they 

 had increased to a considerable size, which proves beyond 

 doubt that they were surrounded by air, at that elevation, 

 whose dew point was below the temperature of the air ; and 

 it may be added, that this is one of the cases where a spout 

 cannot be formed, for the ascending air of the vortex will 

 always, more or less, be mingled with the air through which 

 it passes. 



Again, nothing but an upward or a downward vortex will 

 account for the well known fact, that, in these storms, the 

 clouds are frequently seen to rush together with great rapid- 



