It is a common practice to place two strong logs of wood at the top of the pit, parallel to each other, and at a distance equal to the diameter of the pit, resting on cross sills, laid upon the surface of the ground ; these are termed the hanging sets. From them the cribs or bars are suspended, and prevented from slipping downwards, by a few planks placed in a perpendicular direction along the face of the bars, and to these planks each bar is nailed.
When the cover is of soft mud, which is a common circumstance in many coal districts, it is passed through either by bars laid close to each other, or, if this mode is not found practicable, the operation of tubbing is re sorted to.
When bars are used, each set requires to be put into its place as soon as the pit is sunk the depth of a bar. In this manner the sinking proceeds till the rock head is found, when the masonry is begun and completed as before mentioned. It is necessary to remark, that there is great attention required in this operation; for if the pit is stink below the last placed bars, and not immedi ately secured by additional bars, the mud will swell out into the area of the pit, and, when this begins, it is al most impossible to stop it. The consequence of which generally is, that a void is formed at the back of one part of the bars, the resistance is then lessened, and the pressure operating powerlully on the opposite side, the bars give way, and the pit is frequently lost. It is also found necessary, sometimes, before a bar can be placed into one side, to drive a row of thin sheeting piles, from three to four feet long, into the mud upon the other sides in face of the bars, which prevent the mud coming into the pit while the other bar is fixing; and when this bar is placed, one side of the sheeting piles is removed and driven at the face of the last placed bar, till the adjoining bar is fixed. In this manner the bars are placed pro gressively.
NVhen the mud is found to be so very soft that the operation of tubbing must be resorted to, a circular tub is formed of the requisite diameter. It is made of plank, from two to three inches thick, with the joints bevelled to the radius, inside of which are cribs of hard wood, placed at from two to four feet asunder, according as the circumstances of the case may require. The cribs are made of the best heart of oak, sawn out of the natural curve of the wood, suitable to the radius, in segments of from four to six feet in length, from eight to ten inches in the bed, and five or six inches in thickness.
The reed ef the wood require to be clean, without any twist or cross grain, for the greater strength and security of the work. The length of the tub is made from nine to twelve feet long, if the mud is of that thickness; but if the mud is very thick, a succession of tubs is placed one above another. The tub first to be used has the lower end of the deals made thin all around, and shod with sharp iron in the face. If the pit is previously secured to a certain depth, then the tub is constructed to pass within the cradling, and lowered down with tackles till it rests among the soft mud, it is then loaded with iron at the top, so that it may sink as the mud is excavated. If the first tub does not reach to the rock, then a second tub of similar construction is placed upon the top of it, and the iron weights removed also to the top. In this manner the tubbing is sunk, till the cover is passed through and secured. If the mud is uncommonly soft, and pumps are used to draw the water, they must be hung with tackles, and not allowed to sink down so low as thu bottom of the tub, as they would not only be liable to choke, but they would have a tendency to draw the mud from the back of the tubbing. It is therefore the practice to keep nle bottom of the tub several feet in advance of where the stand, and to keep about a foot of water upon the top of the mud. to this way the workmen stand upon Meet.% of hoard, to prevent themselves from sinking ; and the mud is lifted with scoops nr shovels under the water. If the mud is of great depth, the tuhs must be strengthened with addi tional ribs to resist the pressure.
The most difficult operation in sinking, is when the cover has a bed or beds of quicksand in it, or has a great bed of quicksand from the surface to the rock head. This circumstance is, in all cases, productive of great additional labour and expense. In the early times of mining, a quicksand cover formed an insurmountable obstacle to the miner ; and it is only within the period of a few years past, that the operation could be gone into with the hope of ultimate success. As this is one of the most important points in coal mining, and as very large sums have been lost by unsuccessful attempts to sink through the stud, the different processes which have been adopted, and most approved of, will be now described, so that a distinct idea may be formed of them.