Besides these levels which drain coal-fields of water, and render them workable, there are other levels of more limited operation, used in mining. These ate named off take levels, or drifts. The object of these is, that when a coal is to be won by an engine, the mouth of the en gine pit is so situated as to the ground adjoining, that, at a moderate expence, a level can by brought up to in tersect the engine pit a number of yards under the sur face, so that the water of the mine, in place of being de livered at the surface, is discharged into the off-take drift. The lower down this kind of level, or off-take drift can be procured with ease, the greater will be the advantage in lessening the column of water to be lifted in the pumps by the engine. From 20 to 30 fathoms oil-take is a considerable object, as that depth saves what is termed a lift of pumps ; even 10 fathoms off take is of consequence to have. These levels are not only of use in lessening the Iliad of water on the engine, but if coals are intersected by them, they will intercept all the crop water to the depth of the point of inter section, and prevent it from going down upon the dip part of the coal, where it would be a heavy load on an engine, if, after intersecting the engine pit, the level is carried forward till it .intersect the coals, through which the pit is sunk, all the crop water of these upper coals will also be intercepted. Such levels are not only of use in the winning described, but if a new deep winning is made to the dip, the crop water is not only prevented from descending to the pit bottom, but the advantage of dischai ging the water raised by the engine into the level is also obtained.
The most prominent advantages being thus stated, regarding day-levels, we have now to state the manner in which they are executed.
As, before the discovery of the steam engine, the coal fields which could be drained by hydraulic machinery were comparatively very limited in number, and as the draining of water by men or horses was not only very expensive, but very limited as to the depth at which water could be raised, the driving of day-levels was a primary object with the early miners ; and we are as tonished when we survey the works of this kind execut ed by them, both as to depth and extent, and that be fore the application of gunpowder to the blasting of rocks. Without this powerful auxiliary of the miner, it is not easy to comprehend the extreme labour and patient perseverance required to pass through the very hard rocks which were met with. This seems to be the cause of the levels executed in the early periods of mining being of so small a size that a man has just room to creep through them.
Many of the levels of the present day are only three feet in width, and four and a half in height. Although these dimensions are in general sufficient to carry off the water which may be found in the colliery, they are too small when the mine is to be repaired, or when sediment and obstructions gather in them. They ought not to be less than four feet wide, by five feet six inches or six feet high ; this is abundantly large for carrying off water. But there are some day-levels, which not only are driven For carrying off the water, but as a pas sage at the same time for bringing out the coals from the mine. In this case, the width would require to be at least four feet six inches, or five feet, so as to admit of an iron rail-way to be laid in it ; besides which, there require to be, at proper distances, wider places formed in the side of the mine, to allow the loaded and empty carriages to pass each other. These by-pass roads are regulated as to the distance from each other, according to the quantity of work to be performed. When a day level thus serves a double purpose, the water is either conducted in a covered drain, cut deeper than the sole on which the rail-way is laid, or it is conducted along the side of the rail-way.
In other instances, a clay-level is not only made to carry off the water from the colliery, but is constructed of such size as to form a canal, by which boats can be carried into the coal mine, and loaded for the market. These levels are of various dimensions, according to the extent of traffic calculated upon. The smaller kind is nine feet wide, by nine feet high, having from three to three feet and a half of water in depth. The larger di mensions in practice are nine feet wide, twelve feet high, with five feet depth of water.
In driving a common day-level, for the sole purpose of draining the coal, the point having been fixed For com mencing it, if the ground is flat, part of it at first will he executed as an open cast or ditch, securely laid with flag stones in the bottom, and built in the sides with sufficient stone walls. The sole to be conducted in a line as nearly level as possible. If the alluvial cover continues, and becomes too deep for open cast, the mine or level must be arched, and the work conducted under cover. If the alluvial cover is soft, it will require to be secured in the sides and roof with timber, which can be drawn out, and used again, if the nature of the case admits of this being done ; but if this is not the case, the timber must be so placed as to admit the side walls and arch to be built within it. In this way the level is carried forward until it has proceeded some yards under cover of the rocks, or strata of the coal-field, when, if the strata are strong, no more mason-work is required. The next point is the proper line of direction, so as to reach the coal to be drained in the shortest distance. This line is that of the true dip and rise of the strata ; and that the mine is going correctly in this line is known when the divisions of the strata seen in the forehead of the mine, are parallel to the sole of it, which ought al ways to be level. If the strata form an angle with the sole of the mine, the direction is going oblique to the line of dip ; and the greater this obliquity, the greater will be the protracted distance, before the coal can be in tersected. In all cases where the strata are not soft and friable, the roof of the mine is cut in an arched form, which adds considerably to its strength ; and wherever the strata are soft and friable, or where the level passes slips and dislocations, the sides are built up and the roof arched with stone or bricks. The latter are generally preferred, on account of their occupying less room, are easily carried into the mine, and the work more expeditiously executed ; it being a rule that the mine shall, in all such cases, be made of such a size as to admit the building, without narrowing the fixed di mensions of it carried through the rock. As a day level proceeds forward, the air begins to fail, from the breath of the workmen, the use of gunpowder, and the issue of carbonic acid and hydrogen gas, which are sometimes found in great abundance ; to remedy this, deal boxes or pipes, from eight inches to a foot square, are carried in from the mouth of the mine to the fore head, and lengthened as the mine proceeds. This generally produces a sufficient circulation of air, until it is found necessary to sink an air pit upon the level, by which a strong circulation of air is not only produced, but the mine stuff is more easily drawn up than taken out to the mouth of the mine. Formerly these air pits were sunk directly upon the level, but that mode is now given up ; for this reason, that if any part of the sides of the pit gave away, the rubbish instantly choked up the level, and frequently occasioned much trouble and expence to get it cleared again. The practice now is, to sink these pits about eight or ten yards from the side of the level, and connect them by a side mine. In this manner, day-levels are carried forward for miles, and may be carried to any distance. If, in the progress of the level, workable coals are intersected before reach ing the coal which is the main object of the mining adventure, an air pit may be sunk of such dimension as to be suitable for a coal•pit, by which coals may be drawn. Air pits generally do not exceed seven feet diameter ; and they ought always to be of the form of a circle. Plate CCCXC. Fig. 7. represents a coal-field, where the fitting is made by a day-level; a is the mouth of the level at the surface of the sea, b, c,“, e, are coals intersected, and drained by the level. All the coals lower than the level can only be drained by ma chinery. A is a coal•pit sunk upon the coal e ; if the level is carried forward, the coals f, g, and any other coals which lie in that direction, will also be drained, and may be wrought by means of the pit A. The ef fect of the level Nv oul I be the same if the coals and strata had dipped in an opposite direction to that re presented in the figure.