[ 299 T 



Of faturation, or of. any approach towards it, no fatisfadory 

 knowledge could be obtained previous to the difcovery of a com- 

 parable hygrometer by Mr. SaulTure ; an invention which perhaps 

 is equal in importance, and confidering it conjointly with the 

 inveftigation of the numerous incidents to which it gave birth, 

 was attended with greater difficulties, and required more fagacity,, 

 than any difcovery, (thofe of Scheele excepted^ made during the 

 courfe of- the laft century. 



This inftrument confifts of a (ingle human hair, duly cleanfed,- 

 and kept in a ftate of tenfion by the weight of a few grains, 

 and rolled on a pulley to whifch an index is fixed. The hair is 

 lengthened by moiflure, and the index drawn up; it is contraded 

 by drynefs, and the index drawn down ; the weight prevailing 

 in the firft cafe, and the elaflicity of the hair in the fecond cafe : 

 heat alfo lengthens the hair, but as it alfo promotes the evaporation 

 of the moifture adhering to the hair, it by that means, upon the 

 whole produces a flill greater contradion, and thus makes it 

 [indicate fuperior drynefs. 



FiXED^ points both of the maximum of deficcation, and of the 

 \maximum of moifture, that is faturation therewith, were accu- 

 jrately fettled, and even fome degrees of fuperfaturation noted. 

 [The maximum of deficcation is not undoubtedly the true maximum -y 



yet 



