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Encyclopedia Britannica - Main :: TAV-THE |
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THEODOLITE ,' a surveying instrument consisting of two graduated circles placed at right angles to each other, for the measurement of horizontal
instrument . The size varies from a minimum with circles 3 in. in diameter to a maximum with a 36-in. horizontal
Theodolites are designed to measure horizontal angles with greater accuracy than vertical, because it is on the former that the most important work of a survey depends; measures
measures
star
The instrument is made in three formsthe Y pattern, the Everest and the transit. Certain parts are common to all the forms in use and to the level. The stand is generally made circular in section, each of the three legs being shod at the lower extremity with steel. Their upper ends are hinged to a flat plate provided with a screwed collar of large diameter (fig. I). To the legs is screwed a plate 00, which supports the lower side of the plate PP. This receives the ends of the screws SS by which the instrument is levelled, its 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 pillar which supports the lower circle of the theodolite or the 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 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 axis, otherwise free to move, holds it rigidly in position with respect to the plate PP. To this collar is attached a slow-motion screw, working against a reaction 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 standards tt alle attached to the upper side of the plate for sup-porting the trunnions of the telescope T. The 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 capstan-headed screws. To the telescope the vertical circle for reading angles in' This word has been a puzzle to etymologists. Various ingenious explanations have been given; all based on the apparent Greek form of the word; thus it has been derived from OeauOac, to see, 6.56s, way, and X0-6s, smooth, plain; from BIv, 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 treatise by one Theodulus, probably a mathematician (see Notes and Queries, 3rd series , vii. 337, 428, &c. Skeat, Etym. Diet., 1910).altitude is fixed, and rotates with it; both can be clamped to the standard, and motion can be given by a suitable double
jecting downwards and embracing a projection on the standard t. To the same frame
diagonal telescope nn is provided with cross hairs, and is used for the final centring of the instrument over an object. The use of aluminium in the construction of all parts not liable to much wear is to be commended, owing to the smaller weight. The Y theodolite differs from the transit in that the supports for the telescope are low, that the telescope rests in a cradle the trunnions of which rest on the sup-ports, and that a segment of a circle attached to the cradle replacesthe vertical circle. When it is desired to read a line in the 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 Germany and elsewhere refracting theodolites and transit instruments are sometimes employed. The eye end of the telescope tube is removeda counterpoise to the object end being substituted in its placeand a prism is inserted at the intersection of the visual axis with the transit axis, so that the rays from the object-glass may be reflected through one of the tubes of the transit axis to an eye-piece in the pivot of this tube. In this 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 - - 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
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