Meajiirement of a Bdfe en Hounilow-Heath. 471 



being added to 7.13, we have 32.06 for the number of revolu- 

 tions meafuring a motion of one-tenth at the objed lens, or 

 the expanfion of one-tenth. In this manner Mr. Ramsden ob- 

 tains the fcale of his pyrometer in the eafieft and moil fimplc 

 way im.aginable, without any neceflity for knowing the abfo- 

 iute diftances of the objea; lens from the wires of the mark on 

 one hand, and thofe of the micrometer on the other ; diftances 

 not eafily afcertained by a6lual meafurement, on account of 

 the pofition of that giafs in its cell, which cannot conveniently 

 be come at. Thus, in tab. XX. as well as in the annexed 

 figure, LM being the object at the diftance of 

 the mark, equal to one-tenth of an inch; 3 

 then ml vf'iW be its magnified image, in pro- 

 portion to the former as mo is to oM, And, 

 if through the point p, the place to which 

 the object lens has been carried by the mo- 

 tion of the expanding rod, a line M^ be drawn 

 parallel to L/, we fliall have ml= 24.93 -f- Z^- 

 *j,i^ — mq— '^2.o6y the number of revolutions 

 of the micrometer meafuring op the expanfion. 

 Having thus obtained the total number of 

 revolutions correfponding to mq-, and having 

 iikewnfe meafured the total diftance mM — 

 26.144 inches, a fpace eafily afcertained be- 

 tween the wires of the micrometer and thofe 

 of the mark, the partial diftances mo and oM may t%e!l 

 be readily found by computation : for mq : ml :'. wM : moz:z 

 20.33 i^^^^hes; and mq : wM :: op : oM— 5.814 inches. 



In order to finifli the defcription of the pyrometer, it is 



only neceftary to obferve farther, that the circular ' fcale, 



- feen in the elevation of the micrometer, whofe zero ap- 



VoL. Lxxv. Q q q r^^^s 



