310 PHILOSOPHY OF STORMS. 



ing the air in which the vapor is condensed into water, 

 about seven thousand cubic feet for every cubic foot of 

 water thus generated. And if the drops of rain are con- 

 gealed, the caloric of fluidity ^volved will expand the air 

 about one thousand cubic feet more. Thus an upmoving 

 column of air mingled with vapor becomes a mighty steam 

 power, which may be calculated like any other steam power, 

 if the tension of steam in the atmosphere, and the height of 

 the cloud from its base is given ; for these two being given, 

 the quantity of vapor condensed can be calculated. 



I have not been able to learn what the dew point was at 

 Brunswick, on the day of the spout, but at Philadelphia it 

 was 71, and the temperature of the air was82 at three o'clock. 



If we assume that the dew point at the spout was 71, 

 and the temperature of the air 79, from the principles ex- 

 plained, when the air at the surface of the earth first com- 

 menced its motion upwards in the form of a column, it 

 would have to rise eight hundred yards before the upper 

 end would, by the cold of expansion, begin to condense its 

 vapor into cloud. And this would be the height of the base 

 of the cloud to which the spout belonged, at the moment of 

 its first formation. 



But as the cloud increased in height above, the air below 

 the cloud would be pressed less and less by the superin- 

 cumbent weight, and thus the air at the surface of the earth 

 would begin to condense its vapor before it ascended eight 

 hundred yards, and, therefore, the base of the cloud would 

 seem to descend. And if the cloud above became so lofty 

 arid so light that the barometer sunk two inches underneath 

 it, the visible spout or cloud would reach the earth ; the air 

 under the cloud would be cooled 8, by expansion. And 

 inversely as the cloud in this case did reach the earth, the 

 barometer did fall two inches. Two inches of mercury is 

 equal to about eighteen hundred feet of air in perpendicular 

 height, and this is the diminished head of pressure in the 



