Hygrometry . 403 
<c which extends in a fpace of few inches, dimini (lies rapidly, 
46 and does not interfere beyond that limit with the other laws 
4t of moijiure” 
82. 1 Avail now explain, by an example, what is the funda- 
mental deviation of an hair hygrometer , introducing in it, for 
a moment, thofe two Angular points 98 and 100, which, in 
M. be Saussure’s experiments, were a very natural caufe of 
miftake, A hair hygrometer and mine, being in a clofe vef- , 
fel, at a time when the temperature , fenfibly con flan t, fhall be 
but little above 32 ; if moijiure is firft introduced into that veflel, 
fo as to bring the hair hygrometer, by a very flow diredt motion, 
to 98, my hygrometer will flop between 70 and 75; and both 
inftruments will be fixed , if moijiure and heat remain the fame. 
Let moijiure then be made to increafe very flowly, till the hair 
hygrometer has attained its point 100 ; mine will have arrived 
at 80 ; and they again will remain at thofe points as long as, 
with the fame temperature , the fame quantity of vapour fhall 
remain in the veflel. Laftly, let a fufficient or fuperfluous 
quantity of water be introduced into the veflel, the hair hy- 
grometer will retrograde to 98, and mine proceed to ioo, at 
which points they will flop, whatever be the quantity of wa- 
ter ; and they will remain fixed, as long as the heat fhall not 
increafe. This explains the riddle of the lingular point 98, or 
of a certain point , various in different hair hygrometers, various 
even, at different times, in the fame individual, at which that 
inftrument {lands with very different degrees of moijiure ; confe- 
quently, its little motions round that point may create great 
deception, efpecially from the nature of organized fubftances, 
on which now I fhall add a few words. 
83. The above is the fundamental march of the hair hygro- 
meter, fuch as it is on the whole, and as it would be con- 
G g g 3 ftantly. 
