445.—This instrument has been used in military surveys by the Royal Engineers. The objections that civil engineers have made to Everest's theodolite are that the working parts are made very open, so that the wet and dust intrude; further it lacks the general convenience of the transit principle, which is necessary for astronomical observations. The tripod is sometimes made of the ordinary solid section, art. 216; but for India, where carrying labour is cheap, a heavy framed stand is used, which is special, as follows:—
Fig. 191.—Everest's locking plate tribrach.
Larger image
446.—Everest's Tribrach.—The upper part of the engraving, Fig. 191, shows this tribrach that supports the upper part of the instrument directly upon its vertical axis. The three arms of the tribrach carry each a milled-headed adjusting screw, the nut of which is formed in the arm. The arm is sawn up to admit of adjustment, that the milled head may turn softly but without any shake. The lower points of the milled-headed screws, technically feet, fall into V-grooves in the head of the tripod. The V's are not shown in the engraving. Above the upper surface of the tripod head, a thin, three-armed plate of metal, termed the locking plate, is centred upon the hollow axis of the head, so that it will move laterally. The locking plate has a hole and slot at the end of each of its arms, the holes of which admit the toes of the feet of the tribrach into the V-grooves formed in the head of the tripod. The locking plate when moved laterally locks all the toes in at once, so that the instrument is secured by this means to a certain degree from accident. This locking plate has commonly a milled-headed screw clamp which fixes it in its locked position. The head of this screw is under the tripod head, and consequently cannot be shown in the engraving. It is a defect of this locking plate that the screws, unless they fall perfectly in the V-grooves have a tendency to ride. To avoid this the author has for many years made the ball feet fall upon a plain surface, being at the same time held in their places by a slotted plate which fits over the neck of the balls. This plan, which is not shown in the engraving, is now adopted by other makers. The author uses also his patent tribrach sometimes on this instrument.
Fig. 192.—Stanley's Everest theodolite.
Larger image
447.—The Framed Tripod of Sir Geo. Everest's design is made of straight-grained mahogany, each leg being formed of two side-pieces, with one or two cross-pieces. The engraving, Fig. 191, shows the head of a tripod of this construction. The side-pieces are spliced together at the lower ends, where they form a rather obtuse point, which is shod with a gun-metal shoe. The upper ends of the side-pieces carry strap plates that receive a bolt which holds them firmly by means of winged nuts to the tripod head. The legs can be detached after use and the tripod head be placed in the case with the instrument in a packing provided for it. Some modification of this form of tripod is generally used for all large field instruments. The author's improved Everest theodolite is shown at Fig. 192.