3 y 0 Mr. de Luc on 
6 .. The fundamental procefs of M. de Saussure, with the 
view- of difcovering the effedts of moijlure on the hair hygro- 
meter, was this. He repeatedly caufed fucceffive known quan- 
tities of water to evaporate into a clofe glafs veffel, previoufly 
■reduced to extreme drynefs, and containing that hygrometer and a 
manometer ; he obferved the correfpondent changes of thofe in- 
struments, and, by combining the refults of his experiments, 
he reduced to regular feries the correfpondent motions of the 
two inftruments by equal quantities of evaporated water. 
Having confined himfelf to that only clafs of experiments, 
which, from caufes that 1 fhall explain, could not difeover to 
him the difficulties of his attempt, he thought himfelf war- 
ranted to draw from them the following conclufions. iff. 
That the degrees of moijlure in the inclofed medium, were nearly 
proportional to the quantities of water evaporated in the veffel ; 
and that, confequently, the ratio obferved between thofe quan- 
tities and the march of bis hygrometer, could be confidered as 
giving immediately the march of the inftrument correfpondent 
to moijlure itfelf; which, according to our common opinion, 
is a certain quantity of aqueous vapours fpre'ad in the medium , 
2 dl y , That when no more water could evaporate mthe veffel, 
the inclofed medium was arrived at extreme moijlure ; and that, 
confequently, the point indicated at that time on his hygro- 
meter, was to be the limit of its Jcale on that fide. 3<dly, That 
having, from thofe experiments, a probable, determination of 
the expanfons of the hair by fucceffive equal quantities of moif- 
ture, in beginning from the point were this is. null, and ending 
nt its extreme, his inftrument could not differ eflentially from an 
abfolute hygrometer. 
64. Thefe conclufions were very natural in the ftate of M. 
de Saussure’s experiments ; but before their publication I had 
gone 
