GEOLOGICAL SOCIETY OF PENNSYLVANIA. 



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the vapour, the air containing it is condensed also, as is 

 generally believed. 



This latter, however, is not the fact: for I find by 

 calculation that the quantity of latent caloric given out 

 by the change of vapour to water or cloud, is sujQicient 

 to produce an expansion in the air six times greater than 

 the contraction caused by the vapour turning to water. 

 This calculation is founded on these three principles, 

 which are all demonstrated by experiment. 1st. The 

 latent and sensible heat of steam is a constant quantity, 

 equal to 1212 of Fahrenheit. 2d. The capacity of at- 

 mospheric air is 250, that of water being 1000. 3d. 

 The expansion of air by heat is 1 -480th of the whole, for 

 every degree of Fahrenheit above its bulk at 32°. 



It follows from these facts, that whenever vapour, in 

 an ascending current of air, begins to condense into 

 cloud, there is an expansion of the whole mass of air as 

 far as the cloud extends, caused by the evolution of the 

 latent caloric of the vapour. Moreover, this evolution 

 of caloric prevents the air in ascending from becoming 

 cold as rapidly as it would by expanding if it was dry air. 



It is known that if dry air should be made to ascend 

 in the atmosphere, it would become one degree colder 

 for every 100 yards of ascent, so that at the distance of 

 ninety 100 yards high, it would be 90° colder than when 

 it left the surface of the earth. But if saturated air 

 should be made to ascend ninety 100 yards, it could not 

 sink in temperature even 45° without condensing a large 

 portion of its vapour; in some cases when the dew-point 

 is high, more than enough to heat it 45° above what it 

 would be by ascending to that height if no latent calorie 

 had been given out. 



It follows then, from these principles, that the higher 

 this air ascends, the more will the equilibrium be disturb- 

 ed, and that the equilibrium cannot be restored while 



