TRANSIT INSTRUMENT (Lat. trnasitta, passage over). An astronomical instrument consisting principally of a telescope fixed to a horizontal east-and-west axis, and revolving in the plane of the meridian. It is employed, as its name denotes, in the observation of the meridian transits of the heavenly bodies, i.e. in noting the exact instants when they reach the meridian of the observatory. The axis, which is the most important part of the instrument, and thus demands the utmost care in its con struction, consists of a hollow sphere or cube, to opposite sides of which are tightly fastened the bases of two cones in whoe apices the pivots are screwed; the sphere or cube is pieced for the admission of the telescope, which is firmly fas tened at right angles to the axis. One of the pivots is hollowed so that a stream of light can be directed from a lantern half way along the interior of the axis, and through an aperture in the side, into the telescope tube, where, being received by a small reflector, set at 45° to the axis and telescope tube, it is directed to the eye piece, rendering visible a series of fine spider threads stretched across the field of view. The pivots must be carefully turned to a perfectly cylindrical form, and fitted into the instrument, so that they are accurately in line. One ex tremity of the axis carries one and sometimes two small graduated circles, each supplied with index, clamping screws, and vernier; these circles are capable of indicating angular measures to within 1' or 2'. There are three adjustments
necessary before a transit instrument can be used: the axis must be horizontal; the sight line of the telescope, as marked by the spider-threads, must be at right angles to the axis of motion; and the latter must be placed so as to point accurately east and west. On the perfection of the first two of those adjustments depends whether the telescope sweeps over a great circle of the sphere, and the third is necessary to in sure that this great circle shall be the meridian of the place of observation. These adjustments can never be made quite perfect, and the usual mode is to investigate the amount of error in each, and allow for it by a calculated correc tion applied to the observed time of the star's crossing the meridian. To note accurately this observed time by the astronomical clock is, as we have said, the essential part of a transit ob servation. It is best done by means of a chrono graph (q.v.), an instrument with which an auto matic record of the observed time can he made by means of an electric signal sent out directly from the observer's hand. The transit used by engineers will be found described and illustrated under ENGINEERING INSTRUMENTS.