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Published online by Cambridge University Press: 05 July 2011
By far the most important of what may be termed the miscellaneous astronomical instruments is the Transit, or Transitcircle, the smaller and less perfect kinds being chiefly used for taking the time, and the larger for measuring the positions of stars, &c, for forming catalogues.
We shall only describe the small, or portable, transit. The instrument consists of three principal parts, the telescope, the stand, and the circle: a b, is a telescope of a large field and low power, the tube of which is in 2 parts, connected by a cubical centre-piece, into which, at right angles to the optical axis, are fitted the larger ends of 2 cones, c, c, which form the horizontal axis of the telescope, the smaller ends of each cone are accurately ground to z perfectly equal cylinders, or pivots. These pivots rest on Y's, angular bearings, which surmount the 2 side standards, e and w, of which e may be called the eastern, and w the western. One of the Y's is fixed in a horizontal groove, so that, by means of a screw, a small azimuthal motion may be imparted to the instrument; in like manner a small motion in altitude may be obtained by turning the foot screw g. On one end of the axis is fixed, so that it may revolve with it, a decimation circle, d, divided to degrees and read by verniers to minutes, &c.
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