THEORY DEDUCED FROM PHYSICAL LAWS. 27 



the floor from a stove below. These last were made in 

 Professor W. R. Johnson's parlor, and confirm the accuracy 

 of this manner of finding the specific caloric of air so as to 

 leave no doubt on the subject. The mean of all the experi- 

 ments is 0.259. The table is not without interest, also, as 

 a specimen of the hygrometric state of the air in our climate. 

 The reader will perceive that these tables of dew points 

 and wet bulbs were made previous to Professor Apjohn's 

 formula reaching this country, so they could not have been 

 got up for the purpose of sustaining a theory. 



58. The formula I have used in calculating the specific 



caloric of air is a w ~' e , a being the specific caloric of air, 



w and w' the respective weights of vapor in the atmosphere 

 when saturated at the temperatures of wet bulb and dew 

 point, and d the difference of the temperatures of air and 

 wet bulb when in a brisk current of air, the weight of 

 atmospheric air being taken at unity, when under mean 

 pressure, for I have not found that changes in the barometer 

 affect the question, though I instituted a very extensive se- 

 ries of observations with that view. And e is the latent 

 caloric of vapor at the wet bulb temperature, which I have 

 assumed equal to the difference of the wet bulb and 1212. 



59. Second. I instituted a series of experiments to ascer- 

 tain, independent of chemical laws, whether air is more 

 expanded by the evolution of latent caloric when a portion 

 of its vapor is condensed into water than it is contracted by 

 that condensation. The result, it will be seen by the table 

 below, is abundantly confirmatory of the theory. 



I took a copper vessel containing about a gallon, furnish- 

 ed with a stop-cock, and bent tube mercurial gage. I 

 transferred this vessel with the stop-cock closed from one 

 temperature to another, and carefully waited till the gage 

 became stationary. I then measured the height that the 

 mercury stood in one leg above that in the other ; the stop- 



