491.—Plain Miner's Dial.—The original simple form of specialized miner's dial is shown in Fig. 201. It consists of a compass, divided to single degrees, read by a finely pointed edge-bar needle mounted on a jewelled cap. The needle has a sliding rider placed upon it, [art. 130], so that it may be carefully balanced to horizontality in any locality in which it is used. The divided compass is raised on a step, and the upper surface of the needle is made to be quite level with the division when the compass is horizontal. In erecting the instrument with the needle correctly balanced, the compass may therefore be brought to horizontality by the coincidence of the upper surface of the needle with the plane of the divisions, without the necessity of having spirit levels.
Fig. 201.—Mining dial.
Fig. 202.—Cover to the same.
Fig. 203.—Sight.
Fig. 204.—Section of ball and socket joint.
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492.—The compass-box is extended in one meridian, north to south, by strong arms that carry a pair of sights hinged to turn down to the surface of the cover for portability. The compass-box and arms together are termed the limb. The limb of the instrument is mounted upon a ball and socket joint to be described. The socket is slotted down on one side to permit the limb to be turned to a vertical position. In this position the level shown on the front of the instrument is used for levelling by means of the sights: this level is not, however, put on all plain dials.
493.—The cover of the compass-box, Fig. 202, is fixed on the box to a given position by a stud and slot. It has an arc divided upon its outer surface, which is centred from a small hole placed near the outer edge. A line from the centre of the hole to the zero of the arc is made perpendicular to the central indices of the sights. A piece of silk or a horse-hair carrying a small plummet is fixed to hang from the hole. By this means when the limb is turned down in the slot of the socket and the silk or hair stretched by the plummet to permit it to hang in front of the arc, it will then cut the divisions, and thus form a reading index to the arc, giving thereby approximately the vertical angle at which the sights are set to degrees.
494.—The instrument is mounted on a simple jointed tripod to be described. It will be seen by the above description that this instrument is cheaply made, and is not designed for very exact work. It is now giving way for more exact instruments, but it forms the groundwork on which mining survey instruments are most generally constructed. The height of this dial with sights erect is 11 inches; weight, 6 lbs. Some of the separate parts above enumerated, which are common to many other forms of mining instruments, will now be more particularly described.