47° Major-General Roy’s Account of the 
follows, that there are 71.27 threads of the fcrew in an Inch 5 
that feven resolutions and nearly _y s th parts move the wire of 
the micrometer one- tenth of an inch ; and that _.^ v th part of 
a revolution, or half a divifiou, anfwers to amotion of feme- 
thing; 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 fufficiently obvious, that the number anfwering 
to one-tenth LM at the mark being likewife obtained, and 
added to the former, their fnm will give the meafure of one- 
tenth at the object lens, or the fpace by which the expanding 
;rod has lengthened, as fhewn by the motion of the lens from 
0 to p. This meafure of one-tenth of an inch at the mark 
was afcertained in two different ways, and the refults exadlly 
agreed with each other. In the firft place, a very thin ivory 
Hide, 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 Hide now exifis. A candle being then 
placed behind it at night, while the pyrometer hood within 
doors, and the micrometer wire being repeatedly moved by the 
fcrew, its coincidence with the lines was diftindlly feen through 
the ivory j whereby two of the fpaces were found to be mea- 
fured 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 Aide, where they now remain, at the 
diftance of one-twentieth of an inch on each fide of the inter- 
fection wires, as may be feen by obferving the real mark, or 
rather its magnified image, as fhewn in the oval field of the 
micrometer, in the central figure of coiiftruftion. The revo- 
lutions of the micrometer anfwering to the diftance between 
thefe parallel wires was, as before, found to be 24.93 » which 
