Between the two portions of the axis of the circle, namely, that which works in the socket of the counter and that upon which the socket of the back tele scope turns, there is securely united to the axis a wheel of about two inches diameter;the edge of which is sur rounded by a circle cut into three pieces, and again joined at two pkices in the same manner as the links of a watch chain, mid at the third by a clamping screw, which is seen 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 ears 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 is released from the ac tion of the clamp, the general motion is free. The ap paratus last described is very superior to the means which Borda employed for accomplishing the same object. 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 screw, which, for the sake of quick and slow motion, is thrown off and on at pleasure. This screw, owing to its great distance 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 dan gerous, is at least a very troublesome one, am] produc tive 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 of necessity 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 otherwise 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 pla ced 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 position without having recourse to the original adjustment.
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 quadrants of zenith distance, and a blank of about half an inch sepa rates their zeros from each other. Two small sliders, one of which is seen at i, are made to fit the arc of the semi circle, 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 are set to the approximate zenith distance of a heavenly body about to be observed by repetition, the sli ders and indices will touch each other at their relative po sitions, so as to stop the telescope at the proper point up on the limb for finding the object, near the centre of thc field of view.
The azimuth motion has also its stops ; a small cylin der of steel, pointed at one end, is lodged in a piece 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 slims 360° or I 80,° 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 these 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 out having recourse to the divisions at all during the in termediate observations.
An index carr) ing a microscope at each end, one of which is shewn at k, is applied to the centre of the fore index, by the help of which the four verniers arc lead ; that is, two of them by each, for one will not read all the four, because the arm cannot pass the telescope. A similar arm is centered upon the base of the pillar, which, moving freely all round, its sing-le microscope serves to read off all the three verniers of the azimuth circle.
Having now gone through the description of thc re peating circle, it only remains that there should be given an enumeration of its few and simple adjustments. The azimuth axis is adjusted vertical, or the circle ho•, rizontal, by mean a of the feet screws and levels, in the way practised in the corresponding adjustment of other instruments. 2d, the plane of the upper circle is ad justed vertical for astronomical purposes, by turning it round the cross axis; and the required position is as certained by applying- a plumb-line to the face of the circle. This adjustment is, however, obtained much more correctly, by observing any elevated object at nest, pointing the telescope upon it directly, and then obser ving its image as reflected from the surface of a fluid : the middle vertical wire bisecting, in both cases, is the proof of adjustment, and furnishes an unexceptionable criterion. 3d, The line of collimation of the telescope parallel to the plane of the circle. As both the tele scopes have their positions eccentric, respecting the ver tical axis, choose, for this purpose, a very distant object, and as nearly as can be estimated in the horizon; bisect that object by the middle vertical wire, and then read off what the verniers of the azimuth circle give; now turn the face of the circle to the contrary !rand, bisect the object in this position, and read the azimuth circle a second dine ; now carry .the instrument in azimuth through half the angular difference, (abating I S0°), as shown at the two positions, and then the plane of the circle will pass exactly through the object ; move now the wires of both telescopes by their proper screws, until they bisect the object. The fourth adjustment is of use only, when taking levels is the end of observation ; it is to set the line of collimation of the back telescope parallel to the level ; a very distant object is also required for this purpose, because the back telescope is lower in position than the front one. With every thing adjusted as before, bisect any object nearly in the horizon by the middle horizontal wire of the fore telescope, and read the divisions of the circle ; reverse the instrument, both in altitude and azimuth, and bisect the object, and read the circle as before. The half difference of the readings will be the altitude or depression of the object; set the indices, therefore, to this half difference, and turning the instrument round in azimuth, look for a new object that is bisected by the wire ; this object will be in the horizon, and the end will be accomplished by turning the back index until the level comes to the mid dle, and adjusting the horizontal wire by the proper screw, so that it may bisect that object.