When coal is found, and its quality and the thickness of the seam ascertained to be such as to warrant further expense, a shaft is dug down of some eight or ten feet diameter, cased with brickwork or wood to prevent the falling-in of its sides, and in some cases powerful machinery has to be erected to pump out the water which flows in. On reaching the coal, galleries--called “gates,” or “bords”--are dug in it in opposite directions, forming one long straight passage, and from this other smaller ones, called “headways,” are dug, at right angles, to the depth of about twenty-four feet, and from these other “gates” are carried parallel with the first, forming a series of roadways joined by short passages, and having squares of coal between them; the height of all these passages is determined by the thickness of the seam of coal, usually from three to ten feet. The great masses of coal forming the squares between these passages are gradually dug away (as far as can be done with safety) and the gates continued onwards, but before long the ventilation becomes impeded, and the air foul and dangerous from “fire-damp” (carburetted hydrogen) or “choke-damp” (carbonic acid), gases which are given off from the fissures in the coal. It is therefore necessary to produce a continuous current of fresh air in every part of the mine, which is done by sinking another shaft at the furthest part of the mine and keeping a large fire burning at its mouth, over which a tall shaft is generally erected, from which a column of light air ascends, drawing fresh air down the other shaft and through every part of the mine, to supply this “up-cast” shaft, as it is called. This supply of fresh air is economised and regulated by doors or valves, so placed that any part requiring extra ventilation can obtain it at any moment by shutting these doors and letting the whole current go through that particular part. The removal of coal is effected partly by digging with the “pick,” and partly by blasting with gunpowder; a large square mass is cut all round, and a charge of powder fired behind it, so as to bring down at once sixty or eighty tons of coal, which is brought along the gates on “trams” to the bottom of the shaft, where “corves” or baskets filled with it are drawn up to the “pit’s-mouth” by steam machinery, one corve ascending full while another is descending empty.