C 6 °i ] 
that the bar, which makes the balls of the inftrument, 
Shall in each experiment fufFer the fame degree of 
heat, as the bar to be meafur’d : Of confequence, the 
meafures taken by the micrometer are the differences 
of their expanfion. The expanlion then of the balls 
between two given degrees of heat being once found, 
the abfolute expanfion of any other body, by adding 
or fubtrading the difference to or from the expanlion 
of the balls, according as the body to be meafur’d 
expands more or lefs than the bads, will alfo be de- 
termin’d. 
When the inftrument is made ufe of, it is immerg’d, 
together with the bar to be meafur’d, in a cittern of 
water j which water, by means of lamps apply’d 
underneath, is made to receive any intended degree 
of heat, not greater than that of boiling, and thereby 
communicates the fame degree of heat to the in- 
ftrument, the bar, and to a mercurial thermometer 
immerg’d therein, for the purpofe of ascertaining 
that degree. That this may be truly the cafe, the 
water Should be frequently ftirr’d, that there may be 
no difference of heat in the different parts of the 
water: This being done, the height of the quick- 
filver appearing ftationary, the contad with the ferew 
of the micrometer alfo remaining the fame, for a 
fpace of time, it is to be Suppos’d, that the heat of 
the three bodies will be the fame, as the heat of the 
water, however different they may be in fpecittc gra- 
vity, &c. The whole difficulty is now reduc'd to this 
problem, viz. 
T p find the abfolute expanfion of the bafiis between 
any two given degrees of heat , not greater than that of 
boiling water. 
+ G 
For 
