Transit Instrument

(Ast.) A telescope so constructed as to point always to the meridian. It rotates therefore on an horizontal axis, directed due east and west. The instrument is employed to determine the moment when a star cro es the meridian. As it is of the utmost importance that such observations should be made with extreme accuracy, many contrivances have been adopted to make the instrument work as perfectly as po ible. What is requisite is that the axis should be perfectly horizontal, that it should point due east and west, and that the optical axis of the telescope should be exactly at right angles to it.-A T. circle is a 7. instrument, the telescope of which is fixed between two graduated circles, so that the altitude of a star, as well as the time of meridian pa age, may be accurately noted. [s. 904]