+ 5W X — [ X , 



Oa the Hygrometer by Evaporation. 85 



pour at the temperature t— St, instead of the original cubic 

 foot of mixed air, viz. 



240 X -I— X-^ + 5w X -^. (A) 



I -Loir 30 l-fmr * ' 



b — X 

 i+mr " 30 



the first term only being air, and the rest vapour. 



The air at the reduced temperature t— 8t, and with its 

 new accession of humidity, is in a state of saturation. Its 

 tension g will therefore depend only on the temperature 

 T— St; insomuch that when one of these quantities is given, 

 the other may be thence found by means of the Tables and 

 fonnulas usually given in this part of natural philosophy. 

 Now at the temperature' t — St, and under the pressure b, 

 the weight of a cubic foot of air mixed with vapour at the 

 tension g, will be equal to, 



240 ro X . , / , , X -^ + 5X0 X . , / , -„ (B) 



the first term being air, and the second vapour. 



But, in any two parcels of the same air, there must be the 

 same proportion between the two constituent parts. I there- 

 fore divide the air in the expression (A) by the air in the ex- 

 pression (B); and likewise the vapour of the former expres- 

 sion by the vapour of the latter: and as the two quotients 

 must be equal, I get this equation, viz. 



b — x 

 30 



h 48a.Jf 



so' X 



b — f 5 J 



which, by the usual reduction, becomes 



Now in the 4'th vol. p. 726, of Biot's Traite de Physique, I find 

 a = -2669 = ,V nearly, 5 = '847 = |5 nearly, and, at p. 713, 

 X = 550 : and, by substituting these numbers, it will be found 

 that the coefficient of x is very nearly equal to that of g: the 

 same coefficients also are both little diflerent from unit, because 

 OT is always only a small part of X: and, by means of these 

 considerations, the foregoing equation may be simplified with- 

 out detractuig almost in any degree from its exactness, viz. 



■'•=?- To ""-T- (^) 

 Furtlicr, because --- is in most cases very nearly c(]ual lo unit. 



30 



get 



? r-' (-) that 



