Of the instruments and operations of subterranean operations

—It is by the aid of geometry in the first place that the miner studies the situation of the mineral deposits, on the surface and in the interior of the ground; determines the several relations of the veins and the rocks; and becomes capable of directing the perforations towards a suitable end. The instruments are, 1. the magnetic compa , 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 compa es 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.