Graduation of Agronomical Infruments . 9 
at the point given, he fet off equal fpaces in fucceffion to the 
end of his arch, which was about 75 0 . Thofe were diftin- 
guifhed upon the limb of the inftrument by very fine points, 
which were referred to by a grofler divifion, the whole being 
properly numbered. The fubdivifion of thofe arches of 10 
minutes each was performed by a double microfcope, carried 
upon the arm or radius of the inftrument, the common focus 
being furnifhed with parallel threads of fingle filk, whereof 
eleven being difpofed at ten equal intervals, comprehending 
together one ten-minute divifion, the diftance of the neareft 
threads became a very vifible fpace, anfwerable to one minute 
each, and therefore capable of a much further fubdivifion by 
eftimation. 
The divifions of this inftrument were therefore, properly 
{peaking, not degrees and minutes ; but yet, if exa&ly equal, 
would ferve the purpofe as well, when their true value was 
found, which was done by comparifon with larger inftru- 
ments. 
Now, if it be confidered, that in going ftep by ftep of ten 
minutes each, through a fpace of 75 degrees, there will be a 
fucceffion of 450 divifions, dependant upon each other; if it 
be alfo confidered, that the leaft degree of extuberance in the 
furface of the metal, where each new point is fet down, or 
the leaft hard particle (wherewith all the bafe metals feem to 
abound) will caufe a deviation in the firft impreffion of a taper 
point, and thereby produce an inequality in the divifion ; it is 
evident, that though this inequality may be very fmall, and 
even imperceptible between neighbouring divifions, yet among 
diftant ones, it may and will arife to fomething confiderable ; 
which, in the menfuration of angles, will have the fame ill 
tendency as in near ones. Now, as M. Roemer has given us no 
Vol. LXXVI. C 
means 
