563 



/ 4- fl 



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 



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

 to a temperature i'\ becomes 



461 4- /" 



and if saturated with vapour at this temperature, the tension 

 of such vapour being*'', it will become 

 461 + i" ir 



V X TTTrz 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 461 ±< 461 ±/ ir 



V X 77T-r- X X -7771 T, = f X ,^„ X 



493 TT-s" AQ\+i" 493 tt-s"' 



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. : 



, P -f" 

 m X log. i-; — ^,, 



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

 plained, the barometric formula finally becomes 



n — f" 461 ±^-T- 



D = wj X log. '] ^ „ X , ,,., X 



& P'—f" 493 



^ip-n X ip'-r") 



^ip-f") X (/,'-F'0- i(/"+F") 



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

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



