THE TRANSIT. 17 "Whence, substituting in the formula, we have 2^=29 57' 53" 4- 0.6i3 (1 28' 32") + 0.643 (- ^|*) (11".5) + (0.643) I ( - 0.357) (0.143) (- 1"). = 29 57' 53" + 56' 55".616 - 1".32 + 0".01, = 30 54' 47".31 the required distance. Instruments. The principal instruments used in field astronom ical work are the Transit, Sextant, Zenith Telescope, and Altazi- muth or Astronomical Theodolite. A short description of each instrument will be given in connection with the first problem in- volving its use. But since much relating to the transit is appli- cable also to the zenith telescope and altazimuth, that instrument will be explained first. THE TRANSIT. The Transit is an instrument usually mounted in the meridian, and employed in connection with a chronometer for observing the meridian passage of a celestial body. Since the R. A. of a body is equal to the sidereal time at the instant of its meridian passage, or is equal to the chronometer time plus its error (a = T-- E), it is seen that by noting T, E will be given when a is known, and con- versely a will be given when E is known. The very accurate determination of E is the chief use of the transit in field work. The instrument consists essentially of a telescope mounted upon and at right angles to an axis of such shape as to prevent easy flexure. The ends of this axis called the pivots, are usually of hard bell metal or polished steel, and should be portions of the same right cylinder with a circular base. They rest upon Y's, which in turn are supported by the metal frame or stand. At one end of the axis there is a screw by which its Y may be slightly raised or lowered in order that the axis may be made horizontal. At the other end of the axis is another screw by which its Y may be moved backward or forward, in order that the telescope may be placed in the meridian. The telescope is provided with an achro- matic object glass, at the principal focus of which is a wire frame carrying an odd number of parallel vertical wires as symmetrically disposed as possible with reference to the middle; also two horizon- tal wires near to each other, between which the image of the point
