Meajuremeni of a B-afi on Hoimftow-Hcath. 471 
being added to 7.15, we have 32.06 for the number of revolu- 
tions meafuring a motion of one-tenth at the object lens, or 
the expaMon of one-tenth. In this manner Mr. Ramsden ob- 
tains tho fcaie of his pyrometer in the eafieft and molt fimplc 
way imaginable, without any neceffity for knowing the abfo- 
lute diftances of the objefl lens from the wires of the mark on 
one hand, and thofe of the micrometer on the other ; diftances 
not eafily afcertained by adfual meafurement, on account of 
the pofltion of that glafs in its cell, which cannot conveniently 
be come at. Thus,, in tab. XX. as well as in the- annexed 
figure, LM being the objefl at the diftance, of 
the mark, equal to one-tenth of an inch ; 
then ml will be its magnified image, in pro- 
portion to the former zs mo is to eM. And, 
if through the point p> the place to which 
the object lens 0 has been carried by the mo- 
tion of the expanding rod, a line Mg' be drawn 
parallel to L /, we {hall have ?nl~ 24.93 + /s r== 
7.13 = mq — 32.06, the number of revolutions 
of the micrometer meafuring op the expanfion. 
Having thus obtained the total number of 
revolutions correfponding to mq\ and having 
likewife meafured the total diftance »M- 
26.144 inches, a fpace eafily afcertained be- 
tween the wires of the micrometer and thofe 
of the mark, the partial diftances mo and sM may then 
be readily found by computation \ for mq : ml :: mM i mo-=z 
20.33 inches ; and mq imM :: op : 5.814 inches. 
In order to finifh the defcription of the pyrometer, it is 
only neceflary to obferve farther, that the circular fcale, 
feen in the elevation of the micrometer, whofe zero ap- 
% r 0L» LXXV. Q q q pears 
