Independently of the vast and practically unlimited power which the steam-engine places at our command. it may be considered as uni versal in its application, and entirely unrestricted by those limitations of local circumstances which circumscribe the utility of all other modes of drainage. The steam-engine may be erected on any spot which convenience requires, and it may be made of .whatever degree of power may be considered requisite ; fuel and water alone are needed for its operations; and while successive improvements have reduced the former to a fraction of its earlier consumption, it can always supply itself with the latter from the ground. This independence of local circumstances is of the more importance to the miner, as it is not in his power to make choice of localities : the manufacturer may erect his mill wherever water-power is abundant for driving his machinery, but the miner must carry on his operations on whatever spot nature has de posited her mineral treasures, and make the best of circumstances over which he can have no control. Thus the steam-engine smokes in the narrow valleys of Cornwall, almost at the sea-level, on the verge of the cliff at Botallack, and on the elevated table-land of Mexico.
The steam-engines employed for drainage are erected close to the shaft in which the pumps are fixed, which is called the "engine-shaft;" one end of the beam hangs over the centre of it, and is attached to the pump-rod, which is raised at each stroke of the engine, afterwards sinking with its own weight, which 'is always counterbalanced by a "balance-bob," as before explained, so that the whole power of the engine is exerted in raising the column of water in the pumps. The engine is generally enclosed in a large substantial building, either two or three stories high, which affords convenient access to every part of it. The centre of the beam is supported by the front wall of the house, and a low building attached to it contains the boilers, which in 'Cornwall, together with the steam-pipe and cylinder, are carefully cased and covered up with some non-conducting substances. The arrangement of the engine, with that of the " capstan" and " shears" used in raising and lowering the pit work, is shown in Fig. 7. The engines employed in draining mines have generally cylinders of not less than 40 inches in diameter, and the cylinders vary from that size to a diameter of 80 or 90 inches, the latter being the largest size ever con structed, and estimated at 300 horse-power. The chief peculiarities of the Cornish engines consist in using high-pressure steam (40 or 50 lbs. to the square inch) expansively, by cutting off the communication with the boiler at one-fourth or one-fifth of the stroke ; in allowing a short interval between each stroke for the perfect condensation of the steam, and in carefully preventing the radiation of heat from the boiler, cylin der, &c. The engines employed in our collieries present no peculiarity
worthy of notice, nor has any great attention been paid to their improvement, owing to the cheap rate at which they can be supplied with fuel.
Support.—The support of mines is the next subject which requires our attention, and the extent to which it is necessary will depend very much on the nature of the inclosing rock, which may be either so hard as to stand of itself, or so soft as to crush together unless the exca vations be properly secured. The mode of support used in mines is of three kinds : by leaving pillars of the vein, as before noticed, for which purpose the poorer masses are of course selected ; by timbering ; and by walling either with brick or stone. Timbering is a very common and convenient plan, and is thus practised :—In the case of a shaft, four pieces of timber of the requisite strength are framed into each other and fixed within it at intervals of about four feet apart, the intermediate ground being supported by driving thin boards between each set of timbers and the rock. Levels are supported by three pieces of timber placed in the form of a doorway, rather narrower above than below, and framed together at the top (see Fig. 8), the ground between each of these doorways being supported in the manner above noticed. Shafts and levels are sometimes also supported by walling, and in coal-mines the pits are generally lined throughout with brickwork. The " gunnies," or large open excavations from which the ore has been taken, are kept open by strong pieces of timber placed across them, and pressing against the two walls of the vein, which they thus prevent from closing together, as might otherwise be the case, especially where the vein was much inclined, and the pressure of the unsupported hanging wall is consequently great. These open spaces are very useful for disposing of the deads and rubbish which arc continually accumu lating underground when flab workings are carried on in the rock or in unproductive parts of the vein, and which it would be useless and expensive to raise to the surface. For this purpose a "stull" is formed by placing strong timbers in the backs of the levels, upon which boards are laid, so as to form a close covering on which the deads and attle (or rubbish) arc then thrown, till the space above has been completely filled up. The pressure of this mass gives it sufficient solidity to sup port the walls of the vein in an effectual manner ; but notwithstanding the large quantity of rubbish thus disposed of in mines, a great deal always remains to be taken to the surface, as the waste heaps on the surface of every mine will show.