28 PHILOSOPHY OF STORMS. 



cock was then opened and closed as soon as possible after 

 the mercury came to a level in the two legs ; the mercury 

 in the gage would then instantly begin to change its level, 

 and in a short time become stationary again ; the difference 

 of level was then measured, and the whole recorded in the 

 table under article 64. 



60. This was done both with dry air, and with air sat- 

 urated with moisture, with a view to ascertain the differ- 

 ence of temperature caused by the condensation of the 

 vapor. 



This difference of temperature would be indicated by the 

 difference of rise of the barometer gage from that which 

 took place in dry air allowance being made both for the 

 quantity of vapor discharged and condensed at the moment 

 of turning the stop-cock and letting the air in the urn ex- 

 pand ; for it is manifest that this same quantity would be 

 again evaporated when the air became heated to its original 

 temperature. Now, it will be seen, that when dry air was 

 used, the cooling due to expansion was about one degree 

 for every four and fifteen hundredths that the air had been 

 heated : but when moist air was used at the temperature of 

 about seventy-five, the cooling effect of the expansion was 

 only one degree in six, and $ even without allowing any 

 thing for the rise of the gage by the vapor, and as this is 

 more than one third of the whole at the temperature em- 

 ployed, the real depression of temperature in this case is 

 only about four ninths of that in dry air. 



At the temperature of about 100, which was tried though 

 not recorded, the result was equally striking and confirma- 

 tory of the principle. Here the rise of the gage after open- 

 ing and shutting, was about one in five of the original 

 height, though one half of this effect is due to the vapor, 

 as will be seen by examining a table of dew points, (129) ; 

 and, therefore, at this temperature, the cooling effect of the 

 expansion of moist air when compared to that of dry, is 

 about as four to ten. 



