108 PHILOSOPHY OF STORMS. 



It is also easy to explain why the base of the cloud, dur- 

 ing this rapid motion of the air upwards, remains nearly at 

 the same level. For so long as the dew point of the air at 

 the surface of the earth, which supplies the ascending 

 column with vapor, remains the same, if the temperature 

 of the air is the same also, it will have to ascend to the 

 same height, before cold enough is produced by expansion 

 to cause condensation of the vapor. This height is found 

 to be about one hundred yards for every degree between 

 the temperature of the air and the dew point, unless when 

 the cloud becomes of great perpendicular depth, when the 

 base sinks a little from the levity of the cloud. 



The quantity of depression of the vapor as well as of the 

 air, after due allowance is made for the fall of the dew 

 point from the expansion, is nearly four hundred yards for 

 a depression of the barometer one inch, on the supposition 

 that the temperature of the air sinks 4 for a diminution of 

 pressure equal to an inch of mercury. 



A depression of near two and a half inches has been re- 

 corded in a spout accompanied with hail nine inches deep ; 

 hence the base of the cloud would descend in the centre 

 near one thousand yards ; and of course, if the dew point 

 and temperature of the air were not more than 10 degrees 

 apart, the cloud would reach the earth, and exhibit itself in 

 the form of an inverted cone, expanding as it goes up, and 

 by its rapid motion carrying up large "drops of rain above 

 the region of perpetual congelation, and throwing them out 

 at the sides in the form of hail. 



From the principles here developed, it will be easy, when 

 a great storm springs up, or comes within disturbing influ- 

 ence, to tell in what direction it is raging ; for the direction 

 of the wind points it out. 1 The connection also between 

 volcanoes and rains is no longer mysterious for an upward 



1 An exception to the general principle of wind blowing towards the rain, 

 was mentioned and explained in article 33. 



