344 SURVEY OF THE COAST 



When the mirrors are parallel, the alhidade AB stands un- 

 der the telescope, and the alhidade DC is at right angles with 

 it, so that in the observations, the vernier A comes alternate- 

 ly on the two sides of the telescope. The angle between 

 the collimation line of the telescope, and that from the cen- 

 tre to the small mirror, form an angle of about 17° at the 

 small mirror. The direction of the handles /,/is perpendi- 

 cular to the collimation line of the telescope. 



As all angles are measured at least double, namely on 

 both sides of the parallelisms, the circle is divided like every 

 other into 360°. 



To give to the great mirror the full field of reflection, on 

 the side of the telescope, when large angles are measured, 

 the telescope is not screwed fast in the support at n, but the 

 part commonly made to adjust the collimation line parallel 

 to the plane of reflection is here extended into a tube 7n about 

 four inches long, in which the telescope is slided in and out 

 according to what the angle may admit, and to keep the cir- 

 cle not farther from the eye than necessary. 



On account of the ease of holding it, the screw I in the te- 

 lescope tube slides in the slit of this tube, to prevent it from 

 turning and thereby altering the direction of the collimation 

 line towards the plane of reflection. 



The different eye pieces are fitted in a separate tube which 

 unscrews atj?, for the ease of packing. 



To give to this instrument the same advantage in finding 

 stars by night, as the circles of eighteen inches have, there 

 is a light divided semicircle o, o adapted to the alhidade DC, 

 supported in the middle by a small piece reaching to the 

 telescope a. Upon this two sliding pieces q, q are placed 

 on both sides of the middle, to the proper double altitude 

 of the star, and the light pieces r, r fastened to the other 

 alhidade at A will be arrested by them, whether this alhidade 

 moves alone or with the circle. 



The other parts of the instrument being exactly similar 

 to Mr. Troughton's circle, need not here be mentioned. It 

 is also evident that the construction and shape of this instru- 



