[ 4-17 ] 
in dd e e (fig. i.). Its internal diameter is about *, 
of a line* The reafon why it fhould not be fenfibly 
lefs will be given hereafter (5 2) ; and if it was fen- 
fibly larger, the variations of height in the mercurial 
column would not be confiderable enough. On the 
dimenfions that I propofe, when the hygrometer is 
put into melting ice, in a fine fu miner day, the mer- 
cury falls about fix inches in the tube. The outfiae 
diameter of this tube fhould be about two lines, in 
order that the part g g of a brafs piece through which 
it pafies, and which is to enter into the ivory pipe, ba 
as thin as pofiible. 
35. The glafs tube, as I faid before, fhould have 
belonged to a thermometer. Its extremity widens 
of courfe towards the ball ; which will be of ufe, 
when the mercury is poured into the hygrometer, in 
order that it may drive the air before it, by riling 
from the ivory pipe into the glafs tube. To preferve 
this widening, break the ball of the thermometer by 
firiking againft the bottom ; and with pinchers take 
off the reft little by little, and make the extremity 
cylindrical by grinding it upon the wheel. The fame 
muft be done at the top, which I fuppofe to have 
been made to end in an olive or final! refervoir for the 
filling of the thermometer. This widening is like- 
wife to be faved for the reafons hereafter to be men- 
tioned (52). 
36. The piece /’ f gg is intended to join the ivory 
with the glafs tube. It is of brafs, fhaped as in the 
figure. A cylindrical bole is bored through it, which, 
holds the glafs tube as tight as pofiible, without dan- 
ger of breaking it ; and its lower part is to enter with, 
fom.e degree of difficulty into the ivory pipe. 
37 - T° 
