563 



f I a 

 tion. The mean temperature its usually taken as — - — , and 



this must be very nearly its true value. For the same reason, 



the mean force of vapours may be set down as' ^ — ; and 



let us assume the mean value belonging to the pressure as 



^ip -f") X {p - V"). 

 Now a volume v of dry air at 32° under a pressure tt, if raised 

 to a temperature t", becomes 



461 + t" 

 '' ^ 493 ' 

 and if saturated with vapour at this temperature, the tension 

 of such vapour being*'', it will become 



461 + t" TT 



V X 77-:; X 



493 TT - s" 



This is the volume of the air when raised to t" and satu- 

 rated with vapour at this temperature ; and if this volume 

 of air have its temperature further changed, we shall say to t, 

 then its bulk will be represented by the expression 



461 +t" TT AQ\±t ^ 461±^ TT 



493 7r-«" 461+^" 493 tt-*" 



substituting, then, in this expression instead of v the value 

 of the length of the column of air between the two sta- 

 tions supposed dry, and at 32°, viz. : 

 1 P -f" 



and for tt, t, and s" their proper mean values as already ex- 

 plained, the barometric formula finally becomes 



\/{p-n X ip'-^") 



I may add here, that the correction for moisture is far 

 from being insignificant in its amount, as may be seen by 



