Transit Instrument
an instrument designed accurately to denote the time when a heavenly body passes the meridian. It consists of a telescope supported on a horizontal axis, or pivots, the extremities of which terminate in cylindrical trunnions resting in metallic supports shaped like the upper part of the letter Y, and hence termed the "Y's," and imbedded in two stone pillars. In TRANSIT INSTRUMENT order to relieve the pivots from friction and facilitate the turning of the telescope, counterpoises are provided operated through levers, carrying friction rollers, on which the axis turns. When the instrument is in proper adjustment, the telescope should continue in the plane of the meridian when revolved entirely round on its axis, and for this purpose the axis must lie in a line directly E. and W. To effect this adjustment its ends are provided with screws by which a motion, both in azimuth and altitude, may be imparted. The telescope has a series of parallel wires crossing its object glass in a vertical direction. When a star, designed to be the subject of observation, is seen approaching the meridian, the observer looks at the hour and minutes on a clock placed at hand for the purpose. He then notes the passage of the star across such wire, listening at the same time to the clock beating seconds. The exact time at which the star passes each wire is then noted and the mean between the time of passing each two wires equidistant from the center being taken gives a very close ap- 8 TRANSIT INSTRUMENT proximation to the truth. The transit instrument is the most important of what may be called the technical astronomical instruments. The smaller and portable kinds are used to ascertain the local time TRANSMISSION by the passage of the sun or other object over the meridian, while the larger and more perfect kinds, in first-class observatories, are used for measuring the positions of stars, for forming catalogues, its special duty being to determine with the greatest accuracy the right ascension of heavenly bodies.