Hygrometry. 4,05; 
tenths of a degree difference, from one experiment to another,, 
on its point of extreme moifiure , that anomaly cannot be of any' 
confequence on the determination of what muft be confidered aS'. 
that ftate in the medium ; but if it happens to the hair , which,, 
in approaching extreme moifture , has but very fmall motions,, 
it may reverfe thofe which it had had naturally (as I have ob~ 
ferved it fometimes) and become a caufe of deception. 
84. 1 have now explained, how mere accidental circum- 
ftances have been the caufe of a difference in the ideas that M* 
de Saussure and I had formed on what is to be under- 
flood by extreme moifture in every cafe; and I am going to 
illuftrate the whole of that fubjedt by a Angular fadh An 
hygrometer made with a box thread , or a thin fafcicle of 
the fibres of that wood, being placed in open air, next to a 
hair hygrometer, or to moft of the other inftruments of that 
fort, moves in a contrary way from them ; but we may lay 
a fide that circumftance, by fuppofmg, that the numbers , marked 
on the dial of the firft, are increafing in tf\ e oppofite diredioa 
from thofe of the other inftruments. Let us then fuppofe, that 
fome experimental philofopher had chofen the box thread for 
his hygrometer ; with him I fnould have fallen into no control 
verfy on the point of extreme moifture ; for, either under the 
moift vejfely or in any other cafe approaching extreme moifture 
his hygrometer would have moved like mine. But the box 
thread , at approaching extreme drynefs , firft relents much its 
pace, then becomes ftationary , and afterwards retrograde ; by 
which property, with the concourfe of fome accidental circum- 
ftances as have happened in M. de Saussure’s experiments, 
the very fame queftions that I have examined with fo much 
labour in refpedl of extreme moifture , only becaufe of the hair 
hygrometer, would have been tranfported to the point of ex- 
treme 
