Graduation of 'Agronomical Injlruments * r t 
What I apprehend to be peculiar in it, was the application of 
the arch of 96° ; not only as a check upon the arc of degrees 
and minutes, but as fuperior thereto, being derived from the 
more fimple principle of continual bifettion . 
To make room for this, he has entirely rejected the fubdivifion 
by diagonals, and has adopted the method of the V ernier ; but the 
fubdivifion of the vernier divisions he, as I apprehend for the 
firft time, meafured by the turns of the detached adjufting 
fcrew, making it in fa£t a micrometer, by which the diftanca 
of the fet of the inftrument was to be meafured from the 
perfect coincidence of one of the adtual divifions of the limb 
with the next ftroke of the vernier ; by which means the ob« 
fervation could not only be read off with all the precifion that 
the divifion of the inftrument was capable of, but the two fets 
of divifions could be checked and compared with each other. 
Another thing that I apprehend to be peculiar in this inftru- 
ment, was the more certain method of transferring and cutting 
the divifions, from the original divided points, by means of 
the beam-compafs , than could poffibly be done from a fiducial 
edge , as had doubtlefs been conftantly the practice in cutting 
diagonals; for, placing the fteady point of the beam-compafs 
in the tangent line to that part of the arc where each divifion 
was to be cut, the opening of the compafs being nearly the 
length of the tangent, the other point would cut the divifion 
in the direction of the radius nearly; and though in reality an 
arch of a circle, yet the fmall part of it in ufe would be fo 
nearly a right line, as perfectly to anfwer the fame end ; all 
which advantages put together, it is probable, induced Mr, 
Graham to reject the diagonals. 
Soon after the completion of this quadrant, Mr. Graham 
undertook to execute a zenith feftor for the Rev. Dr. Bradley, 
C 2 which 
