320 SURVEY OP THE COAST 



observed at nij>;ht, two small stops ?;, n are placed on the 

 limb of the horizontal circle, and are fastened to it by a 

 spring beneatli it, and presenting their projecting parts on 

 tlie limb to the sides of the alhidade A, in the two positions 

 of tlie circle in which the star is met by a vertical [)lane pass- 

 ing through it. 



Instead of the arrangement formerly used to produce a 

 small vertical motion in the legs of the instrument, the sciews 

 are here again constructed on tlie same piincij)les of two 

 screws working in each other, exactly equal in all respects 

 to those described in the two-feet theodolite. 



On some Adjustments of the Repeating Circle. 



It will not be expected that I should give here a full de- 

 scription of the adjustment of this instrument : it is too easy 

 and too well known. But I have no where seen mentioned 

 the most proper mode of placing tiie circle accurately in tlie 

 plane of the vertical and of verifying the parallelism of the 

 two motions of the circle and the telescope. 



This consists in observing the pole star, (best in its great- 

 est digression,) both directly and by reflection from a mercu- 

 rial horizon. I do it in the following manner, whenever any 

 more nicety is desired than can be expected from the mere 

 placingof the two semicircles A'Artothe coincidence of certain 

 strokes made upon them at oo, with the sides of the supports 

 of the horizontal axis q. 



Having carefully levelled the horizontal motion of the in- 

 strument by the large level on the circle, so that the verti- 

 cality should take place by the above adjustment, the tele- 

 scope is pointed to the pole star, and the horizontal motion 

 clamped. Then reading the approximate altitude or zenith 

 distance, the telescope is lowered so much below the horizon 



