[ 6oo 3 
With refped to the firft property, this inftrument 
is capable of receiving a bar 2 feet 4 inches long, and 
might be made capable of receiving bars of a much 
greater length, of fome kinds of materials, but not 
of others ; on account of the flexibility brought upon 
them by a degree of heat not greater than boiling 
'water. 
The meafurcs taken by this inftrument are deter- 
mined by the contact of a piece of metal with the 
point of a micrometer-icrew. The obfervation is the 
beft judg’d of by the hearing, rather than that of the 
fight or feeling. By this method I have found it very 
pra&icablc, to repeat the fame meafurement feveral 
times, without differing from itfelf above one twenty- 
thoufandth part of an inch. This principle of deter- 
mining meafures by contad: is not wholly new ; but 
has been employ’d on feveral occafions, as I am in- 
form’d, by the late Mr. Graham : But the prefent 
manner of applying thereof, I believe, is fo ; and the 
degree of fenfibility arifing therefrom exceeds any 
thing I have met with. As the method will eafily 
appear by the draught (fee Plate XXI.), I jflnall avoid a 
farther defcription of it in this place *. 
As no fubftance has hitherto been difcover’d in na- 
ture, that is perfectly free from expan fion by heat, I 
chofe to conftrud: this inftrument in fuch a manner, 
that 
* I have lately feen an inftrument at Mr. Short’s, made by the 
late Mr. Graham, for meafuring the minute alterations, in length, 
of metal bars; which were determin’d by advancing the point of a 
micrometer-ferew, till it fenfibly flopp’d againft the end of the bar 
to be meafur’d. This ferew being fmall, and very lightly hung, 
was capable of agreement within the 3 or 4000th part of an inch. 
