72 PHILOSOPHY OF STORMS. 



some degrees hotter than the air a little above the surface, 

 and thus produces an unstable equilibrium, so that the least 

 agitation would cause an upward motion to commence at 

 the point of greatest heat, especially if that point contained 

 a higher dew point, as it generally does. As soon as this 

 motion commences, other air rushes in below, and the 

 higher and longer the column of heated air becomes, the 

 more rapid does the upward motion become, and finally, 

 after the upper end of the column is as many hundred 

 yards high as the temperature of the air on the ground is 

 above the dew point in degrees of Fahr., the cold produced 

 by the expansion of the air, begins to condense the va- 

 por arid form cloud, and still as other air rises to that 

 elevation it begins to condense likewise, and thus the 

 base of the cloud remains at the same elevation, while 

 the cloud goes on increasing in perpendicular height 

 above. This is the kind of cloud which is formed almost 

 every clear day in the summer when the dew point is not 

 very low, but never forms when it is overcast. When the 

 air is calm, if these clouds are observed carefully when 

 they are forming, they will be seen to increase in perpen- 

 dicular height while their bases remain at the same level. 

 They rise in the form of pyramids or cones, with dense, 

 well-defined outlines, as white as snow. If they do not 

 meet with an upper current causing their tops to lean in the 

 direction in which it is moving, they rise perpendicularly, 

 and as they are broad enough even at their tops to lift up 

 before them a considerable mass of air, it sometimes hap- 

 pens that in reaching strata of air highly charged with 

 vapor it lifts them to a higher elevation, and causes a thin 

 streak of cloud to be formed at some distance above the top 

 of the columnar cloud. This streak so formed I have de- 

 nominated a cap. It is generally a little curved convex 

 above, and concave below, and as it moves slower upwards 

 than the columnar cloud, the latter overtakes it and passes 



