In order to give room for the lower wedging crib, the recess is cut a few inches wider, as at c, flow b to c is laid with sheeting deals all around the circle, or with a thin uniform stratum of oakum. Upon this the wedg ing crib d is laid, and neatly jointed to the radius of the pit, each segment trained exactly to the circle. At each joint of the segment sheeting deal is inserted. This wedging crib is about ten inches in the bed, and six inches deep. The space e, at the back of the crib, about two and a half inches wide, is filled with pieces of clean reeded dry deal, inserted end-ways : and this is regularly wedged with one set of wedges all around ; then with a second and third set in the same regular manner, the object being to keep the crib in a posi tion ti uly circular, the wedges used in this operation are made of dry clean fir, very thin at the head in comparison with their length, openings are made for entering the points of these wedges by a steel wedging chissel ; and the wedging is continued regularly around, until the steel wedging chissel cannot make an opening in the wood. By this operation no water can pass downwards by the back of the crib. The next opera tion is, to fix a spiking crib,f, to the rock, about 10 or 12 feet front the lower crib, according to the length or the planks to be used for the tubs. This spiking crib is about five inches square, and is only used for spiking the deals or the tub to it. They must, host ever, be set true to the sweep of the pit, as upon them the true cir cular figure of it depends. This spiking crib being placed, the tubbing deals, k, are to be fixed. These arc made of deals three inches thick, and about six inches broad, planed on all sides, and the %%fought to the true bevel of the circle of the pit. Sheeting deals are laid under the deal ends, and the tub deals are fir ni ly set together, one by one, in a perpendicular direc tion, their tipper ends reaching to the middle of the spiking crib f Formerly these planks were fixed to the spiking cribs with iron nails or spikes, but as these were soon destroyed by corrosion, particularly when there was any salt in the water, treenails are now used, and found to be much better. When the planks are set around, and within less than thee breadths of a plank from being closed, a plank is placed on each side of the opening, having a square joint next each other in place of being bevelled, which leaves a ieetangular or parallel space for inserting the closer or closing plank. This plank is prepared so as just to enter by force, it is then rubbed on the edges with soft soap, and is, by main force, and heavy mallets, driven flush with the other plank. The sheeting deals under the ends of the plank, and in the joints of the wedging crib, are then wedged tight, in the manner before mentioned. The main cribs gg, are then to be placed as counterforts, for the security and strengthening of the tubbing. These cribs are made in the manner before mentioned ; they are about eight or nine inches in the bed, seven niches thick, and are placed at distances from each other, proportioned to the column of water to be resisted. At great depths they are laid close together, and the distance from each other increases as the pressure decreases, care being taken to have al ways a decided excess of strength, and made to bear equally against the tub deals, by means of wedging at the joints. The flu st main crib is laid close to the foot of the tub deals, and resting on the wedging crib. Dur ing this operation, the growth of water at the back of the tubbing is allowed to flow freely through a ping hole at the lbot of the tubbing, until the whole operation is completed. The upper ends of the first set of tub planks arc then cut square, and level all around. The second spiking crib I, is then fixed, and another set of tubbing deals put around as the lormer, having sheeting deal inserted betwixt the ends of the two sets at f. When this is wedged, the cribs h h are placed in the manner before described. According to this plan, tubbing may be carried upwards to any height, until the water rising at the back of it would discharge itself into the natural reservoir above the rock head, as is frequently done ; but if a tubbing of a few fathoms is to be formed in a at a considerable depth from the rock head, then a wedging crib must be placed at the top of the recess, and firmly back-wedged, as at the lower wedging crib d. When this kind of tubbing is completed, the plug hole is filled up, and the water quickly rises and produces its full pressure. Although, from the nature of the ope ration described, it might be supposed that nothing could shrink even in the least degree, yet, as the pressure ac cumulates, the wood and the joints are heard very dis tinctly creaking by the excessive pressure. From this it is evident that no form but that of a circle could resist such a weight. After the full pressure is obtained, it frequently happens that veins of water, not thicker than a fine hair, will be found springing from the sides of the tubbing; these generally take up in a few days by the swelling of the wood. When all is secured, sheathing deals, inch and quarter thick, are nailed to the cribs all around the pit, not only to preserve the cribs from injury, but to make a smooth wall for the ascending and descend ing baskets with coals. These sheathing deals are put on in regular perpendicular courses, and the but-ends meet flush on one of the cribs.
The solid cribbing, executed with oak cribs only, and no tubbing deals, is a more complete piece of work than the former. If the pressure is great, they are formed three to four feet long, ten inches in the bed, and seven or eight inches deep, of the best oak, as be fore described. The first crib is laid down, and back wedged, in the same manner as the wedging crib in the plank tubbing, with this difference, that it has been found, that when the wedging was made in lines paral lel to the sweep of the cribs, the immense pressure had a tendency to press forward the lower part of the crib ; and it has been found from experience, that, if the wedging is done in a diagonal manner, it is much su perior, and quite effectual. After the back wedging is
completed, the next tier of cribs is laid down in precise position, having sheeting deal along the bed, and at each end joint, the perpendicular joints are overlapped in each set by the position of the next segments alter nately, so that the face of the cribs, with the jointings of sheeting deals, has the appearance of ashler work. A wedging crib is fixed in the upper tier, if next the top of the recess, and the back wedging completed, as before directed. Each crib is set true to the radius of the pit, and the back of it filled up with pieces of wood betwixt it and the rock. When the whole space proposed to be cribbed is filled up, then the wedging of the joints commences while the water flows by a plug hole at the bottom. All the horizontal and perpendicular joints are gone over completely with one set of wedges; and, in the same way, a second and a third set of wedging is done, till wedges can be no longer driven. The face of the wedgings is then made smooth, the plug hole filled up, and the full pressure sustained. In this kind of work, innumerable fine veins or filaments of water issue from the wedgings, the water being forced through the pores of the wood. If these do not takeo_t:4, addition al w edging must be attempted, till not a particle of wa ter can he seen issuing. In this piece of work, much ingenuity is displayed, and it reflects much credit upon the inventor.
The third mnde of tubbing is by means of iron cy linders cast into segments. These are different from the drop or sinking cylenders, because they are placed piece after piece in the circular recess of the pit cut out for them. The flange for the wedging joint is some times towards the centre of the pit, but the improved method is, to turn the flange inwards. In the latest improvements of this plan, executed by Mr. Buddle, where there was a pressure of several hundred feet, the segments were six feet long, and two feet deep, an inch thick, counterforted with raised work on the back ; the lip of the flange was strong, and supported by brackets. These iron segments of a cylinder are set true to the ra dius of the pit ; and every horizontal and perpendicular joint has sheeting deal in it. A wedging crib is fixed at the bottom, and the segments are carried regularly up, having the joints like ashler work. This kind of tubbing can be carried to any height, until the water finds an outlet at the surface, or strata containing wa ter can be tubbed off, as by the other kinds of tubbing before described. A pit completed in this manner pre sents a smooth shaft wall of iron, the flanges being to wards the outside of the cylinders. In this work no screwed bolts are used for joining the segments toge ther. In a pit in the Newcastle district, 70 fathoms have been executed in this manner, under Mr. Buddle's directions.
When a thin bed or parting betwixt two of the stra ta which arc impervious to water, produces much wa ter, or when the cutters or fissures in the strata do so, this water can be completely prevented from flowing into the pit by wedging, for which there is a parti cular method. In place of attempting directly to wedge the fissures, as was the former unsuccessful practice, the fissure is cut open with chissels, and made about two inches wide and seven inches deep, as represented Fig. 16. Plate CCCXC.; the lips are rounded off about an inch and half; pieces of clean deal are then driven in, the face of which comes no farther than the contour of the lips; the whole is then firmly wedged, till all the water is stopped back. By placing the wedging back from the face, the rock is prevented from hursting dur ing the operation, which always happened in the former process.
In sinking engine pits, besides contending with the water, ventilation has to be attended to, on account of the pernicious airs or gases found in the strata, and which at times come off in great quantities. These gases are carbonic acid or fixed air, and carbureued hydrogen or inflammable air. The first of these gases being heavier than the atmospheric air, sinks to the bottom of the pit, and filling it up like water, displaces the common air, until it flows over the mouth of the pit. This air instantly extinguishes flame, and de stroys animal life. The other gas, being lighter than air, instantly ascends the pit, mixed with common air. If this gas is much diluted with the common air, the workmen suffer no inconveniency from it ; but if pure and unmixed, it destroys animal life, or if mixed with certain proportions of common air, it suddenly explodes and burns the workmen. These gases will be parti cularly noticed when the ventilation of mines is treat ed of.
In ordinary cases, while pits are sinking, the brattice walls produce a circulation, by the air descending upon one side and ascending by the other. If this does not take place, the circulation must be produced by fire rarefying the air.
The most approved method is, to cover the engine-pit area of the shaft with deals, having apertures for the pump spears and tackling to pass through, with doors for the men going up and down ; near the top of the pit, and immediately under this scaffolding, a tube of brick, at least three feet square, is carried in a hori zontal direction, and connected with a furnace having a high chimney with double doors, where the person en ter s to throw coals on the fire, as represented Fig. 3. Plate