Fig. 211.—Bracket sight.
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522.—The Telescope is placed on Y's, and is of exactly the same form as that described for a plain theodolite. The Y's in this instrument offer a great convenience for reversing the telescope for back sights in range when the vertical axis is fixed. The level under the telescope is sufficiently good to convert this instrument into a level for drainage, etc., when the rocking ring is locked with the compass. Its examination and adjustment are the same as those last given, except for the telescope, which is the same in all particulars as that of a 5-inch plain theodolite.
Fig. 212.—Improved miner's dial.
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523.—Improved Miner's Dial.—The illustration given, Fig. 212, is of the form of dial introduced by the author, a part of the arrangement only being of his own design. The telescope with Y supports is the same as that just described, and the sights, not shown, are cranked in the same manner as shown Fig. 211. The horizontal circle, instead of being in the interior of the box, is placed on the exterior rim, and reads with two verniers—not for correction, but for convenience of reading in different positions. The compass is divided upon the upper surface of the step to degrees, and in the same manner on the interior cylindrical surface of the step. This last often permits the compass to be read in a close working when the upper surface could not either be lighted or sighted. This plan was used on old circumferentors.[23] The plane of the compass is divided to 10° as usual. The compass adjusts by clamp and tangent motion. The axis of the instrument is supported upon a ball and socket arrangement designed by the author for roughly bringing the compass to level, and a parallel plate adjustment for final setting. The ball is fixed by clamping a pair of plates together by a thumb-screw. Each plate is hollowed in the centre to hold nearly half the ball. When fixed, the instrument is found to be very rigid.
524.—A plan of clamping designed by the author to meet the conditions of the tribrach system of adjustment of equal rigidity to that above described, is shown in elevation, Fig. 213 B. In this the upper half of the socket is screwed down outside the lower half socket by means of three projecting handle pins. This is a somewhat neater arrangement than that shown in Fig. 212. Either of the above-described ball arrangements elevate the instrument, and are better omitted for close working if there is a special adjustment in the tripod attached to the instrument, as that to be described presently, which will be found sufficient in most cases. The height of the instrument from the tripod is about 6½ inches; weight, 11 lbs. for both parallel plate and tribrach adjustments.
525.—Adjustable Tripod for Dials.—The author's improved form of tripod is adjustable to all heights between 30 inches and 57 inches, Figs. 213, 214. Each leg is formed of two stiff bars of mahogany, shown in detail, Fig. 214 G of section, about 1¼ inches by 5/8 inch, and a third bar or leg G′ of about 1¼ inches square, which slides between the other two. The sliding surfaces are grooved and tongued together in V grooves in the solid. Two strap-pieces of brass SS′ are fixed near the ends of the bars. One of these S′ is firmly soldered to a boss-piece that takes a thumb-screw, which has quite sufficient power to hold the leg G′ firmly at any position of extension. It is a rigid stand, which may leave the tripod head nearly vertical upon any inclination of the floor surface.
