Page:Transactions of the Provincial Medical and Surgical Association, volume 2.djvu/115

 the clearest idea we can have of the nature of a metalliferous vein or lode, is that of a narrow rent in the rocky crust of the earth, approaching more or less to a vertical direction, and filled with metallic ores. The object of mining is to break down, and transport to the surface, the contents of this supposed rent; in other words, to cut out from the containing rock this thin metallic plane. To effect this, galleries, called, in Cornwall, levels, are driven horizontally on the vein, one above the other, and the ore, &c. produced by their excavation, are transported to the surface by vertical openings, called shafts, cutting the former at right angles. The horizontal galleries are, in the first instance, about two feet wide and six feet high, but varying, of course, according to circumstances, and being frequently extended much beyond their original dimensions. They are driven, one above the other at intervals of from 10, to 20, or 30 fathoms. When extended to a certain distance from the original shaft, it is necessary for the sake of ventilation, as well as for other reasons, to form a second, which traverses, at right angles, all the galleries in the same manner as the first. The distance between shafts is very various, being from 20 to 100 fathoms. Frequently a communication is made between two galleries only, by a partial shaft (called a wins,) in the interval between two shafts. When there are more lodes than one worked in the same mine, as frequently happens, galleries often run parallel to each other at the same depth. In this case they often communicate by intermediate galleries, driven through the rock, (or country, as it is called in Cornwall) which are termed