[ 344 ] 



But in heats above 167* or 170^ air and vapour are difpofed to 

 feparate. 



50. Hence we may deduce the impoffibility of difcovering a 

 coefficient univerfally applicable to exprefs the rate of expanfion of 

 air in every ftate of moifture, (as Tremley has well noticed. See 

 2 Sauffure voy. aux Alpes 4^^). This muft vary with the mean 

 flate of hygrometers above and below the heights to be meafured; 

 and experiments of this kind have not yet been made. De Luc's 

 coefficient anfwers tolerably well for very dry air, that is whofe 

 faturability is greateft, Sir George Schuckburg's for air much 

 moifter, and General Roy's for air fiill more mcift, that is whofe 

 faturability is fmalleft. Hence each fucceeds in certain cafes, and 

 fails in others. The dilatation or contradion, which air faturated 

 with moifture at any one given degree of temperature receives 

 without the addition of any more moifture, at any higher or 

 lower degree of temperature, has not as yet been difcovered ; for 

 Schmidt, who alone has attempted it, is juftly diffident of the 

 corrednefs of the table he has given of it, and in fad it is not 

 grounded on the indication of any known hygrometer, and im- 

 properly fuppofes the L. degree to indicate the mean betwixt 

 the loweft and faturation. Whereas the LXVth degree on Sauf- 

 fure's indicates that mean j and XCVlil. and not C. indicates 

 faturation. 



According 



I 



