AIR-Shafts, among Miners, denote holes or shafts let down from the open air to meet the adits and furnish fresh air. The damps, deficiency, and impurity of air which occur, when adits are wrought 30 or 40 fathoms long, make it necessary to let down air-shafts, in order to give the air liberty to play through the whole work, and thus discharge bad vapours, and furnish good air for respiration: the expence of which shafts, in regard of their vast depths, hardness of the rock, drawing of water, &c. sometimes equals, nay exceeds, the ordinary charge of the whole adit.
Sir Robert Murray describes a method, used in the coal mines at Liege, of working mines without air-shafts.
When the miners at Mendip have sunk a groove, they will not be at the charge of an air-shaft till they come at ore; and for the supply of air have boxes of elm exactly closed, of about six inches in the clear, by which they carry it down about twenty fathoms. They cut a trench at a little distance from the top of the groove, covering it with turf and rods disposed to receive the pipe, which they contrive to come in sideways to their groove, four feet from the top, which carries down the air to a great depth. When they come at ore, and need an air-shaft, they sink it four or five fathoms distant, according to the convenience of the
breadth, and of the same fashion with the groove, to draw ore as well as air.