288 Mr, Cavallo’s Tlefcription of 
After having conftru&ed and adapted this micrometer to the 
telefcope, it is then neceffary to afcertain the value of the divi- 
fions. It is hardly neceffary to mention in this place, that 
though thofe diyifions meafure the chords of the angles, and 
not the angles or arches themfelves, and the chords are not as 
the arches, yet it has been fhewn by all the trigonometrical 
writers, that in fmall angles the chords, arches, lines, and 
tangents, follow the fame proportion lb very nearly, that the 
very minute difference may be fafely neglected : fo that if one 
divifion of this micrometer is equal to one minute, we may 
fafely conclude, that two divilions are equal to two minutes, 
three divilions to three minutes, and fo on. There are various 
methods of afcertaining the value of the divilions of fuch a mi- 
crometer, they being the very fame that are ufed for afcertaining 
the value of the divilions in other micrometers. Such are the 
paffage of an equatorial ftar over a certain number of divilions 
in a certain time, the meafuring of the diameter of the fun, 
by computation from the focal diftance of the objeft, and other 
lenfes of the telefcope, the laft of which, however, is fubjeft 
to feverai inaccuracies; but as they are well known to aftro- 
nomical perfons, and have been defcribed in many books, need 
not be farther noticed in this Paper. However, for the fake of 
workmen and other perfons not converfant in aftronomy, I 
fhall defcribe an eafy and accurate method of ascertaining the 
value of the divilions of the micrometer. 
Mark upon a wall, or other place, the length of fix inches, 
which may be done by making two dots or lines fix inches 
afunder, or by fixing a fix-inch ruler upon a Hand ; then place 
the telefcope before it fo that the ruler or fix-inch length may 
be at right angles with the dire&ion of the telefcope, and juft 
57 feet 3 § inches diftant from the objedl-giafs of the telefcope : 
this done, look through the telefcope at the ruler or other 
extenfion 
