336 On the Hygrometer. 



In deducing the correction for difference of pressure, the 

 writer has been led to adopt views regarding the specific heat 

 of air, as affected by pressure, differing from those entertained 

 by Mr Meikle. He has been influenced in doing so by the con- 

 sideration_, that while the experiments of Clement and Desormes 

 are opposed to the opinion that the specific heat of a given vo- 

 lume of air is directly as the density, they corroborate the law 

 theoretically deduced by La Place. 



With reference to the experiments made by Professor Da- 

 niell and Mr Meikle^ for the purpose of determining the varia- 

 tion of D with the pressure, it seems evident, that when a ther- 

 mometer with a wet bulb is placed in the receiver of an air- 

 pump, the air cannot be perfectly dry, even when exposed to 

 the action of sulphuric acid ; for the very circumstance of there 

 being a portion of moisture absorbed at the surface of the acid, 

 proves that it pervades also the rest of the receiver. Supposing, 

 therefore, in the two experiments mentioned by Mr Meikle at 

 p. 106, of his paper, t" = — 14° and/_j^, = .031, then the va- 

 lue of D, calculated by means of the formula 



(.22255 — .031) - gggy^g ^'faaTe? .~ ^ ^°-^^' 1st experiment; and 



(.22255 - .031) X .S+S x JCrd = "" •^' "'^ ^"- 

 periment ; the errors being -|- .08 and -|- .8. It appears from 

 the above formula how much the value of D may be affected 

 by the presence of a portion of vapour probably too small to 

 be detected by means of DanielPs hygrometer. 



The author is still unable to show that the modification of 

 the dew-point formula which he proposed, is theoretically de- 

 ducible from the data. However, as one of these data is only 

 hypothetical, namely, the supposition that the air in contact with 

 the wet bulb is fully saturated, he would venture to propose in 

 its stead the condition, that the saturation of the air is not alto- 

 gether perfect, but that, for the same value of D, it is more and 

 more complete, as the temperature of the air increases. It is 

 easy to see, from a consideration of Sir James Ivory's formula, 

 that, on such a hypothesis, the value of D would need to be 



