The instruments are, 1. the magnetic compass, which is employed to measure the direction of a metallic ore, wherever the neighbourhood of iron does not interfere with its functions; 2. the graduated semicircle which serves to measure the inclination, which is also called the clinometer.
3. The chain or cord for measuring the distance of one point from another.
4. When the neighbourhood of iron renders the use of the magnet uncertain, a plate or plane table is employed.
The dials of the compasses generally used in the most celebrated mines, are graduated into hours; most commonly into twice 12 hours. Thus the whole limb is divided into 24 spaces, each of which contains 15° = 1 hour. Each hour is subdivided into 8 parts.
Means of penetrating into the interior of the earth.—In order to penetrate into the interior of the earth, and to extract from it the objects of his toils, the miner has at his disposal several means, which may be divided into three classes: 1. manual tools, 2. gunpowder, and 3. fire.
The tools used by the miners of Cornwall and Devonshire are the following:
[Fig. 700.] The pick. It is a light tool, and somewhat varied in shape according to circumstances. One side used as a hammer is called the poll, and is employed to drive in the gads, or to loosen and detach prominences. The point is of steel, carefully tempered, and drawn under the hammer to the proper form. The French call it pointerolle.