OP TOE UNITED STATES. 343 



in many respects ; but liere I had the reason, as in the other 

 instruments, to leave him at lull liberty in this respect. 



I may therefore be allowed here ajjjain to suppose the con- 

 struction of Mr. Troiighton's reflecting circle fully known, 

 and describe only the alterations made to it, to give it the 

 repeating property. 



Plate IX. fig. 1 is a perspective view, and tig. 10 a section 

 of the centre i)art ol the instrument. The parts «, b, c, d, e, 

 form the frame of the instrument, wijich contains in the cen- 

 tre piece e the axis both of the mirror and the circle ; in a, 

 the support of the telescope, which may be lowered or ele- 

 vated ibr the equalisation of the illumination of the two ob- 

 jects, in the same manner as in Mr. Troughton's circle : in 

 c, the small mirror is fixed, exactly in the same maimer as 

 are also the handles//, and the rectangular piece ^j^', reaching 

 over the large mirror to receive the straight i)andle ; l)ut the 

 frame reaches only so far as to unite all the above parts. 



The circle itself revolves on the side of the frame oppo- 

 site to the mirror, by a bell metal socket /, ^ of half in inch 

 in length, upon an axis turned to the lirass centre piece c of 

 the frame, tlirough the middle of which the axis of the great 

 mirror s, s passes, as far as the upper part where the mirror 

 is fastened to it. 



The alhidade AB of the great mirror is at the end of the 

 axis, opposite to it, and revolves upon the circle so as to read 

 upon the divisions. 



The alhidade DC is fixed to the frame of the instrument, 

 between it and the circle, and forms tviro diametrically op- 

 positf readings upon the circle, for the motion of the mirror 

 and the circle together. 



The two axes have therefore entirely independent revolu- 

 tions, the mirror within, and the circle without th^^ piece //. u. 

 The clamps of the two motions are here bodi outsi(|c of the 

 circle, as it is not necessary that they cross each other in the 

 observation. Both alhidades will give a sepaiatf sf-i'-'^ of 

 angles, corrected for eccentricity by the two opposite read- 

 ings. 



S 



