174 Improvement in dividing astronomical Instruments. 



it, while the beam compass is turned a little on its centre. 

 It is also necessary to lake care thai the point $ is in the 

 arc of the circle, while the bisection is observed by the 

 microscope, which may most conveniently be obtained, by 

 placing a stop on the support on which that end of the 

 beam compass rests. If proper care, however, is taken iiv 

 placing the wire perpendicular, no great nicety is required 

 either in this or in the position of d. 



Another thing to be attended to, in making the wire 

 bisect two scratches, is to take care that it bisects them 

 in the part where they cut the circU- ; for as the wire is 

 not perpendicular to the circle, except in very small arches, 

 it is plain, that if it bisects the scratches ai the circle, it 

 will not bisect them at a diatance from it. 



There are many particulars in which my description of 

 the apparatus to be employed will appear incomplete; but 

 as there is nothing in \e which seems attended with dif- 

 ficulty, I thought it best not to enter further into particu- 

 lars, than was necessary to explain the principle, and to 

 leave the rest to any artist who may choose to try it. 



It is difficult to form a proper judgement of the con- 

 veniences or inconveniences of this method, without ex- 

 perience ; but, as far as I can judge, it must have njuch 

 advantage, both in point of accuracy and ease, over that of 

 dividing by the common beam compasses : but it very 

 likely may be thought that Mr. Troughton's method i? 

 better than either. Whether it is or is not, must be left for. 

 determination to experience imd the judgement of artists. 

 Thus much, however, may be observed, that this, as well 

 as his, is free from the difficulty and inaccuracy of setting 

 the point of a compass exactly in the centre of a division. 

 It also requires much less apparatus than his, and is free 

 from any danger of error, from the slipping or irregularity 

 in the motion of a roller; in which respect his method, 

 notwithstanding the precautions used by him, is perhaps 

 not entirely free from objection ; and, what with some 

 artists mav be thought a considerable advantage, x it is free 

 JTom the danger of mistakes in computing a table of errors, 

 and in adjusting a sector according to the numbers of that 

 table. 



XXIX. Oh 



