32 PHILOSOPHY OF STORMS. 



or one tenth of the whole, and consequently the air which 

 ascended would have expanded into nearly one tenth greater 

 space, and, therefore, each cubic foot of it would contain 

 nearly one tenth less vapor. Now, by examining a table of 

 dew points (129), it appears, that the elasticity of vapor at 60 

 Fah. is .524 inches, and one tenth of this subtracted from 

 it, leaves .472, which corresponds to a dew point of 57. 

 This is the point to which it would be reduced, if it suf- 

 fered no reduction of temperature by expansion. 



But as by the experiments it would be cooled about 

 12, and contracted on this account about ^ of its whole 

 volume, therefore a correction of the above estimate must 

 be made by adding $ of 472 to 472 = 484, which corres- 

 ponds to a temperature of 57 ; and this is the actual dew 

 point of air having ascended from the surface of the earth 

 one thousand yards, having had a dew point of 60 previous 

 to its ascent. 



Now as the temperature of the air in this case would fall 

 about 12J in rising one thousand yards, it appears from 

 this calculation, that the temperature and dew point would 

 at this height almost coincide, and upon a farther ascent 

 and diminution of temperature, a cloud would begin to 

 form. 



67. By a similar calculation for other dew points, I find 

 that the bottom of all cumulus clouds at the moment of being- 

 formed, is about one hundred yards high for every degree of 

 difference of the temperature of the air> and the dew point at 

 the time of formation. 



This rule requires a small correction when the air is very 

 dry, arising in extreme cases to about one hundred and five 

 yards for one degree. 



68. This rule, however, applies only to the base of clouds 

 before they have acquired very considerable perpendicular 

 diameter. For as a cloud goes on increasing in height at 

 the top, the base descends, in consequence of the levity of 



