C 6o 3 3 
the wooden bar be again brought forth apply’d to the 
inftrument, and the degrees of the micrometer and 
thermometer read off, as before : The difference of 
degrees of the micrometer, correfponding to the dif- 
ference of degrees of the thermometer, will exprefs 
the expanfion of the balls between thofe degrees of 
heat j that is, upon the fuppofition that the wooden 
bar was of the fame length, at the time of taking the 
fecond meafure, as at the firft : Indeed a meafure 
can hardly be taken without any lofs of time, as the 
whole of the inftrument, when the hot meafure is to 
be taken, is confiderably hotter than the wooden bar 3 
and, in cafe of boiling water, the fteam being very re- 
pellent and adive, the bar is liable to be fenfibly af- 
feded in its length, before the meafure can be taken, 
both by heat and moifture, which both tend to ex- 
pand the bar : But as the quantity is fmall, and ca- 
pable of being nearly afcertain’d, a wooden bar, thus 
apply’d, will anfwer the fame end as if it was unalter- 
able by heat or moifture. 
In order, therefore, to know the quantity of this 
alteration, let the time elaps’d between the firft ap- 
proach of the bar to the inftrument, and the taking of 
the meafure, be obferv’d by a fecond-watch, or other- 
wife ; after another equal interval of time, let a fe- 
cond meafure be taken ; and after a third interval, a 
third ; and a fourth 3 the three differences of thefe 
four meafures will be found nearly to tally with three 
terms of a geometrical progreftion, from which the 
preceding term may be known, and will be the cor- 
redion 3 which, if apply’d to the meafure firft taken, 
reduces it to what it would have been if the wooden 
bar had not expanded during the taking thereof. 
4 G 2 From 
