517.—The great merit of Hedley's dial is that the rocking centre permits a greater range of open sighting than any other; and the instrument is very low, permitting its use in shallow workings. Further, that it is a very strong instrument to resist accidents, and is very portable. The height of a 6-inch Hedley's dial above the tripod head, in a level position, is 9 inches to the top of the sights. Weight of instrument, 7 to 10 lbs.
Fig. 209.—Hedley's dial with ball clamp.
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518.—In the author's simple dial, Fig. 209, which is of a modern form, the ball is clamped by a capping-piece over it moved to clamp by two stout pins. This form gives a little less height and still holds the dial firmly. The horizontal axis moves rather stiffly, so that no clamp to the arc is required. It is a very cheap form of dial, but substantially made. It answers for a small mine survey.
519.—There have been many variations made and proposed for Hedley's dial. Mr. Casartelli, of Manchester, places the arc over the centre of the compass-box.[22] This plan is intended to make the rocking centre firm; but the arc interferes a little both with the sights and the view of the compass box. Messrs. Davis and Son connect wheel-work with the arc, so as to magnify the scale of motion. Other less important variations in Hedley's dial are common.
520.—Examination and Adjustment of Hedley's Dial.—The general examination of the stand and of such parts of the instrument as correspond with Lean's dial is the same as just given. The rocking ring should be lifted and pressed down at each end alternately to see that there is no loss of time on the axis. The arc should be examined in like manner. The dial should be set up in front of a plumbed line to see that its sights range properly when the instrument is set level by its bubbles. A point should be observed, say through the hole and cross webs at the top of the sight; and with this point kept in view the rocking ring should be moved upwards or downwards so that the point traverses the plumb-line to the extent of the rocking motion. If it does not do so, possibly the transverse level in the plate of the compass-box may be adjusted to make it do so; but in this adjustment it must be particularly observed that the balance of the needle remains so that it still reads the graduation with its upper edge, and that the sights traverse the same plumb-line when turned about, as it is possible to set the level right with one pair of sights and throw other parts out. There are no simple means of adjustment provided, so that if the instrument is not accurate it should be returned to the maker for correction.
521.—Improvement in Hedley's Dial, by Addition of Telescope.—Surface work being generally performed with the theodolite, surveying with open sights following this cannot be effected with sufficient accuracy; therefore there becomes a necessity for the use of the telescope, which was first placed on this instrument by the author at the suggestion of Mr. W. Preece, C.E. In mines, also, although sights present often the only possible means of directing angular positions in cramped and tortuous workings, on the other hand, better work can very often be done and the telescope be conveniently used. Under these conditions, this addition forms an important improvement in the instrument, to be at hand to apply when desired. The telescope of this instrument detaches exactly as with Lean's dial, but the sights are made with an angle piece, so as to extend them to a distance of about 12 inches apart for sighting. Fig. 211 is of one cranked sight. The instrument illustrated Fig. 210 has parallel plates, [art. 193], p. 99, suitable for fairly level workings. A ball and socket joint is sometimes fitted to this instrument in place of these.
Fig. 210.—Hedley's dial with telescope.