36 PHILOSOPHICAL TRANSACTIONS. [aNNO IJSO, 



peculiar in this instrument, was the more certain method of transferring and 

 cutting the divisions, from the original divided points, by means of the beam- 

 compass, than could possibly be done from a fiducial edge, as had doubtless been 

 constantly the practice in cutting diagonals ; for, placing the steady point of the 

 beam-compass in the tangent line to that part of the arc where each division was 

 to be cut, the opening of the compass being nearly the length of the tangent, 

 the other point would cut the division in the direction of the radius nearly ; and 

 though in reality an arch of a circle, yet the small part of it in use would be so 

 nearly a right line, as perfectly to answer the same 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 exe- 

 cute a zenith sector for the Rev, Dr. Bradley, which was fixed up at Wanstead, 

 in Essex, in the year 1727. The very simple construction that he adopted for 

 this instrument, the plumb-line itself being the index, did not admit of the use 

 of a vernier : he therefore contented himself with dividing the arch of the limb 

 of this instrument by primary points, as close as he thought necessary, that is, 

 by divisions of 5' each, and measuring the distance from the set of the instru- 

 ment to the next point of division by a micrometer screw, in the construction of 

 which screw he used uncommon care and delicacy. I have mentioned this in- 

 strument to introduce this observation ; that I think it highly probable, had 

 Mr. Graham constructed the great quadrant after the zenith sector had been 

 fully tried, he would have rejected not only the diagonals but the verniers also 

 as containing a source of error within themselves which may be avoided by a 

 well-made screw. 



It seems also, that Mr. Graham, at the time he constructed both these in- 

 struments, was not aware how much error could arise from the unequal expan- 

 sions of different metals by heat or cold : for in both, the radius, or frame of 

 the instrument, was iron, while the limbs were of brass. They however remain 

 n the Royal Observatory, perfect models, in all other respects, of every thing 

 that is likely to be attained in their respective destinations, and monuments of 

 the superlative abilities of that great mechanician Mr. Graham.* 



Mr. Graham lived till the year 1751 ; and during his time there were few in- 

 struments of consequence constructed without his advice and opinion. They 

 were for many years done by Mr. Sisson, to whom doubtless Mr. Graham would 

 fully communicate his method of division ; and from this school arose that very 

 eminent and accurate artist Mr. Bird, whose delicate hand, joined with great 

 care and assiduity, enabled him still further to promote this branch of division ; 



* I have been informed, that Dr. Maskelyne has caused this objection to the sector to be rectified, 

 since its removal to the Royal Observatory, by substituting an iron limb instead of that of brass, the 

 points being made upon studs of gold, — Orig, 



