H An Account of the Repeating Circle^ 



be of no use. It is true that In proceeding downwards we 

 gradually come to the horizon, where the same effect, that was 

 stated respecting the azimuthal angle, takes place: but here 

 the uncertainty of refraction destroys all confidence. I may 

 also state that three readings to a vertical circle cannot be all 

 equally well illuminated in the night time ; nor at any time are 

 they to be read with equal convenience, as is the case where 

 two readings are placed liorizontally. However, as micro- 

 scopic readings are expensive, and as astronomy is generally 

 tlie chief object of those who procure this instrument, two mi- 

 croscopes to each circle may be sufficient. But were I to have 

 for my own use an instrument of this kind with verniers, the 

 lower circle should have three, and the upper one four. 



When the vertical circle has two readings, and these micro- 

 scopic, they are affixed to the ends of two horizontal tubes fast- 

 ened to one of the columns ; which also support a good hang- 

 ing spirit level. Another level of the best quality is occasion- 

 ally applied to the pivots of the transit axis, in order, indepen- 

 dently of every thing else, to verify its horizon tali ty. 



Adjustment of the Repeating Circle. 



The vertical axis is made perpendicular by means of the 

 feet-screws of the tripod, and the spirit level, in the same man- 

 ner as is required for, other intruments ; an operation so easy 

 and well known, that to mention it is all that seems necessary. 

 To adjust the coUimation of the telescope parallel to the plane 

 of the circle, an object should be chosen as nearly in the hori- 

 zon as can be estimated: the middle wire of the telescope un- 

 der adjustment, being correctly pointed to the object, what is 

 shown on the indices of the azimuth circle must be carefully 

 noted. Reverse the telescope both vertically and horizontally, 

 bisecting again the same object with the same wire, and again 

 read off what the indices give. Take of these readings the 

 mean, or middle point, and set with great care the indices so 

 as to show that mean. Now, by the screws, which act upon 

 the wire-plate, move the wire so as to make it bisect the ob- 

 ject: this being well done, the other telescope, to be adjusted, 

 wants only its vertical wire moved in the same manner till it bi- 

 sects the object. The above, however, is true only when the 

 object is very distant; for, as both telescopes are eccentric, as 

 respects the vertical axis, and unequally so, it becomes neces- 

 sary, when no remtrte object can be seen, to put up marks, say 

 two circles, the radii of which are equal to the eccentricities of 

 the respective telescopes. The next essential adjustment, is 

 to place the plane of the upper circle vertical ; or its axis hori- 

 zontal. The best practical method of doing this, and which is 



quite 



