216 PHILOSOPHY OF STORMS. 



inwards, as it would do if there was a motion in the upper 

 air downwards, seems to spread itself outwards, especially 

 towards the N. E., something in the form of a mushroom, 

 and finally into thin cirrus at the end of the rain. 



On the other hand, if the air did really move inwards at 

 the circumference of the storm, all the phenomena attending 

 it may easily be explained on well known philosophical 

 principles. 



It could not move inwards at the circumference below, 

 without moving upwards in the middle; it could not move 

 upwards in the middle, without becoming colder, some- 

 thing more than 1 for every hundred yards of elevation, 

 until it reached the point where condensation of the vapor 

 would commence ; above which it would cool only about 

 half that quantity, the other half being made up by the 

 latent caloric given out by the condensing vapor the 

 latent caloric cannot be given out without expanding the 

 air in contact with it about 7000 cubic feet for every cubic foot 

 of water generated, and thus producing a highly diminished 

 specific gravity of the air in a rapidly forming cloud. This 

 diminished specific gravity will cause the barometer to fall 

 at the surface of the earth below, and the air will run in- 

 wards and upwards with a velocity which, the fall of the 

 barometer being given, may be calculated where the storm 

 is very narrow, as in spouts, on the principle of spouting 

 fluids. (118, note.) 



This upward velocity, where the barometer falls one inch,, 

 is about 240 feet a second, and is quite sufficient to cause a 

 condensation of vapor great enough to produce those cata- 

 racts of rain which sometimes fall in a short time over a very 

 limited extent (section 8) ; and also to carry up large drops 

 of rain above the region of perpetual congelation, and throw 

 them off at the side of the ascending column, frozen into hail, 

 sometimes 12 inches deep in 12 minutes. Even the shape 

 of the tornado cloud, or water spout, is explained by the 



