512.—To set a line in Azimuth with one taken above Ground.—This is necessary where there is local attraction to the needle below, or there is a suspicion of this, so that the needle cannot be depended upon with certainty. The instrument is placed on staging over the pit and a vertical is taken to its centre either by the means briefly discussed art. 506 by the instrument, or by suspending a plummet, a ball, or a bullet from the centre of the instrument by a thread and burning the thread when the ball is free from vibration. The ball is allowed to fall upon a smooth horizontal surface formed of earth or otherwise, in which it makes a dent which will be vertical to the axis of the instrument if the ball has not been deflected by ventilation currents. Two lights, as distant as possible to be seen to range in line with the dent, are placed at the bottom of the pit. The lights, if thought desirable, may range north and south with the needle; but in whatever direction this may be set the correct azimuth of this may be taken by cutting them by the webs of the nearly vertical telescope of the dial; and this azimuth may be correctly set out on the surface by a pole or other station mark, or its true direction by a pair of these, one on each side of the pit's mouth, the second station mark being set out after a shift of the horizontal vernier exactly 180° on the circle. A straight-edged flooring board painted white may be made to cut the line from light to light, which is more definite for bearing than the lights themselves.
513.—Hedley's Dial, the invention of John Hedley, H.M. Inspector of Mines, in 1850, has now become the most popular form of miner's dial, modified, however, from its original form in various ways. The peculiar feature of this form of dial is that the sights move upon a framework centred upon a horizontal axis, so that they may by a rocking motion take horizontal angles within a wide azimuth without obstruction to the sight of the compass.
Fig. 208.—Hedley's dial.
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514.—For consideration of the general features of Hedley's dial, the tripod and the ball and socket are the same as that described for the plain dial; but the socket is not cut down on one side to change the position of the axis, as the compass-box in this instrument is required to be kept uniformly level. The general appearance is shown Fig. 208. For districts in which the working strata are fairly level, parallel plates are put to this instrument in place of the ball and socket joint. The compass-box revolves, as that described for Lean's dial; but it is more general in this instrument to have a clamp and tangent motion, as in a theodolite, than the rack and pinion motion. Two levels for setting the compass horizontal are sunk into the plate of the compass-dial low enough to miss the edge-bar needle. The step of the compass is divided into degrees and the plate of the dial to 10°. The vernier, which is placed on the opposite side of the box to the vertical arc, reads to 3′, as described for Lean's dial.
515.—The Rocking Centre forms the peculiar feature of Hedley's dial. From opposite points of the under side of the compass two pivots are projected. These are set perpendicular to the vertical axis, which is placed above the ball and socket. The pivots are placed central with the vernier and in line with E. to W. of the compass when this is set to zero (360°). The pivots form the axes of a stout ring—rocking ring—which surrounds the compass-box, with space sufficient to clear it when the ring is rocked about its axis. The ring has two extended arms which carry sights as shown. These turn down upon the compass-box when out of use. One of the pivots is prolonged for about ¾ inch beyond the outer circumference of the ring. The prolongation is made generally of triangular section. This forms a fitting to the vertical arc, which is attached by a milled-headed screw when required, the arc being an encumbrance when this dial is used for making horizontal plans only.
516.—The Vertical Arc, with its index arm, forms a separate piece. The arm is centred upon the arc with a ground fitting, which is retained in its position by a collar fixed with three screws. The arm-piece forms the axis, through the centre of which a triangular hole is made to fit the triangular prolongation of the pivot, so that the index arm remains fixed, and the arc moves with the rocking ring, to which it is held by a pair of dowels. The arc is divided into degrees on the outer edge of its surface, and a scale of difference of hypotenuse and base upon its inner edge. The graduations read to a single index line upon a fiducial edge carried down from an opening in the index arm.
Hedley's dial can be locked by a pin, which is attached to the under side of the compass-box, so as to work by the compass only. The ring can also be locked level with the compass by a sling latch-piece so as to convert it into a plain dial.