t 42* J 
43* I follow the laft fteps of the mercury in its 
fall, by means of a fine filken thread fixed very tight 
around the tube. This is left at the loweft point it 
has been brought to. If this point be too low, rela- 
tively to the frame of the hygrometer, frefh mer- 
cury is poured in, and the thread proportionally 
drawn up higher ; if too high, I take off fome of 
the mercury and lower the thread ; and in both 
cafes make ufe of the horfe hair. This mud be 
done when the mercury ceafes to fall, in order that 
the place where the thread is to remain may be im- 
mediately determined by this operation. 
44. This point thus fixed is named o in my hy- 
grometer; it is that in which drynefs is nothing 
(if I may be allowed to exprefs myfelf fo), fince it is 
that of extreme humidity, in a given heat ; viz. that 
of melting ice. From this point are-reckoned all 
the degrees I am now going to fpeak of ; which 
thus become degrees of exficcation. 
45, The laft eflential operation is that by which 
the fize of the hygrometrical degrees are deter- 
mined ; and this I fhall defcribe by an example. It 
muft be remembered that the hygrometer’s tube was 
originally a thermometer (23). I take it in this firft 
ftate, in the inftance I am going to give. The dis- 
tance between the thermometrical points of melting 
ice and boiling water, at twenty -feven French inches 
of the barometer, was found to be 1937 parts of a 
certain fcale. I broke the bulb of this preparatory 
thermometer, in a bafon, in order to receive care- 
fully all the mercury- that it contained. This be- 
ing weighed in nice fcales, amounted to 2 on. 1 1 dr. 
I i i 2 1 2 gf 
