Theodolite

An instrument, variously constructed, used, especially in trigonometrical surveying, for the accurate measurement of horizontal angles, and also usually of vertical angles. The theodolite consists principally of a telescope, with cro -wires in its focus, mounted so as to turn both on vertical and horizontal axes, the former carrying a horizontal vernier-plate over a graduated plate or circle for aximuthal angles, and the latter a vertical, graduated arc or semicircle for altitudes,—the whole furnished with leveling-screws and levels for adjusting to the horizon, and mounted on a tripod. It is usually so constructed that a horizontal angle may be repeated indefinitely around the limb, and thus a large number of repetitions added mechanically, to secure greater accuracy in the resulting mean.