[ 4.16 J 
31. Prepare after this a brafs cylinder, about 3 A 
inches long, and to one of its extremities fix the 
pully proper to receive the firing of the bow when 
the piece is turning. This cannot be done too care- 
fully, both to make it perfectly round, and to fit it ex- 
actly to the hole of the ivory tube ; its extremity 
muft even be rounded, that it may be applied clofely 
to the bottom of the hole. Having then roughly pre- 
pared the outlide of the ivory tube, and introduced 
into it the brafs cylinder, put both pieces thus united 
upon the turning wheel, and find out on the outlide 
bottom of the ivory tube, the point which anfwers to 
the axis of the brafs piece, in order that this may turn 
exactly upon its axis. It is with this view that the 
brafs cylinder is made longer than the ivory tube. 
32. All thefe precautions are defigned to make 
the fides of this tube of an equal thicknefs, viz. JL. of 
a line, except at the two extremities. At the bot- 
tom b the tube ends in a point, and at the top a a, it 
mufl; for about two lines be left a little thicker, in 
order to enable it to bear the preflure of another 
piece, which is to be put into it. Thus the thin or 
hygrometrical part of the tube will be reduced to 
2'L French inches, including the concavity of the 
bottom. 
33. Before this piece is ufed, put it into water fo 
as that the external part alone be wetted by it, and 
leave it there till the iniide be every where covered 
with the dew I mentioned before (25). This will 
take place in a few hours; I have given the reafons 
for this operation (28). 
34. The glafs tube intended for this hygrometer 
muft be about 14 inches long. Its lower end is feen 
in 
