hygromerer. 



experiment was made under every favourable circumHance^ 

 and the aqueous atmofphere found at 22**. By problem li, at 

 page 13, it would have been determined at2l|°, ufing the 

 fecond column of grains in the table. 



On the fubjeft of evaporation it may be confidered as un- 

 pardonable not to advert to De Sauflure's valuable Eflays on 

 Hygrometry. 

 Experlmenrsof That excellent philofopher determined, by a well conceived 

 wnf are^"with experiment, that dry air of the temperature of 64° or GG'', 

 the preceding iinbibcd aqueous vapour fo as to increafe its elafticity -^ of the 

 theory. atmofpheric prefTure; and that a cubic foot of fuch air required 



II or 12 grains of water to produce the effect. By the table 

 above at page 12 it appears the force of vapour at 61°=. 54* 

 = -yL. of 29 .5 inches nearly. It is probable this difference is 

 occafioned in part at leaft by the want of perfe6l drynefs in 

 the air he operated upon, which caufed the increafe of elaffi- 

 He confided too city to be lefs than otherwife. It was, t think, unfortunate 

 much in is ^^^^ j^^ attached (d much importance to and confidence in his 

 i)ygrometer; and that he adopted the theory of chymical folu- 

 tion of water in air, contrary to the fads he difcovered, which 

 feemed more reconcileable to the notion of aqueous vapour be- 

 ing a diftinct elaflic fiuid. Indeed he is forced to acknowledge 

 in the ift. chap, of his Eflay on the Theory of Evaporation, 

 that in the ordinary temperature of the atmofphere, aqueous 

 vapoiir is formed in the firft inftance a diflant elaftic fluid, and 

 after it has been converted into an elafiic fluid, it is diffolved by 

 the air; " Je crois qu'il ne la diffout que lorfque I'acftion du 

 " feu I'a convertie en vapeur elaftlque.'* Now if it can for 

 a moment exift independently under the preffure of the atmof- 

 phere, why may it not contmue to exift in that ftate? 

 and on that ac- J-Jis table of the weight of aqueous vapour in a cubic foot 

 of air at different degrees of the thermometer, being derived 

 from experiments with his hygrometer, except the ftandard 

 one of 66° [Ij^'Reaumur), is far from accurate; and the in- 

 accuracy increafes with the diflance from the ftandard, whicb, 

 as has Ix^en obferved, appears to be nearly corre6l : in the 

 "higher temperatures he makes the water diffolved too little, 

 and in the lower temperatures too much. He fays (§ 93) that 

 the loweft he has feen the hygrometer in the open air, is 40; 

 and tliat it indicated a reduction of temperature in the air 



amounting 



3 



count his tables 

 are inaccurate. 



