In sinking engine pits, the first point is the mode of passing through the alluvial cover. If it is of a firm and dry consistency, the process is easy, whether the depth be great or not. As all engine-pits require to be particularly well executed, and every part of the work done in a substantial manner, the shaft which passes through the alluvial cover ought to be secured with ma sonry of jointed ashler, with the joints accurately bevel led to the centre of the circle which forms the area of the pit. The stones used are from a foot to sixteen in ches in thickness, about a foot in depth, and of any con venient length. The pit is therefore begun, and sunk of a circular form, through the alluvial cover, of such an extra width as to admit the thickness of the masonry or stone cradling, as it is termed. When the depth is about 12 or 15 feet, the bottom is made level, and a ring or crib of oak or elm, about four and a half inches thick, and ten inches broad, laid in the bottom. Upon this the ashler is built upwards to the mouth, and all the void betwixt the ashler and the earth walls firmly beat up with clay. The next operation is to sink again, keeping the sides about three inches inside of the perpendicular of the ashler walls. This sinking is carried down from three to nine feet, according to the consistency of the cover, and when the distance is fixed upon for another crib, the pit is widened out at the bottom, so as to admit the second crib to be laid exactly in a perpendicular di rection below the other ; and when this crib is laid in its place, about three or four feet in breadth is taken out from one of the sides, and in this a pillar of ashler is built, resting upon the lower crib, and supporting the up per one. The same thing is done on the side immedi ately opposite, and then at the other quarters of the cir cumference, after which the intermediate spaces are widened and built up. In this manner the pit is sunk until the rock head is found, when it is made level, and the lower part of the ashler rests upon it. When stones cannot be easily procured, hard bricks are substituted, ten inches or a foot in length, with a level to suit the ra dius of the pit. When ashler is built in stages, as be fore described, the under building may be sometimes done with three pillars, and then filled up around ; but when no cribs are used, as is sometimes the case, very narrow pillars are built up, to secure the masonry above.
When the cover is not very firm, the stages sunk at a time, and built up with masonry, will not admit of being more than from two to three feet, and when the cover is of such a kind as not to admit of this mode of operation, a different plan must be adopted.
If the cover is of clay, with sand backs and oozings of water, so that it will not stand firm for even a foot or two, it may be sunk through, and secured by the fol lowing process.
For circular pits, circular Tellies of wood, named cribs, and for square or octagon pits, square balks of wood, named bars, are prepared, and used as a tempo rary cradling until the mason-work is built. The width of this carpentry depends upon, whether the timber is to be drawn out as the building advances upwards, or if it is to be allowed to remain, and the mason•work built within it. The shape of the pit where the timber is used is either a circle, a square, or an octagon. The
temporary cribs are formed of oak, ash, or elm, about seven inches in the bed, and five inches deep. The balks are of fir, and from six inches to a foot square, according to circumstances. The joints of the circular cribs are plain, and bevelled to the radius of the circle.
The joints of the balks for a square or octagon pit are half checked.
If the timber is to be left in the pit, and the masonry built within it, the quality of the wood need not be much attended to ; but if it is to be drawn and used again, it requires to be good, in order to bear the fatigue it is ex posed to, without being rendered useless.
With circular cribs of wood, backing deals are used of about inches thick ; the pit is then sunk as far as the cover will admit with safety ; a strong crib is placed at the top or pit mouth, in a level position ; the backing deals are placed all around, and reach to the bottom of the space sunk ; cribs are then placed about two feet apart, and the lower crib of the set is fixed the half of its depth lower than the ends of the backing deals ; the cribs are secured at the joints by thin spars of wood overlapping them, and they are kept in their horizontal position, either by brackets placed under them, and nailed to the backing deals, or by a few deals placed in side of the cribs, and nailed to each. Particular care is requisite, in sinking through the cover, to have it no wider than necessary, in order that the backing deals may bear against it, and if there are any vacuities, these must be filled up with rubbish well packed. When the first or upper space is thus secured, another space is sunk, of such a depth as the cover will admit of ; then backing deals and cribs are placed in the same manner as above described, the end of the second set of deals having a hold of half of the lower crib of the first set. In this manner is the cover sunk through and secured, until the rock head is found, when the masonry is be gun, and carried upwards to the top ; if the cover is of such a kind as to admit the cribs and backing deals to be drawn, these are taken out in such lengths at a time as are judged safe, and the space betwixt the masonry and cover filled up with any kind of rubbish, firmly beat in. If there is any risk of the cover slipping, then the car pentry, or the greater part of it, must be allowed to re main in the sides of the pit.
In sinking square pits through a firm cover, the same kind of process is adopted as that with the circular cribs, the only difference being in the joints, which are half checked.
In sinking through a firm cover with a pit of an octa gon form, the bars or balks are half checked in the joints, and put in close to one another ; this is termed being placed skin for skin. If the cover is of a very firm consistence, their scantling or dimensions arc small ; if it is less firm, they are made proportionally strong. The process carried on in sinking to the rock head is the same as before described, the depth of cover passed through and secured, each stage being regulated accord ing to the tenacity of the cover. The masonry is car ried upwards from the rock head in the same manner as before described.