The joints between the rings of tubbing are made with sheet lead one-eighth of an inch thick, coated with red-lead. The lead is allowed to obtrude from the joint one-third of an inch, and is wedged up by a tool which has a face one-twelfth of an inch thick. The mode of suspending the tubbing to the rods will be understood by referring to [Figs. 152 to 154]. The rods are attached to a ring by the bolts connecting one ring of tubbing with another. The bottom ring of tubbing and the ring carrying the moss box have their top flange turned inwards, but their bottom flange outwards. A strong web of iron, forming the base of a tube 161⁄2 inches in diameter, is attached to the tubbing. The object of this tube is to cause the water in the shaft to ease the suspension rods, by bearing part of the weight of the tubbing. Cocks to admit water are placed at intervals up the tube, by which means the weight upon the rods can be easily regulated, so that not more than one-tenth to one-twentieth of the weight of the tubbing is suspended by the rods at one time. The ring holding the moss box is hung from the bottom joint in the tubbing by sliding rods.
Figs. 152-154.
The arrangement of the moss box which forms the base of the tubbing is one of the most important points requiring attention in this system of sinking. Ordinary peat moss is used. It is enclosed in a net, which, with the aid of springs, keeps it in its place during the descent of the tubbing. When the moss box, which hangs on short rods fixed to the tubbing, reaches the face of rock, it is dropped gently upon it, and the whole weight of the tubbing is allowed to rest upon the bed. This compresses the moss, the capacity of the chamber holding it is diminished, and the moss is forced against the sides of the shaft, thus forming a water-tight joint, past which no water can escape. This completes the third process.
It may be noted that up to this point the following important differences between this and the ordinary system of placing tubbing are to be observed;—The tubbing, on reaching its bed, bears the aggregate pressure of all the feeders of water which have been met with in the shaft. The tubbing, having been passed down the shaft in the manner described, no wedging behind, or other modes of consolidating it in the shaft, have been carried out. The connection between each ring of tubbing is so carefully made, that the repeated wedging of the joints, as in the ordinary system, is rendered unnecessary. The pit is still full of water up to the ordinary level.
Under these conditions the next process is;—The introduction of cement behind the tubbing to complete its solidity.
Figs. 155, 156.