SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 399 



initial temperature may have been. Conversely, a descending current 

 of air grows warmer at the rate of 1° O. per 100 meters. 



If the rising air contains aqueous vapor, and if the latter is not con- 

 densed because the air remains relatively dry, and its ascent has not 

 continued sufficiently high, then will the diminution of temperature be 

 somewhat slower, since the specific heat of aqueous vapor (0.4S05 accord- 

 ing to Eegnault) is somewhat higher than that of the air, but to so slight 

 an extent that we can entirely neglect this retardation, and be entirely 

 satisfied with the above adopted value of 1° C. per 100 meters. The 

 weight of vapor contained in one kilogram of moist air is at the atmos- 

 pheric temperatures so slight in comparison with the weight of the air, 

 that the somewhat greater specific heat of the damp air can exert but 

 little influence. 



If 2 represents the weight of aqueous vapor contained in a kilogram 

 of moist air; 



1 — q = the weight of the dry air: 



c' = the specific heat of the moist air, will be given by the expression 



c' = 0.2375 (1-2) + 0.4805 q = 0.2375+0.2430 q. 

 The value of q, which always remains a very small fraction, is found 

 from the following equation : * 



or very nearly 



<Z = 0.623!.; 



where e represents the tension of vapor, and ^9 the total pressure or height 

 of barometer when both quantities are measured by the height of a 

 column of mercury. For instance, if the ascending air has at 30^ C. a 

 relative humidity of 60 per cent., for which e = 18.9 millimeters, then 

 will q = 0.01564 ; whence c' = 0.2413, and the quotient 



~ = ~= -0.009751. 



dh J (J 



The change in temperature is therefore, for every 100 meters, only 0^.016 

 smaller than for dry air. The entire error introduced by extending the 

 computation to 900 meters above which condensation follows amounts 

 therefore only to 0^.14 C 



We may be allowed here to add some further remarks upon the rela- 

 tions between the temperature and pressure of the air. The formula 

 previously found gives directly the change of temperature as dependent 

 upon the change in altitude of a mass of air ; we will now seek for a 

 relation which expresses the change of temperature dependent upon a 

 change of pressure. From the fundamental equation first laid down, it 

 follows directly, if dQ = 0, that — 



dp~cJ^ 2) -'^-23-, 

 if for i?, c, J, the values already given bo introduced, and for T the value 



