78 MB. W. N. SHAW ON HYGROMETRTC METHODS. 



from the air and determining the weight of aqueous vapour thus absorbed by the 

 tubes. Then, if f be the amount of moisture per cubic metre in the air which enters 

 the tubes ; A, the density of dry air at C. and 760 mm. pressure ; d, the specific 

 gravity of aquecus vapour referred to dry air at the same temperature and pressure ; 

 t, the temperature of the entering air ; a, its coefficient of expansion per degree 

 Centigrade ; e, the pressure of the aqueous vapour, then 



In order to determine the volume, v, of air which enters the tubes, a known volume 

 of water is allowed to flow out of an aspirator, its place being taken by the air which 

 passes through the drying tubes. Let V be the volume of water which is allowed to 

 run out of the aspirator, this volume will then be occupied by saturated air, the dry 

 part of which has passed through the drying tubes ; if T be its temperature when in 

 the aspirator, E the corresponding pressure of aqueous vapour, the pressure due to 

 the dry air will be B E, where B is the height of the barometer at the time. Its 

 pressure before entering the tubes was B e, and its temperature was t ; its volume, 

 therefore, was 



T , B - E 1 + a t 



V = V - - - 

 B-e 1 +T 



Hence, if w be the number of grammes of moisture absorbed, 



w B - e 1 + T 



= V'B -E' i + < ' 



and we get for determining e the following equation : 



_ 760 (1 + a. t) w B - e. 1 + aT 



Arf V B -E 1 + t 



or 



e 760 1 + a T w 



(2) 



B -e " B -E V Ad 



I + T) w 



A d (B - E) V 



= x 



therefore 



or 



e = 



approximately, neglecting squares not likely to be larger than 



( E - X wfV\ 



\ ~* ~B / a PP roximate 'y- .iff,-. " ", : :- (4) 



