THEODOLITE

(Yunanca köken — orijinale bakınız.)A surveying instrument for measuring the angular distances between objects projected on the plane of the horizon. In accurate surveying, when the instrument used for observing the angii ll or reflecting circle, or such that Its plane must be brought into the plane of the three inch form the angular points of the triangle to be measured, the altitudes of the two distant objects above the horizon of the observer must be determined, and a calculation is then nece ary to reduce the observed angles to the plane of the horizon. The object of the theodolite is to measure tile horizontal angles at once, and thereby render the preV tO US calculation, and even the observation of the altitudes, unnece ary.. -y to conceive, in a general way, how this object may be effected. A telescope with cro wires in its focus must be mounted so as to be moveable both about n vertisal and horizontal axis, in order that it may be brought to beai upon any object, whether in the horizon, or above or below It; am! the proper means applied for measuring, with the utmost accuracy, the angle described aliout the vertical axis (which is the horizontal angle) in turning the telescope round from one object to another. It is consequently neces sary to tix a graduated circle to the vertical axis, so that its plane may be exactly horizontal, and its centre coincident with the vertical axis; and about the same axis a radial bar, firmly connected with the telescope, must revolve parallel to the plane of the circle, for the purpose of indicating the divisions pa ed over on the limb of the circle when the telescope is turned from the first object to the second. And in order to render the instrument subservient to the purpose of measuring altitudes, a graduated vertical arch is attached to the telescope; but as it seldom happens, in the practice of surveying, that the objects observed are very much elevated above or depre ed below the horizon, or that the vertical angles are required to be taken with the same degree of accuracy as the horizontal angles, the vertical arch is generally only a portion of a circle of smaller radius, and le minutely divided than the horizontal circle. The theodolite, as now generally constructed for the pur ordinary surveying, may be described as follows: The horizontal limb or circle consists of two circular plates, which turn freely on each other. The lower or graduated plate receives the divisions of the circle, and the upper or vernier plate has two vernier divisions diametrically opposite. The vertical axis consists of two conical parts, one working within the other. The external part is attached to the graduated plate, and the internal to the vernier plate. Tin diameter of the under plate is somewhat larger than that of the vernier plate, and its edge is sloped otTto receive the graduation; and portions of the opposite edges of the vernier plate are sloped otT in like manner to receive the vernier divisions. The graduation is usually to thirty minutes ol a degree, but it is subdivided by the verniers into single minutes; and in a well-made instrument quarter ■mutes may be estimated by the eye. For the purpose of adjusting the plane of the circle to the horizon, the external Una Is tilted into a ball, which works in a socket between two parallel plates held firmly together by the ball and socket, the under plate being connected with the staff head supporting the Instrument. But this adjustment may also be made tand in larger Instruments is usually made) by a tipod support, having a I lot screw at each extremity acting [s. 1251]