THEODOLITE, a most important in strument for measuring horizontal and vertical angles, but particularly adapted for accurately measuring the former. Its principle is identical with that of the altitude and azimuth instrument; the construction and purpose of the two, however, differ, the latter being employed for astronomical purposes, while the theodolite is used for land surveying; but the better instruments of this class may be employed for observing the al titude of celestial bodies. The vertical circle is not generally, however, of suffi cient size, nor so graduated as to be available for very accurate astronomical observations.
When a point is to be viewed with the telescope, the telescope is moved so that the image of the point coincides with the intersection of the cross wires. The vertical limb is divided into degrees, and is capable of being read by means of the vernier and the microscope to thirds of a minute. A pair of plates, constituting at their edge the horizontal limb of the iustrument, are free when unclamped, to move independently of each other. The plate carries a magnetic compass and two spirit levels at right angles to each other, by means of which the circle may be brought accurately into the horizontal plane by raising or depressing it by means of the screws. The plate is fur
nished with two verniers, diametrically opposite to each other, the degrees marked on which are read off by the mi croscope. By the motion of the tele scope, on the horizontal axis of the ver tical limb, altitudes and vertical angles can be measured, while, by its motion on the vertical axis, the angular dis tances between two objects can be ascer tained by the readings on the horizontal circle.
Before using a theodolite, it should be properly adjusted; that is, the different parts should be brought to their proper relative positions. The theodolite is in adjustment when the following condi tions are fulfilled: (1) When the inter section of the cross wires is in the axis of the telescope. (2) When the axis of the attached level is parallel to the axis of the telescope. (3) When the axes of the levels on the horizontal limb are per pendicular to the axis of the horizontal limb. (4) When the axis of the vertical limb is perpendicular to the axis of the horizontal limb.