Very light 3-inch and 4-inch plain theodolites of from 5 lbs. to 7 lbs. complete are made occasionally for travellers.
Fig. 189.—Stanley's new model plain theodolite.
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442.—The author has recently modified the plain theodolite, Fig. 189, for which there still remains a small demand in the Colonies, by making the construction much more solid by shaping the work out of single castings in gun-metal for parts formerly screwed together in many pieces, which formerly was necessarily arranged to permit facility of construction by hand-work. There are also in the new instrument some improvements made in detail. The limb dividing is covered for protection. The readers are joined through the vertical axis and are hinged to turn up. The compass has an aluminium ring with a microscope which permits it to be read at a convenient height and much more accurately. The tangent screws are covered to exclude dust, and some other improved details.
Fig. 190.—Everest's theodolite.
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443.—Everest's Theodolite, Fig. 190, designed by the late Sir George Everest, and used for details of the great trigonometrical survey of India, is built up very much upon a well-known common French model. In service in India it has proved an excellent instrument. The horizontal circle or limb of this instrument consists of a single plate, upon which the silver is inlaid flat upon the surface, upon the plan shown, [Fig. 128]. In place of the ordinary vernier plate three arms are extended from the central axis, which carry each a vernier at its end, reading to a fiducial edge, [Fig. 127], p. 186. The verniers trisect the circle, and are marked A, B, and C. A fourth arm, proceeding from the same relative position of the centre as the arms of the vernier, carries a clamp and tangent which is similarly constructed to that of the ordinary theodolite described. The instrument has also an under clamp and tangent for setting the telescope to bearing, or for repeating, as in the ordinary theodolite.
444.—The horizontal axis carries the telescope in a cylindrical fitting as in the transit theodolite, terminating in two pivots which are set to permanent position as in the plain theodolite. The pivots rest in bearings upon short standards carried out from the centre upon a flat horizontal bar to which a spirit level is attached for adjustment of the pivots to horizontality. Vertical angles are read off upon two arcs which have a horizontal axis as their centre attached to the telescope, so as to move with it in the vertical plane, with clamp and tangent adjustment. An index, upon the same centre carries two verniers and has a spirit level attached to it. The verniers are read by a pair of microscopes. Upon the upper side of the telescope a trough needle is placed.