C 418 ] 
37. To hinder the part of that tube, which en- 
closes the brafs piece, from being affefted by the 
variations of the humor, which might fometimes 
prevent a Sufficient preffure, I cover this part of the 
tube with a brafs.verrel, reprefented in h h i i. It 
muff enter with force, and will henceforth be con- 
fid ered as part of the ivory pipe. 
38. To unite thofe pieces together, I make ufe of 
gum lac, or of maflich, which melts by the heating 
of the glafs and the brafs. I firft cement the brafs 
piece with the glafs tube by introducing the tube, and 
leaving it at firft at an inch diftance from the place 
where it is to be fixed •, I then' hold this end of the 
tube over live coals, by bringing it nearer and nearer, 
and turning it, that both that and the brafs piece be 
every where equally heated ; and when they are hot 
enough to melt the gum lac, I rub the glafs tube 
with it, and pufh the brafs piece to its place by means 
of a hollow bit of wood, drawn beforehand over the 
tube for this purpofe. As the brafs piece advances, 
the lac accumulates towards the end of the tube; I 
take away the fuperfluous part, but leave a flight 
coat of it over the end of the brafs piece, in order to 
prefer ve it from the contact of the mercury that 
might corrode it. When this piece is properly 
placed, and ftill warm, I cover with lac its cylindrical 
outlide, and introduce it into the ivory tube, which 
has been fomewhat warmed by holding it near the 
fire, in order that the lac may flick more clofely 
£0 it. As foon as theie pieces are cold, they are 
found very ftrongly cemented together, and neither 
mercury nor water can make their way between 
them. 
39. The 
3 
