Besides these general levels of drain age, there are subsidiary levels, called off-takes or drifts, which discharge the water of a mine, not at the mouth of the pit, but at some depth beneath the sur face, where, from the form of the coun try, it may be run off level free. From 20 to 30 fathoms off-take is an object of considerable economy in pumping ; but even less is often had recourse to ; and when judiciously contrived, may serve to intercept much of the crop water, and prevent it from getting down to the dip part of the coal, where it would become a heavy load on a hydraulic engine.
Day levels were an object of primary importance with the early miners, who had not the gigantic pumping power of the steam-engine at their command. Levels ought to be no less than 4 feet wide, and from 5 feet and a half to 6 feet high; which is large enough for carry ing off water, and admitting workmen to make repairs and clear out depositions. When a day-level, however, is to serve the double purpose of drainage and an outlet for coals, it should be nearly 5 feet wide, and have its bottom gutter covered over. In other instances a level not only carries off the water from the colliery, but is converted into a canal for bearing boats loaded with coals for the market. Some subterranean canals are nine feet wide, and twelve feet high, with five feet depth of water.
If in the progress of driving a level, workable coals are intersected before reaching the seam which is the main ob ject of the mining adventure, an air-pit may be sunk, of such dimensions as to serve for raising the coals. These air pits do not in general exceed 7 feet in diameter ; and they ought to be always cylindrical.
When a coal-basin is so situated that it cannot be rendered level free, the winning must be made by the aid of machinery. The engines at present employed in the drainage of coal-mines are : 1. The water wheel, and water-pressure engine.
2. The atmospheric steam-engine of Newcomen.
3. The steam-engine, both atmospheric and double-stroke, of Watt.
4. The expansion steam-engine of Woolf.
5. The high-pressure steam-engine without a condenser.
The depth at which the coal is to be won, or to be drained of moisture, regu lates the power of the engine to be ap plied, taking into account the probable quantity of water which may be found, a circumstance which governs the diameter of the working barrels of the pumps. Experience has proved, that in opening collieries, even in new fields, the water may generally be drawn off by pumps of from 10 to 15 inches diameter ; excepting where the strata are connected with rivers, sand-beds filled with water, or marsh-lands. As feeders of water from
rivers or sand-beds may be hindered from descending coal-pits, the growth proceeding from these sources need not be taken into account. and it is ob served, in sinking shafts, that though the influx which cannot be cut off from the mine, may be at first very great, even beyond the power of the engine for a little while, yet as this excessive flow of water is frequently derived from the drainage of fissures, it eventually be comes manageable. An engine working the pumps for 8 or 10 hours out of the 24, is reckoned adequate to the winning of a new colliery, which reaps no advantage from neighboring hydraulic powers.
When the engine-pit is sunk, and the lodgment formed, a mine is then run in the coal to the rise of the field, or a crop ping from the engine-pit to the second pit. This mine may be 6 or 8 feet wide, and carried either in a line directly to the pit bottom, or at right angles to the backs or web of the coal, until it is on a line with the pit, where a mine is set off, upon one side, to the pit bottom. This mine or gallery is earned as nearly par allel to the backs as possible, till the pit is gained. The next step is to drive the drip-head or main-levels from the en gine-pit bottom, or from the dip-hand of the backset immediately contiguous to the engine-pit bottom. In this business, the best colliers are always employed, as the object is to drive the gallery in a truly level direction, independently of all sinkings or risings of the pavement. For coal seams of ordinary thickness, this gallery is usually not more than 6 feet wide ; observing to have on the dip side of the gallery a small quantity of water, like that of a gutter, so that it will always be about 4 or 6 inches deep at the forehead upon the dip-wall. When the level is driven correctly, with the proper depth of water, it is said to have dead water at the forehead. In this operation, therefore, the miner pays no regard to the backs or cutters of the coal ; but is guided in his line of direction entirely by the water level, which he must attend to solely, without regard to slips or disloca tions of the strata throwing the coal up or down.