[ 3o6 ] 



'To form this table from adual experiment from the XL. of the 

 hygrometer to XCVIII. and from the 9th of Fahr. to gg'', would 

 , require 5220 experiments, of the niceft and moft delicate kind. 



Saussure has given various other ufeful tables, for which I 

 muft refer to his treatife ; but he has given one, to the principle 

 of which I cannot entirely affent. He is of opinion, that the 

 menflrual power of air diminiflies with its denfity, § 146; and 

 hence infers, that its power of receiving vapour is alfo diminifhed 

 when it is rarefied. This lall point, however, is contradidted by 

 the quick evaporation that takes place in vacuo and on lofty 

 ^mountains. 



His own experiments feem to me to prove this increafe of the 

 receptive power of air when its denfity was diminilhed ; for he 

 conftantly found his hygrometer to move to drynefs during every 

 •fucceffive exhauftion, and permanently fo, when the accefs of new 

 vapour was prevented, § 145. Hence it appears, that the greateft 

 part of the moiflure, originally contained in his hygrometer, being 

 no longer confined by the denfity of the ambient air, gradually 

 evaporated. 



Mr Pictkt's experiments on the manner in which hygrometers 



are affected by mere vapour excite much furpriarc, and deferve 



much attention. EfiTais de Phyf. §111. 



Be 



