&is Thermometer with the common mercurial ones. 361 
this canal, ‘be afterwards diminithed in its bulk by ‘fire, as 
the thermometer’ pieces are, it will then pafs further in the 
canal, and more and more fo according as the diminution is 
greater ; and converfely, that if a body, fo adjufted as to pafs 
on to the narrow end, be afterwards expanded by fire, as is the 
cafe with metals, and applied in that expanded ftate to the fcale, 
it will not pafs fo far; and that the divifions on the fide will be 
the meafures of the expanfions of the one, as of the contrac- 
tions of the other, reckoning in both cafes from that point to 
tie the body was adjufted at firft. 
Tis the body whofe alteration of bulk is ‘thus to be mea- 
fured, which, in the prefent inftance, isa piece of fine filver : 
this is to be gently puthed or flid along, towards the end FH, 
till itis {topped by the converging fides of the canal. 
K is a little veffel formed in the gage for this particular feries 
of experiments, the ufe of which will appear hereafter. 
Thecsntraétion which thethermometer pieces receive from fire, 
isa permanent effect, not variable by an abatement of the heat, 
and which accordingly is meafured commodioufly and at leifure, 
when the pieces are grown cold. But the expanjfion of bodies 1s 
only temporary, continuing no longer than the heat does that 
produced it ; and therefore its quantity, at any particular de- 
gree of heat, muft be meafured in the moment while that heat 
fubfifts. And further, if the heated piece was applied to the — 
cold gage, the piece would be deprived of a part of its heat on 
the firft conta&t; and as the gage receives fome degree of expan- 
fion from heat as well as the piece, it is plain that in this cafe 
the piece would be diminifhed in its bulk, and the gage en- 
larged, before the meafurement could be taken. It is therefore 
neceflary that both of them be heated to an exact equality; and 
in that ftate we can meafure, not indeed the ¢rue expanfion of 
Vor. LXXIV. Bbb either, 
