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THEODOLITE
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THEODOLITE ,' a See also:surveying See also:instrument consisting of two graduated circles placed at right angles to each other, for the measurement of See also:horizontal and See also:vertical angles, a See also:telescope, which turns on axes mounted centrically to the circles, and an See also:alidade for each circle, which carries two or more verniers. The whole is supported by a See also:pedestal resting on footscrews, which are also employed to level the instrument. The See also:size varies from a minimum with circles 3 in. in See also:diameter to a maximum with a 36-in. horizontal and an r8-in. vertical circle. Theodolites are designed to measure horizontal angles with greater accuracy than vertical, because it is on the former that the most important See also:work of a survey depends; See also:measures of vertical angles are liable to be much impaired by atmospheric See also:refraction, more particularly on See also:long lines, so that when heights have to be determined with much accuracy the theodolite must be discarded for a levelling instrument. When truly adjusted the theodolite measures the horizontal See also:angle between any two See also:objects, however much they may differ in See also:altitude, as the See also:pole See also:star and any terrestrial See also:object. The instrument is made in three forms—the Y See also:pattern, the See also:Everest and the transit. Certain parts are See also:common to all the forms in use and to the level. The stand is generally made circular in See also:section, each of the three legs being shod at the See also:lower extremity with See also:steel. Their upper ends are hinged to a See also:flat See also:plate provided with a screwed See also:collar of large diameter (fig. I). To the legs is screwed a plate 00, which supports the lower See also:side of the plate PP. This receives the ends of the screws SS by which the instrument is levelled, its See also:annular portion being larger than the collar in 00, so that, until clamped by the screwed plate above it, the whole of the instrument except the legs can be moved horizontally in any direction to the extent of about i in. This facilitates centring over a point. The upper plate PP is bored centrally to receive a parallel or conical See also:pillar which supports the lower circle of the theodolite or the See also:arm of the level which carries the telescope. In the theodolite the edge of the plate rr is bevelled and divided into 36o or 40o degrees, and to See also:half degrees, or to 20 minutes or to minutes, according to the size of the instrument. A collar is provided, which when tightened on the vertical See also:axis, otherwise See also:free to move, holds it rigidly in position with respect to the plate PP. To this collar is attached a slow-See also:motion See also:screw, working against a reaction See also:spring, by which the plate rr can be rotated through a small arc. The upper plate carrying two, three or four verniers vv is attached to a vertical coned pillar passing through the centre of the larger pillar and rotating in it; this plate can be clamped to the lower plate by means of the screw C, and can be rotated with respect to it by the slow-motion screw d. On the upper plate are placed two small levelling bubbles, and two See also:standards tt alle attached to the upper side of the plate for sup-porting the trunnions of the telescope T. The See also:bearings for receiving these trunnions are V-shaped; the V on one side is fixed, while the other is cut through and can be narrowed or made wider, thus lifting or lowering the trunnion by means of two See also:capstan-headed screws. To the telescope the vertical circle for See also:reading angles in ' This word has been a See also:puzzle to etymologists. Various ingenious explanations have been given; all based on the apparent See also:Greek See also:form of the word; thus it has been derived from OeauOac, to see, 6.56s, way, and X0-6s, smooth, See also:plain; from B€Iv, to run, and SoXcxbs, long, and in other ways equally fanciful. Another imaginary origin has been suggested in a corruption of " the 0 deleted," i.e. crossed out, the circle being crossed by diameters to show the degrees; others have found in it a corruption of " the alidade " (q.v.). It would appear, however, to he taken from the O. Fr. theodolet or theodelet, the name of a See also:treatise by one Theodulus, probably a mathematician (see Notes and Queries, 3rd See also:series, vii. 337, 428, &c. See also:Skeat, Etym. See also:Diet., 1910).altitude is fixed, and rotates with it; both can be clamped to the See also:standard, and motion can be given by a suitable See also:double-ended motion screw. The verniers are attached to arms uu bearing on an enlargement of one trunnion of the telescope, one arm See also:pro- jecting downwards and embracing a See also:projection on the standard t. To the same See also:frame is attached a bubble, which should be parallel with the centre See also:line of the verniers. The See also:diagonal telescope nn is provided with See also:cross hairs, and is used for the final centring of the instrument over an object. The use of See also:aluminium in the construction of all parts not liable to much See also:wear is to be commended, owing to the smaller See also:weight. The Y theodolite differs from the transit in that the supports for the telescope are See also:low, that the telescope rests in a See also:cradle the trunnions of which See also:rest on the sup-ports, and that a segment of a circle attached to the cradle replaces the vertical circle. When it is desired to read a line in the See also:reverse direction the telescope is lifted out of the cradle, turned end for end, and replaced in the Y bearings of the cradle again. In the Everest theodolite the supports are low and the telescope cannot be transited. The instrument is similar to that described above except that the vertical circle is not continuous, but is formed of two arcs. In See also:Germany and elsewhere refracting theodolites and transit See also:instruments are sometimes employed. The See also:eye end of the telescope See also:tube is removed—a counterpoise to the object end being substituted in its See also:place—and a See also:prism is inserted at the intersection of the visual axis with the transit axis, so that the rays from the object-See also:glass may be reflected through one of the tubes of the transit axis to an eye-piece in the See also:pivot of this tube. In this See also:case the pillars need only be high enough for the counterpoise to pass freely over the plate of the horizontal riffle: but the observer has always '11'11" 111111111 - - See also:ems I!l !mum /I= qyAg A.= .— 111MVa'its\ (1111111111: d 1I11'I~ ~I~ ~ 'C~I~IIIO• 11 h to place himself at right angles to the direction of the object he is observing. Levelling Instrument.—This is another surveying instrument consisting essentially of a telescope bearing a level and mounted horizontally upon a frame. To the upper side of the parallel plates it is similar in construction to the theodolite. No pro-See also:vision is made for centring over a point. The upper plate is bored through the centre and carries a conical pillar, which rotates freely in it and supports a horizontal plate, to the extreme ends of which are attached, by means of capstan screws or otherwise, two vertical supports, on which the telescope, which is constructed to be perpendicular to the vertical axis of the instrument, rests and rotates with it. The level bubble, by which the instrument is brought into a position at right angles to the axis of the See also:earth, is generally placed on the See also:top of the telescope. In the best telescopes, whether for theodolite or level, the See also:diaphragm on which the See also:image is formed is made of glass, and the cross hairs are engraved thereon. In the level the eye-piece and object-glass are inter-changeable, to facilitate See also:adjustment for collimation. Additional information and CommentsThere are no comments yet for this article.
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