The practice of keeping the shafts dry, as now de scribed, is very generally adopted in Great Britain, when the pits are of a moderate depth, and when all the water found in sinking can be drawn by an engine of a moderate power. But in very deep pits, such as those in the counties of Northumberland, Durham, and Cumberland, where they are from 80 to 150 fathoms in depth, the preventing of the water found in sinking, from being a burden upon the engines, is most care fully attended to, and is one of the chief points in the science of mining. In this operation, much skill and resolute determination are displayed. The mode of passing through the cover, and the various modes for preventing the water from that source descending into the pit, have already been described. The next thing is, to point out the plans which have been successfully adopted for preventing the water found in sinking from flowing into the pit, and becoming a burden upon the engine. Those strata which generally produce the greatest feeders of water, are the sandstones, particu larly those of an open porous texture, or those which have very open cutters. Sometimes both these quali ties are combined, and then the growth of water is un commonly great. Several of the sandstones are, how ever, impervious to water, and almost all the beds of light-coloured argillaceous schistus or fire-clays are par ticularly so, being very close in their texture. But some of the sandstones, though impervious to water in their texture, have wide and open cutters which pro duce much water. Feeders of water are also frequent ly found at the beds or partings betwixt two of the strata. Those strata which are impervious to water are said to be capable of turning water.
While the sinking of one of these deep pits is going forward, a regular journal is kept of every part of the operation, and each feeder of water is measured, not only after it is found, but daily, to find if its growth increases, abates, or is regular. The mode of measur ing is by receiving the water into a vessel of 30 or 60 gallons content, and finding, by a seconds watch, the time required to fill it. Three experiments of this kind are made at each trial, and the average time of the three entered in the journal. \Vhen the feeders of water in one or more of the strata sunk through are producing such a quantity of water, as to render the stopping of it back absolutely necessary, the first thing to consider is, whether any of the strata passed through under the feeders are impervious, or capable of turning water; and if there are none, the pit must be sunk till a stratum of that kind is found ; then the strata are to be examin ed upward, until a similar stratum is fixed upon. There are three methods of keeping back the feeders ; viz. by plank tubbing, iron tubbing, or by oak cribs. As the pressure to be resisted is the next consideration, it be comes necessary to explain its extent or limits. To il lustrate which, Plate CCCXC. Fig. 14. represents a pit sinking through the various strata, having a cover of sand with much water resting on the rock head. It is evident, from what was explained when treating of the forms of coal-fields, that each stratum sunk through in any pit, whether near the surface, or at the greatest depth, rises in one direction until it meets the alluvial cover ; for which reason, the pressure of water at the bottom of the tubbing which rests on the rock-head, is as the depth of the water found in the cover ; and if a stratum a is found to yield a great quantity of water, and that the stratum b above, and the stratum c beneath it, are impervious to water, if the porous stratum a is twelve feet thick, and no water found in the strata passed through from the rock-head, until that depth which is supposed to be fifty fathoms from the surface of the wa ter in the cover, in this case, the tubbing or cribbing has not to resist the pressure of the water of twelve feet only, but must be made of strength sufficient to resist a column of water equal to a perpendicular altitude of fifty fathoms; because the stratum a meets the alluvial cover at d, the fountain-head from which all the water found in sinking proceeds. Upon this principle, though
no feeder of water was found of any magnitude until the pit had been sunk 100 fathoms, if this water re quired to be tubbed off in a stratum only three feet in thickness, the tubbing would require a strength equal to the resisting of 100 fathoms of pressure ; for although the water may only come through the open pores of the stone, yet, when the water is resisted, these innu merable tubes act upon the tubbing with the full effect of the whole hydrostatic column ; for it is lound from experience, that whatever water occurs in pits, or in the working of mines, proceeds front the surface. A clear understanding of this principle is an essential point in milling. Upon this principle, also, if the cover which rests upon the rock-head is of impervious clay, very little water will be found in the strata ; but if sand beds rest upon it, or rivers run along it, the water will be very abundant.
From this view of the great pressure of water found in sinking, the tubbing it off is an op( ration of the first and requires much skill, attention, and practice. If several fathoms of the strata are to be tubbed, in order to resist the flow of water, the pit has to be widened regular ly, to admit the kind of tubbing proposed to be put in. The greatest width being re quired for plank tubbing, and the least width for iron tubbing. Plate CCCXC. Fig. 15. represents a pit widen ed out for plank tubbing, where a, a, a, a, are the impervious strata, b, b, the porous strata yielding the wa ter, the bottom of the recess c, c, is made level, and as smooth as possible, by means of mason irons. The same points are attended to in working off the upper part of the recess d, d. In this operation, wedging cribs, spik ing cribs, and main cribs are used ; besides the plank which forms the tub, a quantity of fine dry clean reeded deal is prepared for forming the joints, which is known by the name of sheeting deal. This sheeting deal is always applied in pieces laid end-ways; that is, with the end•reed of the wood towards the area of the pit. As much of the security depends upon the tub being water tight at its jointing with the rock, several plans have been adopted for this purpose, the most approved of is represented, Fig. 2. Pale CCCXCI.