CIRCLE. 



495. 



Girds, absolutely necessary, (except azimuths are to be obser- 

 "Y" * Ted) but which will generally be found convenient. It 

 is performed, by bringing any one of the levels of the 

 instrument into a parallel line with two of the feet screws. 

 The bulb of the level being made to rest in a central po- 

 lition by the action of either of these screws, the instru- 

 ment is then to be moved half round in azimuth, and if 

 the bulb of the level has changed its place, it is to be 

 restored to its former position, half by the action of one 

 of the fret screws, and the remaining part by the action 

 of the screw, which changes the position of the points of 

 rjspension of the level itself. If, upon a second trial, 

 the level remains unchanged, then the level itself is ad- 

 jus'ed, and the instrument is to be now brought into a 

 position perpendicular to the first position. If the level 

 now is no longer in its central position, the whole devia- 

 tion is to be corrected by the foot screw ; a circumstance 

 pa-ticularly to be attended to, for the level once adjust- 

 ed is always adjusted, whereas the position of the azi- 

 muth circle may be correct in one position, but erroneous 

 in all the rest. 



The axis of the vertical circle should next be made ho- 

 rizontal. This is known to be accomplished, when the 

 hanging level remains stationary during a whole revolu- 

 tion of the vertical circle round its horizontal axis ; but, 

 in many instruments, the level is not capable of revolving 

 with the circle, in which case it must be applied and re- 

 versed end to end, till the bulb remains in the same posi- 

 tion at each application. 



In large circles intended to be used as transit instru- 

 ments, the exact verification of the horiznnal axis is of 

 great importat.c?. If the level be very sensible, it is usu- 

 ally furnished with a scale. In that case, it is better to 

 read off the deviation each way. than to attempt to bring 

 the level to a central position. This verification is per- 

 formed in the following manrer : Suppose the divisions 

 of the scale to represent seconds of space, and the cen- 

 tral division, both of the east and west end, to be mark- 

 ed zero, then apply the level, and take three or more 

 readings as follow* ; 



East End. 

 S".2E. 

 2 .7E. 

 2 .8E. 



West End. 

 2M E. 

 1 .7E. 

 1 -3E. 



Mean '2 .9 E. 1 .7 E. 



Mean of the two 2".3 E. 



Now reverse the level, and take three readingi. Sup- 

 pose 



8".5 W. 5"..<5 W. 



8 .2 W. 6 .0 W. 



7 .3 W. 6 .5 W. 



Mean 8 .0 W. 6" W. 



Mean of the two 7",0 west. 

 But in the first position it was 2".3 east. 



Hence the mean of two positions is 2".3 west. This 

 small quantity may be corrected by the adjusting screw, 

 particularly if furnished with a micrometer head, and a 

 similar trial repeated. 



It is in this manner that the transit instrument is level- 

 led at the Royal Observatory, and the result is register- 

 ed once a week in the transit book, and published with 

 the annual observations. 



The next adjustment, if not previously attended to by 

 the maker, is to cause the image of a star to be formed 

 accurately on the micrometer wire, which should be pla- 



ced exactly in the focus of the object glass. This re- 

 quires a very careful examination. Both a star and the 

 micrometer should be seen at the maximum of distinctness 

 at the same time. A double star, such as Castor, */ Ari- 

 etis, or y Virginis, should be chosen for this purpose. If, 

 when ttiese stars are seen distinctly double, the eye-glass 

 requires to be drawn either in or out to improve the 

 distinctness of the micrometer wire, it is a proof that 

 the object glass itself requires to be drawn out a similar 

 quantity, that the image may be thrown at its pro- 

 per distance. It thould be remembered, that this ad- 

 justment is precisely the tame for either long or short- 

 sighted persons, the alteration required for either is to 

 be effected by the eye-glass only. The position of the 

 micrometer wire should next be examined, to ascertain 

 if it be parallel to the horizon. For this purpose, let 

 any terrestrial object be bisected, and the instrument 

 turned slowly in azimuth, if the same object remain bi- 

 sected when at each extremity of the field of view, it is 

 a proof that the wire is horizontal. If the circle doe* 

 not move in azimuth, the vc'rtical wire may be rendered 

 perpendicular by a similar process. In each case, the 

 wires are supposed to be perpendicular. Should this not 

 happen to be the case, the horizontal wire may be exa- 

 mined by a star in the equator, which should remain bi- 

 sected during the whole of its passage, and any error in 

 the vertical wire will be rendered of no importance, by 

 always observing the transit of a star. On the centre, 

 indeed, the observation must necessarily be so made, when 

 zenith distances and right ascensions are determined with 

 the same instrument. What we wish to point out, is 

 the importance of paying the greatest attention to the 

 horizontal wire. If the two wires are not exactly per- 

 pendicular to each other, one must be erroneously placed, 

 and this had better happen to the vertical than to the 

 horizontal wire. 



The line of collimation should next be made to de- 

 scribe a great circle of the sphere. Thi^, when the instru- 

 ment admits of it, is done by reversing the vertical circle 

 in its Y's, and observing if the same terrestrial object be 

 accurately bisected in each position ; or the instrument 

 may be turned half round by the azimuth circle, which 

 is an equivalent operation. In very large instrument* 

 between stone piers this cannot often be done. The ob- 

 server in that case, must be provided with two accurate 

 meridian marks, one to the north, the other to the south, 

 and when these are once adjusted, there will be no ne- 

 cessity to have recourse to this troublesome operation. 

 We shall again have to recur to these verifications under 

 the article TRANSIT INSTRUMENT. 



SECT. 1 1. On the Method of conducting a series of As- 

 tronomical Observations with an Astronomical Circle. 



Almost all the instruments above described, resemble 

 each other in this common principle, that they determine 

 the zenith distances of the heavenly bodies by a mean of 

 two observations, one with the face of the instrument to 

 the eaff, the other towards the west. 



Ramsden's and Gary's instruments generally require 

 some previous adjustments of the microscopes, that is, 

 each microscope is supposed to mark zero, when two 

 points or dots, marked on the limb of the instrument, 

 are brought into a vertical position, by means of a plumb 

 line passing over and bisecting each of them. As these 

 dots are as permanent as the divisions themselves, the ob- 

 server need not be anxious to change frequently the po- 

 sition of his instrument, because the error of collimation 



Circle. 



