TRANSIT INSTRUMENT

This important instrument, called by the French instrument des pa ages, lunette meridienne, consists e entially of a telescope firmly attachdirecteded to a transverse to the east horizontal and west "axis,points the of ends the horizon.of which The are extremities of the axis are formed into cylindrical pivots of exactly equal diameters, which rest in notches (technically called Y's, from their resemhance 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 pos i tion 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 note3 on a clock or chronometer, which is an indispensable accompaniment of the instrument; and ir. 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 and 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 iriven in the Nautical Almanac, the comparison of the time1259 indicated [s. 1273]