88 On dividing Instruments. 



application of the eonipasses, no such application being 

 nccess^'v. 



i will now di^nllss 'his method of dividing, with observing, 

 that it is tedious in the extienie; and did I not know the. 

 contrary bevoad a doubt, I should have supposed it to have 

 surpassed the utmost limit of hviman pilience*. When I 

 made my fitst essay at subdividing with the niller, 1 used 

 this n-'.'thod, according to the improvement suggested above, 

 of correc liig a few primitive points; but even this was too 

 •slow for one who had too much to do. Perhaps, however, 

 had UiV instruments been divided for me by an assistant, I 

 ijughl not have grudged to have paid him for the labour of 

 going tbrivugh the wht)!e work Oy the method of adjustment; 

 nor have felt the necessity of cuitriving a better way. 



I might now extend the account of my methoji of dividing 

 to a great length; by relating the alterations which the ap- 

 paratus has undergone during a long course of years f, ancl 

 the various manner of its app'ication, before I brought it to 

 its present state of impiovenjcnt ; but I think 1 may save 

 niyscif that trouble, for truly 1 do not see its use : 1 will, 

 therejbre, proceed immediately to a disclosure o': the me- 

 thod, as practised on a late occasion, in the dividing of a 

 four feet meridian circle, now the property of Stephea 

 Groombridge, esq., of Blackheath, 



The surface of the circle which is to receive the divisions, 

 as well'^as its inner and outer edges, but especially the latter, 

 should be turned in the most exact and careful manner ; the 

 reason for which will be better understood, when we come 

 to describe the mode of applying the roller : and, as no pro- 

 jection can be admitted beyond the limb, if the telescope, as 

 is. generally the case, be longer than the diameter, those 

 parts which extend filrther must be so applied, that they 

 may be removed during the operation of dividing. Fig. 1 



* At the time alluded to, the double microscopic micrometer was unknown 

 to me, and 1 did not learn its use, for these purposes, till the year 1790, from 

 general Roy's description of the large theodolite. Previous to that time, I 

 had used a frame which carried a sinp;le wire very ne:ir the surface to be di- 

 vided ; this wire was moveable by a fine micrometer screw, and was viewed 

 by a sin.jle lens in ;erted in the lower end of a tube, which, for the purpose 

 of taking oft" the parallax, was four inches long. The greatest objection to 

 this mode of constructing the apparatus is, that the wire being necessarily 

 exposed, is apt to gather up the dust; yet it is preferable to the one now in 

 use, in cases whei e any doubt is enttrtained of the accuracy of the plane which 

 is to receive the divisions. 



f The full conception of the method iiad occupied my mind in the year 

 1778; but as my brother could not be readily persuaded to relinquish a 

 branch of the business to me in which he himself excelled, it was not until 

 September 17R.5 that I produced my fust specimen, by dividing an astronomi- 

 cal quadrant of two feet radius. 



and 



