OF TEMPERATURE IN THE ATMOSPHERE. 131 



and shows tlie height^ dx, that must be reached to get a 

 lowering of temperature, —dtj when air saturated with 

 moisture ascends. The pressure, p, is taken as 2 1 1 7 lbs. 



per square foot ; and the value of -, which is the same for 



if 



the same pressure, whatever is the temperature, is — ~ . 



274 



The results, for temperatures from 0° to 35° Cent., are 

 exhibited in the last column of the Table. For the tem- 

 peratures 0°, 5°, and 10°, they agree very well with the 

 height in which Mr. Welsh found a lowering of tempera- 

 ture of 1° Cent. ; and we may conclude that at the times 

 and places of his observations the lowering of temperature 

 upwards was nearly the same as that which air saturated 

 with moisture would experience in ascending. 



It is to be remarked that, except when the air is satu- 

 rated, and when, therefore, an ascending current will 

 always keep forming cloud, the effect of vapour of water, 

 however near saturation, will be scarcely sensible on the 

 cooling effect of expansion. Hence the law of convective 

 equilibrium of temperature in upward or downward cur- 

 rents of cloudless air must agree very closely with that 

 investigated above, and must give a variation of 1° Cent, 

 in not much more or less than 330 feet. 



It appears, therefore, that the explanation suggested by 

 Dr. Joule is correct ; and that the condensation of vapour 

 in ascending air is the chief cause of the cooling effect 

 being so much less than that which would be experienced 

 by dry air. 



k2 



