696
THEORY ANI) PRACTICE OF ENGINEERING.
Book II.
The ventilation of coal mines is effected by means of rarefaction ; a furnace is placed atthe bottom of the pit, which produces a rapid ascending column of warm air, and as thisrises up the shaft its place is supplied by a current of cold air, which passes down one ofthe other compartments of the shaft, and is made to traverse the several workings of themine before it is permitted to arrive at the furnace, where in its turn it becomes rarefied.In the Wallsend colliery the quantity of fresh air thus admitted varies from 2000 to 3000cubic feet per minute ; in some of the coal mines the air has so many miles of gallery totraverse that it is 12 hours before it arrives at the furnace, the rate at which it progressesbeing about 2 feet per second or a little more. The wall of separation in the shaft is calledthe brattice, and is of two kinds : one is permanent, and usually 2 or 3 feet in thickness;the other is of a more temporary kind, composed of S-inch deals, so attached to a skeletonframe that it can be easily removed. Where the galleries are shut off, to prevent the cur-rent of air from passing, the wall is called a stopping; this is sometimes effected bytrap-doors, made cither single, double, or triple, as may be most convenient; the aircourse is called either the intake or return as it receives or emits the air.
Lighting Coal Mines. — The means of effecting this, without subjecting the workmento the dreadful calamities arising from the explosive nature of the gas when mixed withatmospheric air, had long occupied the anxious attention of scientific men, but withoutsuccess, until the experiments of Sir Humphry Davy happily led him to construct alamp by which the lives of incalculable numbers' have been preserved. lie found that nomixture of fire-damp would explode in tubes with a diameter of less than | of an inch ;and also that a much stronger heat was required to effect this than with mixtures ofcommon inflammable gas, and that neither charcoal nor iron made red-hot would producean explosion. Upon these principles Sir Humphry formed the lamp called after him:the flame is enclosed within a wire gauze of very small meshes, there being as many as784 to the square inch; the security which it affords to those for whose use it was inventedis not, however, sufficiently estimated, in consequence of its not affording a great abundanceof light, and in many instances there has been much difficulty in enforcing its introduction.Besides the fire-damp there is another noxious gas, called the choke or black damp ; thisis a carbonic acid, and from its density will not rise to the surface, hut sinks to the bottomof the mine. The heavy carburettcd hydrogen is also found; this, when mixed with certainproportions of common air, is exploded by either charcoal or iron heated to a dull red heat.
The Scotch Coal Fields are about 100 miles in length, and 30 in breadth; and thenumber of beds are said to be 337, 84 of which are coal; the total thickness of all theselayers is computed at not less than 5000 feet.
The Coal Fields in Ireland are nearly 150 miles in length, and 120 in breadth, coveringa surface of more than 10,000 square miles. The coal is anthracitic, being often found inthe state of pure anthracite.
The first charter for the licence of digging coal was granted by Henry III. , in the year1239, and it is there called sea-coal; it became a common fuel soon afterwards, and aconsiderable quantity appears to have been exported to France . When Mr. Taylor, a fewyears ago, gave his evidence on the coal mines before the Committee of the House ofLords, he stated that the annual consumption of coal in Great Britain amounted to19,540,OCX) tons.
Coking of Pit Coal. — The most common method is to throw up the coals in heaps ofabout 40 tons as loosely as possible, and cover them with the smaller pieces; fire is thenapplied at various parts, and the mass is suffered to burn until the whole is ignited; whenthis is effected, it is covered up with dust and ashes to exclude the air, and the heap issuffered gradually to cool: water is often poured over it to accelerate this last part of theprocess, which is said to improve the quality of the coke by making it harder.
Ovens are used for making the better quality of coke; the best are usually elliptical intheir form, about 12 feet in their longest diameter, and 11 feet in the other; the walls are3 feet in thickness; the mouth is usually lessened towards the inside to about 2 feet6 inches, that on the outer side being a foot more. At the back is the entrance into the fluewhich conducts to the chimney; this is regulated by means of fire bricks, which can bemoved to increase or diminish the draught, so as entirely to consume the smoke.
A series of eighteen ovens, with a flue at the back 1 foot 9 inches wide, and 2 feet 6inches high, have been constructed at the station of the Birmingham railway ; and the fluesare conducted into a chimney 11 feet clear diameter at bottom, 17 feet outside, and 115feet in height. These ovens are each charged with 3^ tons of good coal, and entirelyconsume the smoke from the moment they are first lighted up; in about 40 hours it issufficiently coked, when it is thrown out upon the ground and watered ; it is then putinto iron canisters and covered up: good coal will yield 80 per cent, of coke, weighing14 cwt. per chaldron. The loss of weight when coals are coked in the ordinary manneris about 25 per cent., and coal which thus loses \ in weight gains \ in bulk.