theodolite

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An optical instrument consisting of a small mounted telescope rotatable in horizontal and vertical planes, used to measure angles in surveying, meteorology, and navigation.

Century Dictionary and Cyclopedia

1. n. A surveying-instrument for measuring horizontal angles upon a graduated circle. It may also be provided with a vertical circle, and if this is not very much smaller than the horizontal circle, the instrument is called an altazimuth. If it is provided with a delicate striding level and is in every way convenient for astronomical work, it is called a universal instrument. A small altazimuth with a concentric magnetic compass is called a surveyors' transit. A theodolite in which the whole instrument, except the feet and their connections, turns relatively to the latter, and can be clamped in different positions, is called a repeating circle. The instrument shown in the figure follows the system of the United States Coast Survey of attaining simplicity of construction by adaptation to a single purpose—in this case to the measurement of horizontal angles only. This instrument is low and consequently very steady. Within the upright pillar is a truncated cone of steel, and upon this and fitting to it turns the hollow brass pillar carrying the telescope and microscopes. Except for an excessively thin layer of oil, the brass movable part bears directly on the steel, and its weight tends to keep it centered. The pressure is relieved by a small plate of some elasticity fastened to the movable part over the axis and adjustable with screws. It is thus made to turn, as nearly as possible, about a mathematical line. This is the conical bearing of Gamhey. The base, which is as low as possible, consists of a round central part, and three arms having screw-feet with binding-screws. A circular guard for the circle (indistinguishable from the latter in the figure) forms a part of the base. The graduated circle is made slightly conical, so that the microscopes may be more convenient. This circle, with its eight radii and interior ring, forms one solid casting, which bears upon the steel axis conically. It is held in place, in imitation of an instrument by Stackpole of New York, by the pressure of a ring above, which can readily be loosened so as to permit the circle to be turned round alone. The telescope is provided with a filar micrometer, with a view of facilitating reiterated pointings—a new principle of much value. The instrument is leveled by means of a striding level. There are four micrometer microscopes (although some geodesists insist upon an odd number), made adjustable so that one division of the circle Shall be very nearly covered by two and a half turns of the micrometer-screw. The illumination for these microscopes is made through their objectives by light brought, according to the plan of Messrs. Brunner, by prisms from a point vertically over the axis, where a horizontal ground glass is hung in the daytime and a lamp with a porcelain shade at night, so that the images of the lines plowed by the graver in the polished surface of the circle shall not be displaced by oblique illumination. The clamp is attached to an arm from a ring about the brass upright, and bears upon the circular guard outside the circle proper. The tangent screw is contrived so as to eliminate dead motion. The arm carrying the clamp is balanced by another bearing a small finding microscope. Theodolites are made upon manifold models; but the one figured in preceding column is a good example of a modern first-class instrument.

Wiktionary

1. n. A surveying instrument, consisting of a small mounted telescope, used to measure horizontal and vertical angles.

GNU Webster's 1913

1. n. An instrument used, especially in trigonometrical surveying, for the accurate measurement of horizontal angles, and also usually of vertical angles. It is variously constructed.

WordNet 3.0

1. n. a surveying instrument for measuring horizontal and vertical angles, consisting of a small telescope mounted on a tripod