343 SURVEY OP THE COAST 



motions is one of the principal properties required, as stated 

 before. 



This is evidently to be applied in this instrument to the 

 plane of reflection, which is itself determined by the posi- 

 tion of the large mirror. 



In Borda's construction, the axis upon which the mirror 

 revolves being short, the plane of reflection is too much 

 guided by the plane of the hmb, which artists well know 

 can not be executed with the same accuracy and ease as 

 the axis. From this cause, the English artists abandoned 

 the principle of repetition ; and Mr. Troughton gave to the 

 circle a construction, in which the motion of the mirror is 

 determined by a longer axis, and the eccentricity corrected 

 by three readings ; without repeating which, from the excel- 

 lence of his work, and his great care in the choice of the mir- 

 rors, has given most excellent results. 



Reflecting instruments being indispensable in the survey 

 of the coast, for the observations, to determine soundings, 

 and others to be made on board of vessels, ^'c, I consider- 

 ed it proper again to turn my thoughts to the improvement 

 of this instrument, as I had done long before, so as to pre- 

 serve both the multiplying principle and the stability of the 

 plane of reflection. 1 considered the circle itself as of mi- 

 nor influence, and therefore allowed it to be moveable, and 

 alternately clamped to the soHd part of the instrument, which 

 bears the small mirror and the telescope, or to the alhidade 

 of the large mirror, and moving with it. 



Having made a description of such an instrument with a 

 drawing of full size, before I left the country, I presented it 

 to Mr. Troughton, who said immediately that he would make 

 me one upon the same principles, though different in shape, 

 as he wished the instrument to be lighter. He showed me 

 at the same time the ideas of Mr. Mendoza, which had com- 

 pletely failed in a similar attempt, and of which I then ob- 

 tained the first information. 



Mr. Troughton gave to the instrument a shape similar to 

 that of his reflecting circle, from which my drawing differed 



