Trāns'it-in'stru-ment, n.

n. A telescope mounted at right angles to an horizontal axis, on which it revolves with its line of collimation in the plane of the meridian, used in connection with a clock for observing the time of transit of a heavenly body over the meridian of a place.

Prime vertical transit-instrument, a transit-instrument placed with its horizontal axis in the plane of the meridian, and used for observing the transit of bodies over the prime vertical.