meafuring Heights with the Barometer . 659 
part of an inch, by means of a nonius moved with rack- 
work. A thermometer is placed near the ciftern, whofe 
ball heretofore, was ufually inclofed within the wood 
work, a defedt that hath been lince remedied. The three- 
legged Hand, fupporting the inftrument when in ufe, 
ferves as a cafe for it, when inverted and carried from 
place to place. Two of thefe barometers, after the quick- 
filver in them hath been carefully boiled, being fuffered 
to remain long enough in the fame fituation, to acquire 
the fame temperature, ufually agree in height, or rarely 
differ from each other more than a few thoufandth parts 
of an inch, which were conftantly allowed for in calcu- 
lating altitudes, as well as in eftimating the rate of expan- 
fion, in the courfe of the following experiments. 
SECTION I. 
. Experiments on the expanjion of quickjilver. 
THE experiments made for this purpofe were nu- 
merous as well as various, and were therefore fubdivided 
into feveral claffes. To give a minute detail of them all, 
would be extremely tedious, and now wholly ufelefs, 
hnce it was from thofe of the third dafs alone, that the 
" 4 O 2 . rate 
