122 



MR. W. N. SHAW ON HYGROMETRIC METHODS. 



The differences thus tabulated lie all in the same direction, the observed weight of 

 moisture being in each case too small. They may be accounted for in four different 

 ways : (1) the pressure of aqueous vapour in the saturated air may be slightly less 

 than that given by the table of pressures in vacuo ; (2) the density of vapour in 

 saturated air may be less by about 1 per cent, than the theoretical density ; (3) the 

 temperature readings of the saturated space may be slightly inaccurate, errors in the 

 temperature readings of from 0'06 to 0'15 C. being sufficient to account for the 

 differences ; (4) the differences may be due to the incidental errors of the experiments. 

 The irregularity in the numbers for different temperatures tends rather to show that 

 either of the first two suggestions is insufficient completely to account for the 

 observations, and there is evidence that suggestion (2) is directly contradicted by 

 experiment.* The amount of thermometric error required in each case to account for 

 the differences is very small, and REGNAULT gives no details as to the method of 

 correcting the thermometers he employed in the experiments, but his known familiarity 

 with thermometers of every kind and their errors makes it highly improbable that 

 he could have overlooked these in the case before us. The precautions adopted in 

 order to secure the observations against the possible sources of error that are suggested 

 by the arrangement of the apparatus are not definitely stated. 



A table of results .of some experiments to determine by the same method the 

 specific gravity of saturated steam at temperatures above that of the air of the room 

 is also given, but their author does not regard the result as satisfactory. t 



The second part of the memoir considers the various methods employed to determine 

 the fraction of saturation of the air. The chemical method is passed over, as being 

 fully discussed in the previous part of the memoir. 



With regard to the behaviour of DE SAUSSURE'S hair hygrometer, REGNAULT set 



* For another suggestion as to the cause of the difference based upon my own observations, see 14 

 (ante). 



f REGNAULT sums up this part of his work in the following words : " Quoi qu'il en soit, on voit qn'en 

 prenant pour base ma table des forces 61astiques de la vapeur aqueuse dans le vide, et admettant que la 

 densite de la vapeur est constamment dgale a 0'622, celle de 1'air dans les memes circonstances etant 1, 

 le poids de la vapeur d'eau caleule ne pent differer de la quantite reelle que d'une fraction tres petite, un 

 centieme environ." 



