Theodolite, a surveying instrument for measuring vertical and horizontal angles and taking levels, combining the uses of the ordinary transit, the quadrant, and the level. In the American form of the instrument, the telescope turns over and the vertical angles are read on a graduated circle. In the English form the vertical angles are read on a semicircle beneath the telescope and level; the telescope cannot therefore turn over, but is reversible. The American form is preferable by reason of the greater facility and precision of the adjustments. In common with all such kinds of instruments, it is made of brass. The principal parts are the vertical circle A and the horizontal circle F, which rests upon the plate II. A magnetic needle also rests upon the horizontal plate, which may be used when desired for ascertaining the earth's meridian or the deviation of a line from it. The telescope, B, revolves on a horizontal axis, also the axis of the vertical circle, and which rests upon the supports S S. Beneath the telescope and attached to it by adjusting screws is the long spirit level L, with a scale attached for marking the position of the air bubble. The small spirit levels D and E serve to level the horizontal circle.
The vertical axis of the instrument is held by a socket in the plate immediately above the tripod, and is furnished with a clamp C, and slow-motion screws T T. The horizontal circle revolves upon the plate H, upon which there is a vernier the divisions of which are sometimes read by means of an attached microscope, although it is preferable to employ a pocket microscope for the purpose. The vertical circle is also supplied with a vernier, and both circles have clamps and slow-motion screws. It is evident that if the vernier of the vertical circle in the adjusted instrument reads zero when the telescope is level, and then is moved through an arc of 30° to bring the cross hairs upon an object, such object will have an elevation of 30° above the point of observation; and also that if the horizontal circle is moved through an arc of any number of degrees to bring the cross hairs of the telescope from one object to another, the lines passing through such objects will make corresponding angles with each other at the point of observation.
When used for important surveys the circles are 30 in. or more in diameter; in the smaller instruments they are 5 or 6 in. - See Gillespie's " Treatise on Levelling, Topography, and Higher Surveying " (new ed., New York, 1875).