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Thirdly, As the fame change, with refpeCt to the 
pofition of the index and fcale, will enfue, upon the 
fuppofttion, that the materials, compofing theinftru- 
ment itfelf, are expanded in a certain degree, and the 
bar applied to be meafured remains unchanged in its 
length ; as if the inftrument were fuppofed to fuffer no 
expanfton, and the bar to be meafured were fuppos’d to 
expand in the former degree : It is therefore neceftary, 
that, in the making ufe of an inftrument of this kind, 
the materials, of which thofe parts are compofed, upon 
which the meafure depends, and whicfi may be called 
the bafts thereof, fhould be fubjeCt to no expanfton 
or contraction during fuch trial, or that the expanfton 
or contraction thereof fhould be capable of being 
known, and accounted for. 
Fourthly, That as all bodies grow ftill longer by 
the application of a greater degree of heat to com- 
pare the expanftons of different bodies, we ought to 
have fome method of heating them in the fame de- 
gree, notwithftanding their difference of texture, fpe- 
cific gravity, &c. 
Fifthly, The feveral parts, upon which the mea- 
furements depend, ought to be fufficiently large, to 
be themfelves adually meafured ; that not only the 
proportions of increafe of length in different metals, 
by the fame degrees of heat, may be known ; but 
alfo the quantities of thofe expanftons, in real mea- 
fures : Or, in other words, the proportions, that their 
increafe of length, between certain degrees of heat, 
bears to the length of the bodies: By which mean?. 
We are enabled to afcertain the changes, that bodies 
undergo in their dimenftons by the application of any 
given degrees of heat. 
With 
