MEASUREMENT OF HEIGHTS. 527 



(in which case the moistened bulb is coated with ice), m = 'J15. 

 This formula is given by August, the inventor of the psychro- 

 meter, and rests on the comparison of experiments with certain 

 physical considerations*. 



The expression for cp t for different values of t is deduced by 

 August from observations on the pressure exerted by aqueous 

 vapour at different temperatures, employing a mean result, 

 which Kamptz has derived from the obsei'vations of Dalton, Ure, 

 Schmidt, and Aitzberger. But these, with the exception of 

 the two first-named series, are so little accordant with each 

 other, that it may be doubtful whether all the four should be 

 combined. I prefer to adhere to the expression already given, 

 which Laplace derived from Dalton's observations, to which 

 those of Ure approximate. It is my opinion generally that 

 formulae which are well known and extensively applied ought 

 not to be altered until the necessity for the alteration becomes 

 decided, which is by no means the case in the present instance. 

 The researches since made by Arago on the same subject were 

 confined to the elastic force of aqueous vapour at very high tem- 

 peratures, and we cannot be sure that a formula of interpolation, 

 which represents those satisfactorily, is applicable in much lower 

 degrees of temperature. 



By applying Laplace's expression, we obtain the pressure 

 exerted by the aqueous vapour contained in the atmosphere ac- 

 cording to the formula which has been already given. 



p, = 0-0064707 .10" ■''-'"'' - 0-0016562 ilnZil^; 

 ^ ' m — r^ 



and if we divide it by 



{p;) =0-0064707 . 10"^-'"", 



or by the pressure which the vapour would exert if the air were 

 saturated with it, we obtain the proportion denoted above by a, 

 thus: 



(13.) a = i2_: ; - 0-2457 * ^ ~ ^' 



jQOT — CT- m — Tf' 10"'' ~ '''^' 



To facilitate the calculation of a I subjoin a table, the first sec- 

 tion of which is for all values of t from — 20° to + 30°, con- 

 taining 



logl0<"-^''=/^, 



• Poggendorir Ann. der Physik, vol. Ixxxi. p. 69, and vol. xc. (xiv. of the 

 new ieriee), p. 137. 



