-1859] WRITINGS OF JOSEPH HENRY. 267 



mountain, while a light cloud, which crowns its summit and 

 perhaps hangs over its sides, remains apparently immov- 

 able. The truth is that this cloud, which appears station- 

 ary, is in reality a succession of clouds constantly forming 

 and constantly dissolving. Every portion of air which as- 

 cends the mountain, tends (by its expansion and cooling) to 

 form a new portion of cloud, and in its descent tends (by 

 its condensation and increase of temperature) to dissolve a 

 similar portion. The cloud is consequently forming on one 

 side and dissolving on the other, and in this condition may 

 aptly represent the dynamical equilibrium of the human 

 body; which, by every expiration of breath is wasting away, 

 and by every pulse of the heart is renewed. 



What we have given may be considered as the more ob- 

 vious inferences from the first and simplest propositions of 

 Mr. Espy's theory. The phenomena as they occur in nature 

 however are more complex, and another effect is produced 

 by the upward motion of the air, which very essentially 

 modifies the results; we allude to the great amount of heat 

 which is evolved during the condensation of vapor into 

 water. We have stated that the heat evolved from the com- 

 bustion of 20 pounds of dry pine wood is absorbed by a cubic 

 foot of water at the ordinary temperature of the air in its 

 conversion into vapor, and it is evident that this vapor can- 

 not be re-converted into water without giving out to the sur- 

 rounding bodies an amount of heat equal to the combustion 

 of 20 pounds of dry wood. 



In order to give an idea of the importance of this prin- 

 ciple, which is an essential element in the theory of Mr. 

 Espy, it will be necessary to dwell somewhat longer on other 

 points before considering more minutely the results to which 

 it leads. 



Statical equilibrium of the compound atmosphere. — Before 

 proceeding to discuss the subject further, it will be necessary 

 to consider the question, which appears to be in a very un- 

 settled state, as to the effect of vapor in the atmosphere while 

 in the act of diffusion. On the one hand, the resistance 

 which air offers to the diffusion of vapor has been too much 



