[ 6o 7 3 
with and without the thin plate ; again advance the 
former fcrew, fo as to make that of the micrometer 
recede another quarter of a turn, and repeat the mea- 
fures with and without the thin plate. This method 
being purfu’d as far as neceffary, it is evident, that, 
the thicknefs of the plate being always the lame, if 
the difference of meafures, taken with and without it, 
are not always the fame in the different parts of a re- 
volution of the micrometer-fcrew, that this fcrew is 
not equiangular ; but from the differences of the 
meafures correfponding to the thicknefs of the fame 
plate, in the different parts of a revolution, the errors 
thereof may be nearly affgn’d. For greater certainty 
in this examination, left the heat of the obferver’s 
body fhould affedt the bar or inftrument during the 
obfervation, let the whole be immerg’d in the ciftern 
of water, which ought to ftand a fufficient time be- 
fore the obfervation is begun, to acquire the fame 
temper as the air, which all'o ought to be in a fettled 
ftate. 
In this manner I examin’d fuch threads of this 
fcrew as were made ufe of in the following expe- 
riments, but did not find any material errors. 
The refult of the experiments made with this in- 
ftrument agrees very well with the proportions of ex- 
panlion of feveral metals given by Mr. Ellicott ; 
which were deduced from his pyrometer publifh’d in 
the PhiloJ'ophical Pranfaftions : And, considering the 
very different condrudtion of the two inftruments, 
they abundantly tend to confirm each other. 
References 
