[ 423 ] 
naturally to be determined by the fimplicity of a pro- 
portion between the degrees of the thermometer, and 
thofe of the hygrometer, becaufe this laft was to be 
corrected by the firft, on account of the effects of 
heat (24). My firft thought was to divide this line 
into 80 parts, agreeably to the divifions of the fun- 
damental interval in what I call the common ther- 
mometer in my book upon the air, which I (hall al- 
ways be underftood to mean in this paper. But as 
the minutenefs of thefe degrees was found to be in- 
convenient and fuperfluous, I determined to make 
them double, by putting only 40 in the length of 
my hygrometer’s fundamental line. It is eafily un- 
derftood that thefe degrees, thus fettled, begin to be 
reckoned from the place of the thread, which indi- 
cates upon the tube of the hygrometer extreme hu- 
midity, by the heat o of the common thermometer , or 
of melting ice. 
49. The inftrument with its frame is feen fig. 2. 
the dimenfions of which are every way one half of 
thofe of the original. It is mounted on deal, that 
being the wood, which fuffers the leafi: change 
in the length of its fibres. The lower part of the 
frame is flit through the whole length of the ivory 
pipe, in order that the air may circulate freely round 
this pipe, and the bulb of a thermometer which I 
fhall mention prefently. The hygrometer is faftened 
in three parts; viz. at bottom on a fmall bracket, 
at top by a tube paffing through a piece either of 
hard wood or of metal faftened by fcrews ; but 
chiefly by means of a brafs wire on the neck of the 
brafs piece, which unites the glafs with the ivory 
pipe. This piece is laid in a fmall plate of a hard 
wood. 
