ON DIVIDING INSTRUMENTS. ^ 



tractcd choid of 60° and its subdivision ; and the uncer- 

 tainty, which is introduced into the work by the sparing use 

 which is made of* subdivision by 3 and 5, is, in my opinion, 

 likely to be much exceeded by the errours of a divided 

 scale*, and those of the hand and eye, in taking off the 

 computed chords, and applying them to the arc of the in- 

 strument to be divided. 



Ramsden's well known method of dividing by the engine Ratnsden'sme- 

 unites so much accxiracy and facility, that a better can engine! 

 hardly be wished for : and I may venture to say, that it will 

 never be superseded, in the divisions of instruments of wo- 

 derale radii. It was well suited to the time in which it ap- 

 peared ; a time when the improvements made in nautical 

 astronomy, and the growing commerce of our country, 

 called for a number of reflecting instruments, which never 

 could have been supplied, had it been necessary to have di- 

 vided them by hand ; however, as it only applies to small 

 instruments, it hardly comes within the subject of this pa- 

 per. 



The method of Hindley, as described by Smeatonf, I Hindley's me- 

 will venture to predict will never be put in practice for di- *^*^ * 

 viding astronomical instruments, however applicable it 

 might formerly have been for obtaiijing numbers for cut- 

 ting clockwork, for which purpose it was originally intended. 

 It consists of a train of violent operations with blunt tools, 

 any one of which is sufficient to stretch the materials be- 

 yond, or press them within their natural state of rest; and, 

 although the whole is done by contact, the nature of this 

 contact is such, as, I think, ought rather to have been con- 

 trasted with, than represented as being similar to, the na- 

 ture of the contact used in Smeaton's Pyrometer, which 

 latter is performed by the most delicate touch ; and is re- 

 presented, I believe justly, to be sensible to the T-o^^^^r part 

 of an inch. Smeaton has, however, acquitted liimself well, 

 in describing and improving the method of his friend ; and 

 the world is particularly obliged to him for the historical 



* That Bird's scale was not without considerc^ble errourSj-wiH be showa 

 towards the end of this paper. 



t Phil. Trans, for 1788. 



part 



