fnljijling In the Atmofphere. 253 



Vapour when this i;: \.\ a nafcent flate. But in heats above 

 167' or 170" airraid vapnar aredifpofed to feoarate. 



510, Hence ^^t inav deduce the impoffibihty of difcovering 

 a co-efiicient uriiverfally applicable to exprefs the rate of ex- 

 panfioii of air in every (late of moifturc, as Tremley has well 

 noticed. (See SaiiJJ'urt- ^ov. aux Alpes, ii. 4to.) This muft 

 vary with the mc.ui flalc of hvjromcters above and below the 

 heights to be meafured : and experiments of this kind have 

 not yet been nridc. De Luc's co-efficient anfwers tolerably 

 well for very dry air, that is, whofe faturabiiitv is greateft; 

 fir George Schuckburgh's, for air much nioiller; and gene- 

 ral Roy's, for air ftill nioift, that is, whofe faturability is 

 fmallefl:. Hence each fucceeds in certain cafes, and fails in 

 others. The dilatation or contra6lion which air faturated 

 with nioifture 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 difco- 

 vered : for Schmidt, who alone has attempted it, is juftly 

 diffident of the correclnefs of the table he has given of itj 

 and, in faft, it is not grounded on the indication of any 

 known hygrometer, and improperly fuppofes the 50th degree 

 to indicate the mean betwixt the loweft and faturation : 

 whereas the 65th degree on Sauflure's indicates that mean j 

 and 98, and not 100, indicates faturation. 



According to Mr. Watt (as ftated by De Luc, Mefeorolo- 

 g)', iii. p. 145), the fpecifjc gravity of pure vapour is to that 

 of air as 4 to 9. I fuppofe he compares it with air at the 

 ufual denfity of 30 or 29, and at fome particular temperature 

 which is not mentioned ; for at high temperatures the differ- 

 «nce muft be much greater, as appears by the foregoing tables. 



M. SauflTure (Hygrometer, p. 284,) has given us the fpe- 

 cific gravity, not indeed of pure vapour, but of vapour dif- 

 folvea in air, with more precifion ; for he tells us, imo. That 

 a cubic foot of perfeftly dry air has its volume augmented by 

 Tj':jth, when faturated with ten grains of moifture at about 

 65° Fahr. of heat, and barometer 2877 inches (Engliffi). 



ado. That a cubic foot of pure or perfectly dry air of that 

 denfity and at that temperature weighs 751 grains (French) j 

 and after dilTolving 10 grains of moifture, by which it is di- 

 lated s'^th, this new volume weighs 751 + 10 = 761 grains : 

 but a cubic fool of pure air, augmented by an acceffion of 

 I -4th of its bulk of pure air, would weigh 751 4- -i^'JS^ — 

 765 ^ains, lliat is 14 grains more. Hence he infers that, 

 in this cafe, the f|)ecilic gravity of the dilfulved moifture is to 

 that of dry air "as Jo to 14; for V^^th of a cubic foot in the 

 one cafe w«iehs ic grain j, and in the other 14 grains nearly. 



