THEODOLITE. A most important surveying instrument for measuring hor izontal angles, or the angular distances between objects projected on the plane of the horizon. Tins instrument is various ly constructed, and provided with sub ordinate apparatus, according to the price, or the particular purposes to which it is to be applied. One of the most gen erally useful, consists of two concentric horizontal circular plates A and 13, turn freely on each other. The lower or graduated plate B, contains the divisions of the circle, and the upper or vernier plate has two vernier divisions a, dia metrically opposite, only one of which is short n iu the cut. The vertical axis C consists of two conical parts, the one working within the other. The external part is attached to the graduated plate B, and the internal to the vernier plate A.
The plane of the circle is adjusted to the horizon by the screws b b b, acting against a plate of metal resting on the staff-head supporting the instrument. The vernier plate cornea two spirit levels, c c at right angles to each other, with their proper adjusting screws, by which the circle is brought accurately into the horizontal plane indicated by the levels. The horizontal axis of the vertical limb of the instrument is supported by a frame attached to the vernier plate, and turn ing along with it about the vertical axis. To the horizontal axis D, a telescope, with cross wires in its focus, is attached, which moves in the vertical plane, by the graduated circle E, and is used for observing the objects whose angular dis tance is to be measured, and also for tak ing altitudes, or measuring vertical an gles; a spirit level is fixed beneath the telescope for its adjustment. F is a mi
croscope for reading off the vernier divi sions. The screws g, h, are for regulating and fixing the external part of the vertical axis C. To measure the angular distance between any two objects, the telescope is turned round along with the vernier circle (the graduated circle remaining fixed), until it is brought to bear exactly upon one of the objects; it is then turned round until it is brought to bear on the other object, and the arc which the ver nier has described on the graduated cir cle, measures the angle required. The observation may be repeated any number of times in order to insure accuracy, by means of a repeating stand, which turns round concentrically with the vertical axis of the theodolite. The theodolite is riot only a most essential instrument in tri gonometrical surveying for determining stations, and running base-lines, but also in geodetical operations, for assisting in determining the length of an arc of the meridian. For this latter purpose it re quires to be constructed on a large scale.