54 ON DIVIDING ASTRONOMICAL INSTRUMENTS. 



cessary to take care, that the point Sis in the arc of the cir- 

 cle, 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 in placing the wire perpendi- 

 cular, no great nicety is required either in this or in the po- 

 sition of rf. • 



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 circle ; for as the wire is not perpen- 

 dicular to the circle, except in very small arches, it is plain, 

 that if it bisects the scratches at the circle, it will not bisect 

 them at a distance 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 it which seems attended with difficulty, I 

 thought it best not to enter farther into particulars, 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 judgment of the conveni- 

 enciesor inconveniences of this method, without experience; 

 but, as far as I can judge, it must have much advantage, 

 both in point of accuracy and ease, over that of dividing by 

 the common beam compasses; but it very likely may be 



Mr. Trough* thought, that Mr. Troughton's method is better than either. 



ton's method. Whether it is or is not, must be left for determination to ex- 

 perience and the judgment 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 er- 

 rour, from the slipping or irregularity in the motion of a 

 roller; in which respect this method, notwithstanding the 

 precautions used by im, is perhaps not entirely free from 

 objection ; and what with some artists may be thought a con- 

 siderable advantage, it is free from the danger of mistakes 

 in computing a table of errours, and in adjusting a sector 

 according to the numbers of that table. 



XL 



