[ 606 ] 
which is the only part of this inftrumentthat requires 
exadtnefs in the execution ; and how difficult thefe 
are to make, perfectly good, is well known to every 
perfon of experience in thefe matters ; that is, that 
the threads ot the fcrew may not only be equidiftant, 
in different places, but that the threads fhall be equally 
inclin’d to the axis in every part of the circumfe- 
rence. 
As nearly the fame part of the fcrew is made ufe 
of in thefe experiments, the latter circumftance is 
what principally needs enquiry. For this purpofe, 
let a thin flip of fteel, or other metal, be prepar’d, 
whofe thicknefs is about j of the diftance of the 
threads : Let the edges of this thin plate be cut into 
fuch a fhape, as exactly to fit into the fix’d notch in 
which one end of the bar is laid : Let a fcrew pafs 
thro' the ffandard of brafs, on which that notch is 
fupported, in fuch a manner, that the end of the bar 
to be meafur’d that is fartheft from the lever, may 
take its bearing againft the point (or rather the fmall 
hemifpherical end) of this fcrew: Let one of the 
brafs bars, us’d in the other experiments, be apply’d to 
the inftrument, and a meafure taken ; then let the 
thin plate be put in between the end of the bar and 
the p^int of the fcrew la ft mention’d, and again take 
the meafure j but firft obferve, that the plate is put 
down to the notch, fo that the fame place of the plate 
may always agree with the point of the fcrew, and, 
confequently, no error may arifc from a different thick- 
nefs in different places of the plate : Obferve alfo, 
that the whole comes to a true bearing; then advance 
the fame fcrew till the micrometer-fcrew is pufh’d 
backward ~ of a revolution 3 again repeat the meafure 
with 
4 
