CIRCLE. 



tcte. loog, but only about two third* of ill length it employ - 

 *^Y"^^ ed Tor that purpose, working in the socket of the coun- 

 terpoise i the ntt form* a bearing round whicli the 

 tockrt of the back telescope revolves. 



PI.TI An index with four branches, the vernier* of which 



! t. subdivide the circle to 10", ha* an axis of iteel attached 

 to it* centre, which pa*tci through the whole length of 

 the axi* of the circle, the latter being a socket wherein 

 it turn* round, in the tame manner that the axis of the 

 circle turn* in the *ocket of the counterpoise. This in- 

 dex is furnished with the usual apparatus for quick and 

 alow motion, which attache* to, and forms a part of, its 

 chief branch, as at g . There is another index, the posi- 

 tion of which is behind the circle, having but one branch, 

 and which carries no veniicr ; it is furnished with the 

 means of being clamped, and put in gradual motion 

 like the other : in clamping it attaches to the edge- bar 

 of the circle : it is united to a socket, which embraces 

 the part of the axis of the circle not occupied by the 

 socket of the counterpoise. 



The instrument has two telescopes similar to each 

 other in almost every respect. The one in front of the 

 circle is fixed to the index at half right-angles between 

 two of it> four branches; the one behind has a position 

 forming the same angle with the only branch of the 

 othtr index. The connection between the telescopes 

 and indices is formed near the centres, so that the former 

 may take the unbiassed direction which is produced by 

 the axis alone, without being at all influenced by the ir- 

 regularities of the limb. In the circles of Borda, the 

 telescopes are made to take the direction of the circle, 

 and the accuracy of their motions depends upon the truth 

 of its plane. These telescopes are about two feet in 

 length ; the aperture of the object-glasses nearly an inch 

 and three quarters, and they are provided with powers 

 of from 30 to 60, according to their different eye-pieces. 

 The telescope behind the circle has, in its form, only two 

 cros* lines: the plate in which they are fixed is adjustable 

 respecting the vertical and horizontal position of the 

 lines, and is also acted on by two drawing screws for 

 placing the line of collimation parallel to the plane of the 

 circle : two others at right-angles draw also, and are in- 

 tended for placing the collimation parallel to a level, 

 which is to be described. In the front telescope there 

 are three lines crossing other three ; these are adjustable 

 exactly like the former, only the adjustment last men- 

 tioned is not necessary. As the telescopes of the repeat- 

 ing circle are designed to view objects very differently 

 situated respecting distance, the eye-tubes of both have 

 a motion given them, by teeth and pinions, whereby they 

 may be brought outwards, in order to procure distinct 

 vision of near object*. 



Between the two portion* of the axis of the circle, 

 namely, that which works in the socket of the counter- 

 poise, and that upon which the socket of the back tele- 

 scope turns, there is *ecurely united to the axis a wheel of 

 about two inches diameter, the edge of whichis surround- 

 ed by a circle cut into three pieces, and again joined at 

 two places in the same manner as the links of a watch 

 chain, and at the third by a clamping screw, which is 

 **en in the Figure at h. To the middle piece of the cut 

 circle, and opposite to the screw, is fixed an ear extend, 

 ing about five inches from the centre, and upon the front 

 of the horizontal axis, a similar one is fixed : these two 

 c*" are connected by an adjusting screw, which, when 

 the jointed circle is made by its screw to embrace the 

 axis, furnishes the means of gradual motion to the circle 

 and telescopes ; but when the axis il released from the 

 action of the clamp, the general motion i* free. The 



5 





apparatus last described is very superior to the mean* rirel*. 

 which Dorda employed for accomplishing the same ob- 

 ject. In the French construction, a wheel of about five 

 inches diameter, with an indented edge, is fixed, beyond the 

 remote end of the counterpoising cylinder, to the axis of 

 the circle, and is acted on by an endless scrrir, which, 

 for tin- sake of quick and slow motion, is thrown off and 

 and on at pleasure. This screw, owing to its great dis- 

 tance from the parts which require its use, (the circle 

 and telescopes,) must twist the centre work, and of 

 course leave the parts at liberty to move after its action 

 ought to have ceased ; an impediment which, if not a very 

 dangerous, is at least a very troublesome one, and pro- 

 ductive of much loss of time. 



For astronomical purposes, the telescope behind is of 

 no use, instead of which a level is employed ; and in or- 

 der that it should apply immediately to its work, it is 

 fixed, not upon the stand, which is the usual application, 

 but upon the back index of the circle, and has the ad- 

 vantage of its quick and slow moving screws. The po- 

 sition of the back telescope being ofnecessity eccentric, 

 the level is placed parallel to it on the opposite side of 

 the axis, and becomes a counterpoise to the telescope, 

 for which purpose it is made rather heavier than is other- 

 wise required. The tube of this level is ground inside 

 to a radius, which gives the bubble a range of about an 

 inch and a half to a minute. Two ivory divided scales 

 are placed over the tube, which indicate single seconds, 

 and by which the position of the index is ascertained to 

 about double that quantity. 



Another level, the use of which regards the plane of 

 the vertical circle, has one end affixed to the cross axis, 

 and the other to the counterpoising cylinder : if, when 

 that plane is adjusted perpendicular, the level is brought 

 to the middle, it will, in future, ascertain the true po- 

 sition without having recourse to the original adjust" 

 ment. 



The upper part of the back index is formed into a se- 

 micircle of about half the radius of the circle itself, and 

 upon the fore part of it are roughly divided two qua- 

 drants of zenith distance, and a blank of about half an 

 inch separates their &ros from each other. Two small 

 sliders, one of whicli is seen at i, are made to fit the arc 

 of the semicircle, and their fiducial edges may be set so 

 as to point to any division. To the object end of the 

 fore telescope is occasionally attached an index j, which 

 reaches down to the divisions of the semicircles, and at 

 that end covers the space between the zeros. By this 

 contrivance, when the sliders arc set to the approximate 

 zenith distance of a heavenly body about to be obser- 

 ved by repetition, the sliders and indices will touch each 

 other at their relative positions, so as to stop the tele- 

 scope at the proper point upon the limb for finding the 

 object, near the centre of the field'of view. 



The azimuth mstion has also its stops ; a small cylin- 

 der of steel, pointed at one end, is lodged in a niece of 

 brass, and urged forwards by the force of a spiral spring 

 against the edge of the azimuth circle when used, but at 

 other times it is put out of action. When the leading 

 vernier of the circle shews 360 or 180', the pointed 

 end enters a shallow hole, which, for that purpose, is 

 made in the edge of the circle, but so as only just to be 

 felt on turning it round ; for the point will retire of it- 

 self when a slight additional pressure is applied, and then 

 it allows the instrument to turn freely round. With 

 thee contrivances, a bright object may, by repetition, 

 be observed in the day time, or a faint one by night, 

 the former without fear of losing it, and the latter with- 

 out danger of mistaking another for it ; and that with- 



