Fig. 213.—The author's adjustable ball joint and socket tribrach stand.
Fig. 214.—Adjustment to leg of tripod.
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526.—Hedley's Dials, with Pastorelli's and Hoffmann's Ball Arrangements.—By either of these arrangements the ball and socket is brought down close into the parallel plate adjustment so that the dial is of less total height. Hoffmann's is now becoming the most popular system, as practice has shown it to be the most perfect for mining survey. By either of these arrangements the ball and socket is clamped by the same screws that bring the instrument to final position. In Pastorelli's arrangement[24] the socket is drawn down upon the ball by the adjusting screws. In Hoffmann's[25] the ball is pressed up into the socket, which is the exact mechanical equivalent. When the screws are lightly clamped the ball can be moved with moderate force, or even quite loosely by careful adjustment; and in either case, when the ball is once set, care must be taken to keep pressure constantly upon it during the final adjustment by the screws. The general arrangements are shown in two Figs. 215, 216, which are taken from the drawings of the respective patents. In Fig. 215, va, the axis of the instrument terminates in a ball e which works in a cup f. The axis has also a portion of a ball of greater radius b concentric with the lower ball e. The upper parallel plate d is cupped over this ball. When the parallel plate is moderately free on b, the axis va may be set to any angle within the range of the central opening of d; and as the friction upon bd is greater than that upon fe, the axis moves by the adjustment of the parallel plate screws aa. In Fig. 216 the action is precisely the same, except that the pressure is upwards instead of downwards. In Fig. 215 there are springs s under the parallel plate screw heads to keep contact when the screws are loosened. In Fig. 216 the spring is a plate under the screws s, the action being the same in both cases.
Fig. 215.—Pastorelli's ball and socket adjustment.
Fig. 216.—Hoffmann's ball and socket adjustment.
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527.—Some objections have been made to this class of arrangement, over the simpler one of clamping the ball independently and then adjusting by the screws, as being more complex. On the other hand this compound arrangement has the merit in underground instruments of being lower and more compact, which is very important. The author has somewhat modified the arrangements of Hoffmann's head, as shown in the engraving on next page, to render it still more compact for mining instruments.
