TRANSIT INSTRUMENT

This important instrument, called by the French instrument des pa ages, lunette miridienne, consists e entially of a telescope firmly attached to a transverse horizontal axis, the ends of which are directed to the east and west points of the horizon. The extremities of the axis are formed into cylindrical pivots of exactly equal diameters, which rest in notches (technically called Y's, from their resemblance to that letter), formed in metallic supports, susceptible of nice adjustment both horizontally and vertically, so that the axis can be placed perfectly horizontal, and at right angles to the plane of the meridian in which the telescope moves. In the focus of the eye-piece is placed a system of three, five, or sometimes seven vertical and equidistant wires or spider lines, generally cro ed by two horizontal ones, between which it is convenient that the pa age of objects over the vertical wires should be observed. By means of adjusting screws the diaphragm, or plate to which the wires are attached, is brought into such a position that the middle vertical wire intersects the optical axis of the telescope, in which position it is permanently fixed. When the system of wires is brought into this position, the middle one will be a visible representation of that part of the meridian to which the telescope is directed; and when a star is seen to pa this wire, it is in the act of culminating, or transiting the celestial meridian. The instant of the transit is noted on a clock or chronometer, which is an indispensable accompaniment of the instrument; and in order to render the observation more certain, the instant at which the star pa es each of the vertical wires is noted, when practicable, and the mean taken as the true instant of pa ing the vertical wire. The times at which the sun and certain principal stars pa the meridian of Green wich (and consequently the meridian of any other place whose longitude from Green wich is known") being given m the Nautical Almanac, the comparison of the time indicated by the clock and the time in the almanac gives the clock error; and by observing the same stars from day to day the clock rate is determined. In this manner we are enabled to a ign the exact interval of sidereal time between the transits of the different stars, and consequently the difference of their right ascensions. For determining the absolute place of a celestial body it is nece ary to know also its polar distance, which is given by the mural circle; so that the [s. 1263]