On Allan's Dividing Inst nnnent. 121 



others with. The plan wa^j soon abandoned by the Doctor 

 on account (!f its inaccuracy. As hr as f was ever able to 

 learn, I believe rev oid master, Hendiy of York, was the 

 first that ever made an engine to work all the way round by 

 a screw, to divide with. It was about 13 inchc's diameter, 

 and cut into 36o teeth or degrees. I have worked with it 

 many a day, both in the dividing and cuttinof down way- 

 Mr. Snieaton saw it in the year 174I, andlie has told me 

 since that he believed it was made about tuo years l)ef'ore 

 that time. Now 72 or 73 years. 



Now I will on your plan, just in a few words, run over 

 the whole of the operation of making the teeth. All the 

 apparatus being prepared and quite ready for the cuttino- 

 part, 1 would first start on Ramsden's plan by the dots^ 

 one half or nearly of the whole depth of the teeth. This 

 done, the screw has now got a firm hold of the teeth, and 

 plenty of room for correcting by the ring, before the finish, 

 any inequality that can exist in the complete circle. Havino- 

 arrived at this point, I consider the work in a forward stated 

 and the difficulty all over. So nnich accomplished, I would 

 proceed no further on Ramsden's plan, but would have re- 

 cour>e to the correcting ring, |)rocecd with the rackinsi and 

 shifting in the ti)anner"tlescribed above. Towards thetast I 

 would shift the ring every time it was once worked round 

 by the screw cutter. By this moving the rin<r to half and 

 to quarters alternately, all those errors wilfbe perfectly 

 corrected without nmch attention of the workman. 



The Retrospect says you have given no proof, but bold 

 enconiiums on the perfection of your instrument, and denies 

 its self-evidence. I really do not know any ereater test 

 that it can be put to than mav be done by the uuvr, which 

 IS certainly a part of the machine. 



I am confident in my own mind, that with makinc: some 

 little change, the whole of this jiiece of business, f mean 

 cutting or racking the teeth truly, might be accomplished 

 to the gre.'Ucst degree of exactness, wiilvjut steady ])ins or 

 ever lines (only a few pencil marks') by contact itself, which 

 Mr. Smeaton says, and seems to have' proved, in the l^hilo- 

 M.-phical Transactions of the year 1783, is fifteen times 

 nearer the truth than coincidence. Respecting your im- 

 provement in the dividing engine, by what 1" have said 

 above, you* will perceive thai I stifl support my first 

 opinion, that it is a great and important discovery ; 

 and remain, sir, vours truly, 



' '"" S^llr!^;^"''"'' J^'^^ S TANCLII^PE. 



P.S.-I 



