254 



On the State of Vapour 



But I flronffly fufpeft that the origuial experiment, on 

 ■tthich this caTciilation is founded, is erroneous, chiefly by 

 realon of the ftrong adherence of moifture to cold glafs, as 

 will hereafter be feen in treating of dew. From Schmidt's 

 experiments it may be inferred that the fpecitic gravity of 

 ■vapour, difTolved iu air at this tempcratnre, is much lower 

 with refpecl to that of pure air than SautVure has ftated ; for 

 he tells us that about io65 mcafures of dry air in tempera- 

 ture 65°, would, if faturated with moifture at that tempera- 

 ture, occupy the fpace of about 1100 meafures, and confe- 

 quentlv receive an augmentation amounting to about ^A of 

 their bulk : now, transferring this ratio to the cubic foot in 

 SauflTure's experiment, it appears that y\d of a cubic foot thus 

 added to the cubic foot of dry air weighs 10 grains; but a 

 cubic foot of dry air, augmented by an accellion of v\d of 

 limilar air, would weigh 751 + 23*46 grains, which ap- 

 proaches nearly to Mr. VVatt's ratio: therefore the fpecific 

 gravity of vapour dilfolved in air at this temperature is to that 

 of perfertlv dry air as 10 to 2y^ nearly. It fliould however 

 be recolleded that M. Saufliire found that a cubic foot of 

 drv air in reality took up i]'o69 grains of moifture when 

 laturated at this temperature, and that it was only by way 

 of conceilion to thofe againfl whom he argued, that he ftated 

 the weight taken up at 10 grains; then we Ihould have of 

 11-069 to 2i*i95, or, in round numbers, as 11 to 21, or 10 

 to 19. And it Ihould further be remarked that the tempe- 

 rature is given very loofely, for it is ftated to be from 14 to 

 j^*^ or 16"^ of Reaumur. See Hygrometer, p. 104 and 284. 



Saufture has given us a table, Ijy the help of which the 

 abfolute quantity of vapour at any barometrical height, in a 

 cubic foot of air, being known, the proportion and abfolute 

 quantity in a cubic foot, at another barometrical height ^'6 

 inches lower, may be known from the mercurial height 28-77 

 to that of three inches and one- half nearly. 



This table I here give, adapting it to our meafures. 



Thus, fuppofing the abfolute quantity 

 of diflblved vapour at any temperature, 

 and barometer 28-77 to be 10 grains 

 per cubic foot; then the quantity of 

 vapour at a height at whicii a baro- 

 meter would ftand at 25* 17 inches 

 woiJd be 10 X 0-9.528 = 9*528, and 

 at the height at which a barometer 

 would ftand at fcven inches, the quau- 

 titv in a cubic foot would be only lo 

 But iUU it is fuppofed that at thofe great 



heights. 



