THEORY DEDUCED FROM PHYSICAL LAWS. 31 



caloric of vapor, causes the air to occupy much more space 

 when it is imparted to the air, than when it is united with 

 water in the form of vapor. 



66. But as it is very desirable to know the exact amount 

 of this expansion of air by the latent caloric of vapor, and 

 also how much vapor at a particular dew point is condensed 

 by ascending into the air a given height ; and also how 

 high air of a given dew point will ascend before it forms a 

 cloud, let us see what information our experiments give us 

 on these interesting points. 



Now it appears from the experiments on dry air, that on 

 opening the cock, and letting the air expand into equilibrium, 

 it was cooled nearly one fourth the number of degrees to 

 which it had been heated after the cock had been closed. 

 And this law appears to be observed at all the different 

 temperatures. 



It appears, therefore, at least for small elevations, that 

 when air ascends, it becomes colder about l for every one 

 hundred yards : and it will also be found by calculation, 

 that the dew point falls about one quarter of a degree on 

 account of the greater space occupied by the air and vapor 

 for every hundred yards of ascent ; and from these elements 

 it follows, that when air ascends from the surface of the 

 earth on account of greater heat or greater moisture than 

 surrounding columns of air, it will begin to form cloud 

 when it rises about as many times one hundred yards as 

 the temperature of the air is above the dew point in degrees 

 of Fah. 



The method of obtaining this result will be understood 

 by the following example : suppose the temperature of the 

 air to be 70 and the dew point 60 suppose that a portion 

 of air at the earth's surface should rise ten hundred yards, 

 it appears by experiments, that it would sink in tempera- 

 ture, from the expansion due to diminished pressure, 12j 

 and as the barometer would at this height fall three inches 



