ON DIVIDIN® INSTRUMENTS. Q 



by the future application of the compasses, no such appli- 

 cation being- uecessary. 



I will now dismiss this method of dividing, with observ- Its tedlousn«8. 

 ing, that it is tedious in the extreme , and did I not know 

 the contrary he} ond a doubt, I should have supposed ft to 

 have surpassed the utmost hmitof huma-a patience*. When 

 I made my first essay at subdividing with the roller, I used 

 this method, according to the improvement suggested above, 

 of correcting a few primitive points ; but even this was too 

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

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

 might not have grudged to have paid him for the labour 

 of going through the whole work by the method of adjust- 

 ment ; nor have felt the necessity of contriving a better 

 way. 



1 might now extend the account of my method of di- Mr. Edward 

 viding to a great length; by relating the alterations which method °"* 

 the apparatus has undergone during a long course of yearsf, 

 and the various manner of its application, before I brought 

 it to its present state of improvement; but I think I may 

 save myself this trouble, for truly I do not see its use : I 

 will, therefore, proceed immediately to a disclosure of the 

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



* At the time alluded to, the double micrascopic micrometer was un- 

 known to me, and I did not learn its use, for these purposes, till the 

 year 1790, from General Roy's description of the large theodolite. Pre- 

 vious to that time, I had used a frame, which carried a single wire very 

 near the surface to be divided; this wire was movable by a fine microme- 

 ter screw., and was viewed by a single lens inserted in the lower end of a 

 tube, which, for the purpose of taking off the parallax, was 4 inches 

 long. The greatest objection (o this mode of constructing the apparatus 

 J5, 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 where any doubt is 

 enteitained of the accuracy of the plane which is to receive the divi- 

 sions. 



■j- The full conception of the method had occupied my mind in the 

 year 1778 ; but as my brother could not be readily persuaded 'to relin- 

 quish a branch of the business to me in which he himself excelled, it 

 v/as not until September 1785 that I producv=d my first specimen, by di- 

 vidi.ig an astronomical quadrant of two feet radius. 



a four 



