470 Major-General Roy's Account of the 



follows, that thereare 7i.:?7 threiids of the fere w in an mch-; 

 that feven revolutions and nearly -j-V^th parts move the wire of 

 the micrometer one- tenth of an inch ; and that ^-^^th part of 

 a revolution, or half a divifion, anfwers to a motion of fome- 

 thlng more than 0.00014 of an inch. 



Having thus obtained the number of revolutions and parts of 

 the micrometer (7* 13) correfponding to one- tenth of an inch at 

 the wires, it is fufEciently obvious, that the number anfwering 

 to one-tenth LM at the mark being likewife obtained, and 

 added to the former, their fum will give the meafure of one- 

 tenth at the obje(Sl lens, or the fpace by which the expanding 

 rod has lengthened, as {hewn by the motion of the lens from 

 to p. This meafure of one-tenth of an inch at the mark 

 was afcertained in two different ways, and the refults exa6lly 

 agreed with each other. In the firft place, a very thin ivory 

 Aide, whereon feveral twentieths of an inch were nicely divided 

 by exceeding fine lines, was prepared, and made to move in the 

 mark where the brafs fiide now exifls. A candle being then 

 placed behind it at night, while the pyrometer flood within 

 doors, and the micrometer wire being repeatedly moved by the 

 fcrew, its coincidence with the lines was diftindtly ieen through 

 the ivory ; whereby two of the fpaces were found to be mea- 

 sured by 24.93 revolutions of the head. The fecond method 

 'was, by means of two exceeding fine wires placed parallel to 

 each other on the brafs ilide, where they now remain, at the 

 diftance of one-twentieth of an inch on each fide of the inter- 

 feiStion wires, as may be feen by obferving the real mark, or 

 rather its magnified image, as fliewn in the oval field of the 

 micrometer, in the centralfigure of conftru6lion. The revo- 

 lutions of the micrometer anfwering to the diftance between 

 ithefe parallel wires was, as before, found to be 24.93 » which 



being 



