QUADRANT. As an astronomical instrument, the quadrant has within a few years been so completely superseded by the entire circle, that it will not be worth while to describe particularly its construction or adjustments. Still so much of the very groundwork of modern astronomy depends on data furnished by the quadrant, that it cannot be properly passed over without some notice.
We have already said [CIRCLE] that the earliest form of instrument for measuring celestial altitudes was also the best, namely, the solstitial or meridian circle described by Ptolemy. After showing how the proportion which the arc between the tropics bears to the whole circumference was to be determined by this instrument, Ptolemy proceeds to say (‘ Amalgest,' book L, chap. 10), " We have made this sort of observation more conveniently by using, instead of circles, a stout quadrangular block of wood or stone, having one side plane and smooth. Upon this aide we described a fourth part of a circle from a centre near one of the angles, and having drawn from the centre two radii including a right angle, we divided the circumference into ninety degrees with subdivisions. We then inserted two perfectly equal cylinders at the extremities of the vertical radius, so as to be exactly concentric with the centre and extreme point, and set the block vertical by a plumb-line passing over the cylinders, and also in the plane of the meridian by a north and south line described on the horizontal plane. We observed at noon the shadow of the central cylinder, having applied something to the divided arc to show the place more clearly, and, marking the middle point, we took the corresponding division of the quadrant as showing the elevation of the sun on the meridian." It would seem that on the revival of learning, when Ptolemy was thought infallible, tho quadrant came into use on this authority, to the exclusion of the circle. In the Astronombe Instauratre 3techanica ' of Tycho Brand, figures and descriptions will be found of quadrants and sextants of various forms and sizes. We do not think justice is done at the present day to the merits of Tycho, who is better known as the perplexer of the Copernican hypothesis, than as the first great practical astronomer after Hipparchus. Among his numerous instruments Tycho had a large quadrant fixed on a wall, which he calls a mural quadrant, with which he observed meridian altitudes, noting the time of transit by a clock. There are several
other quadrants figured and described, which revolve on a vertical axis, and some have a horizontal circle by which the azimuth was observed at the same time with the altitude. These may be considered as the precursors of the modern astronomical quadrant, and still more perfect altitude and azimuth instrument, just as his mural quadrant led to the mural arc of Flamsteed, the mural quadrants of Bird and Ramsclen, and finally to the mural circle of Troughton. The imper fection of his clocks (for the pendulum was not applied till nearly a century later) compelled Tycho to adopt an instrument which has long been out of use. This was his astronomical sextant, which was stronger, more convenient, and lighter than the quadrant. It was supported at the centre of gravity on a ball and hemispherical cup, and could consequently be easily placed in the plane passing through two stars, and so used for measuring their distance from each other.
To verify the value of the arc, and to test the powers of his sextants and quadrants, the distances of a chain of stars near the equator were taken, and their declinations also observed, when it was found that the sum of the angles at the pole which resulted from observation was 360° very nearly. (Tycho Brahe, Astronomite Instauratn Progymnasmata,' pp. 138, 145.) Hevelius has described his quadrants and sextants in the first volume of his Machin Cmlestis,' a work which is not uncommon ; the second volume, containing his observations, was burnt soon after it was printed, with the exception of a few copies, and is one of the scarcest and dearest astronomical books in existence. In the convenience of his instruments, and perhaps in the accuracy of their graduation, he surpassed Tycho, but he never could understand the advantage of telescopic over plain sights, though one of the best practical opticians and industrious star-gazers of his time. This unfortunate prejudice not only rendered the labour of his long life labour in vain, but embittered his latter years to a painful degree by involving him in a dispute with Hooke.