OBSERVATORY. 



pole ; for supposing a o p b c, to be an 

 arch of the meridian, make the angle a 

 f P> equal to half the angle a/c, and 

 'the linefp, will point to the pole ; and 

 if the angle o f p, which is {he error of 

 the axis, will be equal to half the angle, 

 bfc, ormfu, found by the observation ; 

 because the difference of the two angles, 

 afh, a/c, is double the difference of 

 their halves, afo, and a f p. Unless 

 the star be very near the pole, allowance 

 must be made for refractions. See QUAD- 



BAUT. 



VI. Transit and Equal Altitude Instru- 

 ments. The transit instrument is used for 

 observing objects as they pass over the 

 meridian. It consists of a telescope fixed 

 at right angles to a horizontal axis ; which 

 axis must be so supported, that what is 

 called the line of collimation, or line of 

 sight of the telescope, may move in the 

 plane of the meridian. This instrument 

 is made of various sizes, and of large di- 

 mensions, in our great observatories; but 

 the following is one of a size sufficiently 

 large and accurate for all the useful pur- 

 poses. The axis, A B (fig. 2), to which 

 the middle of the telescope is fixed, is 

 about two feet and a half long, tapering 

 gradually toward its ends, which termi- 

 nate in cylinders well turned and smooth- 

 ed. The telescope, C D, which is about 

 four feet, and an inch and a half diame- 

 ter, is connected with the axis by means 

 of a strong cube or die, G, and in which 

 the two cones, M Q, forming the axis, are 

 fixed. This cube G, serves a the prin- 

 cipal part of the whole machine. It not 

 only keeps together the two cones, but 

 holds the two sockets, K H, of fifteen 

 inches length, for the two telescopic 

 tubes. Each of these sockets has a square 

 base, and is fixed to the cube by four 

 screws. These sockets are cut down in 

 the sides about eight inches, to admit 

 more easily the tube of the telescope ; 

 but when the tube is inserted, it is kept 

 in firm by screwing up the tightening 

 screws at the end of the sockets at K and 

 H. These two sockets are very useful 

 in keeping the telescope in its greatest 

 possible degree of steadiness. They also 

 afford a better opportunity of balancing 

 the telescope, and rectifying its vertical 

 thread, than by any other means. In or- 

 der to direct the telescope to the given 

 height that a star would be observed at, 

 there is fixed a semicircle, A N, on one 

 of the supporters, of about eight inches 

 and a half diameter, and divided into de- 

 grees. The index is fixed on the axis, at 

 the end of which is a vernier, which sub- 



divides the degrees into twelve parts of 

 five minutes. The index is moveable on 

 the axis, and may be closely applied to 

 the divisions by means of a tightening 

 screw. Two upright posts of wood or 

 stone, Y Y, firmly fixed at a proper dis- 

 tance, are to sustain the supporters of 

 this instrument. These supporters are 

 two thick brass plates, R R, having well 

 smoothed angular notches in their upper 

 ends, to receive the cylindrical arms of 

 the axis. Each of these notched plates 

 is contrived to be moveable by a screw, 

 which slides them upon the surfaces of 

 two other plates immoveably fixed upon 

 the two upright pillars ; one plate mov- 

 ing in a horizontal, and the other in a ver- 

 tical direction ; or, which is more simple, 

 these two modes are sometimes applied 

 only on one side, as at V and P, the hori- 

 zontal motion by the screw P, and the 

 vertical by the screw Y. These two 

 motions serve to adjust the telescope to 

 the plane of the horizon and meridian .- to 

 the plane of the horizon by the spirit- 

 level, E F (fig. 4) hung by D C on the 

 axis M Q, in a parallel direction, and to 

 the plane of the meridian in the following 

 manner : Observe by the clock when a 

 circumpolar star seen through this instru- 

 ment transits both above and below the 

 pole : and if the times of describing the 

 eastern and western parts of its circuit 

 are equal, the telescope is then in the 

 plane of the meridian : otherwise the 

 screw, P, must be gently turned, that it 

 may move the telescope so much that the 

 time of the star's revolution be bisected by 

 both the upper and lower transits, taking 

 care at the same time that the axis re- 

 mains perfectly horizontal. When the 

 telescope is thus adjusted, a mark must 

 be set at a considerable distance (the 

 greater the better) in the horizontal di- 

 rection of the intersection of the cross 

 wires, and in a place where it can be il- 

 luminated in the night-time by a Ian- 

 thorn hanging near it; which mark be- 

 ing on a fixed object, will serve at all 

 times afterwards to examine the posi- 

 tion of the telescope by the axis of the 

 instrument being first adjusted by means 

 of the level. 



To adjust the Clock by the Sun's Transit 

 over the Meridian. Note the times by the 

 clock when the preceding and following 

 edges of the Sun's limb touch the cross 

 wires. The difference between the mid- 

 dle time and twelve hours, shows how 

 much the mean time, or time by the 

 clock, is faster or slower than the appa- 

 rent or solar time for that day-; to which 



