The price of a small Equatorial instrument, such as that described p. 454, is about 16 guineas, exclusive of some of the eye-pieces, which were afterwards added for the purpose of making particular observations. Instruments of a larger size, and with more complicated machinery, sell from 50 to 100 guineas and upwards. Messrs. W. and S. Jones, Holborn, London, construct such instruments.
ON THE QUADRANT.
figure 87.
Every circle being supposed to be divided into 360 equal parts, or degrees,—it is evident, that 90 degrees, or the fourth part of a circle, will be sufficient to measure all angles, between the horizon of any place and the line perpendicular to it which goes up to the zenith. Thus, in fig. 87, the line CB represents the plane of the horizon. ACBH, the quadrant, AC the perpendicular to the horizon, and A the zenith point. If the lines BC and CA represent a pair of compasses with the legs standing perpendicular to each other, and the curved lines AB, DE and FG, the quarter of as many circles of different sizes—it is evident that although each of these differs from the others in size, yet that each contains the same portion of a circle, namely a quadrant or fourth part; and thus it would be from the smallest to the largest quadrant that could be formed,—they would all contain exactly 90 degrees each. By the application of this principle the comparative measure of angles may be extended to an indefinite distance. By means of an instrument constructed in the form of a quadrant of a circle, with its curved edge divided into 90 equal parts, the altitude of any object in the heavens can at any time be determined.
There are various constructions of this instrument, some of them extremely simple, and others considerably complex and expensive, according to the degree of accuracy which the observations require. The following is a description of the Pillar Quadrant, as it was made by Mr. Bird, for the observatory of Greenwich, and several continental observatories.
This instrument consists of a quadrant E E H G L (fig. 88.) mounted on a pillar B, which is supported by a tripod AA, resting on three foot screws. The quadrant, the pillar, and the horizontal circle all revolve round a vertical axis. A telescope H is placed on the horizontal radius, and is directed to a meridian mark previously made on some distant object for placing the plane of the instrument in the meridian, and also for setting the zero, or beginning of the scale truly horizontal. This is sometimes done by a level instead of a telescope, and sometimes by a plumb-line G, suspended from near the centre, and brought to bisect a fine dot made on the limb, where a microscope is placed to examine the bisection. The weight or plummet at the end of the plumb-line is suspended in the cistern of water b, which keeps it from being agitated by the air. A similar dot is made for the upper end of the plumb-line upon a piece of brass, adjustable by a screw d, in order that the line may be exactly at right angles to the telescope, when it is placed at o. The quadrant is screwed by the centre of its frame, against a piece of brass e with three screws, and this piece is screwed to the top of the pillar B with other three screws. By means of the first three screws, the plane of the quadrant can be placed exactly parallel to the vertical axis, and by the other screws the telescope H can be placed exactly perpendicular to it. The nut of the delicate screw L is attached to the end of the telescope F, by a universal joint. The collar for the other end is jointed in the same manner to a clamp which can be fastened to any part of the limb. A similar clamp-screw and slow motion is seen at n for the lower circle, which is intended to hold the circle fast, and adjust its motion. The divisions of the lower, or horizontal circle, are read by verniers, or noniuses, fixed to the arms of the tripod at l and m, and, in some cases three are used to obtain greater accuracy.
figure 88.
In using this quadrant, the axis of the telescope H is adjusted to a horizontal line, and the plane of the quadrant to a vertical line, by the means already stated. The screw of the champ L is then loosened, and the telescope directed to the star, or other object, whose altitude is required. The clamp screw being fixed, the observer looks through the telescope, and with the nut of the screw L he brings the telescope into a position where the star is bisected by the intersection of the wires in the field of the telescope. The divisions are then to be read off upon the vernier, and the altitude of the star will be obtained. By means of the horizontal circle D, all angles in the plane of the horizon may be accurately measured—such as the amplitudes and azimuths of the celestial bodies.