When the quadrant was thus reduced into the plane of the meridian by the holdfasts above described, that radius of it which terminates 90 degrees, was placed exactly vertical (by the movement above mentioned,) with a plumb line of very fine silver wire; so suspend ed as to play exactly over the middle of the central point o (in the pole of the arbor o i,) and also over the stroke at 90 degrees upon the limb below. This posi tion of the quadrant being once found, another plumb line was suspended by the side of the quadrant, quite clear of the centre work, so as to play exactly over the middle of a fine point made.in the limb below, in order to examine afterwards with more expedition, whether the quadrant has kept its place. For this purpose an oblong piece of brass ab, laid flat upon the square plate at the centre of the quadrant, was gradually moved to the right or left by two screws c, d, working against the ends of it : a slit a b being cut lengthwise through the plate to slide along two other screw-pins, e, f, fixed in the back plate. The wire of the plummet was hung by a loop upon a pin g, and lay in a very fine angular nick, filed in the edge of a little plate h, which projected a little farther than the loop for the wire to bear upon it. This plate h, and the pin g, were both fixed to the oblong plate a b ; by whose gradual motion, above described, the wire h i was brought to play exactly over the middle of the hole i in the limb ; and then the plate a b was pressed to the quadrant by the screws e,f.—Smith's Optics, Vol. II. p.
In the year 1753 a quadrantal arc of 96°, with its ' subdivisions, was inserted in this quadrant by Bird, be tween the two original divided arcs, and all the obser vations since made with that instrument are referred to this new arch.
2. Description of Bird's Mural Quadrant The Mural Quadrant of Mr. Bird was constructed on the model of Graham's, and was made entirely of brass. It was erected in the time of Dr. Bradley in 1750, and was used by that astronomer and Dr. Al askelyne in their most valuable observations. The radius of that instru ment is eight feet. Near the eye-piece of the telescope there is a good micrometer, for giving it a slow motion, and for measuring the number of seconds that the read ing lines of the vernier want of coincidence in any ob servation. As Mr. Smeaton recommended, however, the vernier is no longer used, and the last 5' space in the observed arc is subdivided by the screw.
Mr. Pond having had reason for suspecting the accu racy of the total arc of the quadrant, Mr. Troughton ex amined it by an apparatus contrived for the purpose. Ile found the total length of the quadrantal arch 7" too small, and he discovered a similar error of 2", arising from the wear of the axis of motion. In the intermediate division he could not detect more than one second of inequality. The difference of the read ings of the two arcs has never been observed to exceed 4", which proves the great accuracy of Bird's gradua tion. The graduation of 7" in the total arch seems to
be occasioned solely by a change of figure in the space of 48 years, as Dr. Bradley found the arc to be exactly 90° in 1759.
Mr. Troughton has not examined the quadrantal arch of Graham's quadrant, but he is of opinion that, being made of iron, it has preserved its form better than Bird's, which was made of brass.
3. Cole's Quadrant by a single Reflection.
This instrument, invented by Mr. Benjamin Cole, is composed of a moveable radius AB, Plate CCCCLXXVI. Fig. 10. a quadrantal arch DE, three vanes A, B, C, and a vernier FG. The moveable radius is a piece of wood about 2 inches long and 12 thick. The quadrantal arch is also of wood, and has a radius of 9 inches, being divid ed into degrees and third parts of a degree. The sight vane A, a thin piece of brass about 2 inches high and 1 broad, is placed perpendicularly at the end of the radius A, and through the middle of it is a small hole, through which the coincidence of the horizon and solar spot is to be viewed. The horizon vane B, about 1 inch broad, and 2a high, has a slit cut through it about q inch long and 4 broad. It is fixed in the. centre pin of the quadrant perpendicularly, and is always inclined to the sight vane. The shade vane C consists of two brass plates, one of which revolv ing on an arm, is about inches long, and aths of an inch broad. It is pinned at one end to the upper limb of the quadrant by a screw, about which it has a small motion. The other end lies in the arch, and the lower edge of the arm is directed to the middle of the centre pin. The other plate, which is properly the vane, is about two inches long, and is fixed at right angles to the first plate, at the distance of about half an inch from the end next the arch. This vane may be used either by the shadow which it casts, or by the solar spot formed by a convex lens placed within it. In order to rectify this vane, set the line C of the vernier opposite to a degree on the upper limb of the quadrant, and turn the screw on the back side of the limb forward or back ward till the hole in the sight vane, the centre of the glass, and the sunk spot in the horizon vane, all lie in the same straight line. In order to take the altitude of the sun by that quadrant, turn your back to the sun, and hold the instrument of the radius AB with your right hand, till it is in a vertical plane passing through the sun, then looking through the sight vane and the hori zon vane till you see the horizon, move the quadrantal arch upward with the left hand, till the shadow of the sight vane or the solar spot formed by the lens fall di rectly on the slit in the horizon vane. When that is done, the part of the quadrant raised above G or S, ac cording as the solar spot or shade is viewed, will be the altitude of the sun required.