After the coal is discovered, whether through surface indications or by borings, the preliminary working begins by means of a shaft and levels. Generally the first step is to sink a shaft or pit. When the ground is soft, the pit must be walled with brick, stone, or timber, as fast as the descent is made. When the pit is sunk through limestone and sandstone, the progress is slow, but the walls sustain themselves, and do not require either masonry or timbering. A great inconvenience in sinking a shaft arises from springs and small streams of water. In many places where this inconvenience occurs, the shaft is fitted with a wooden lining, or tubbing, as it is called, which is made of thick staves somewhat resembling those of casks, the joints being carefully fitted, in order to keep out all water, and to withstand great pressure. Sometimes this tubbing is made of iron, wrought or cast. Where the ground is loose, or composed of sand and water, the tubbing is forced down from the top, or sinks by its own weight. When this tubbing consists of masonry, it is built in a circle at the surface, and as fast as the earth is removed the masonry sinks. A fresh circle is added at the surface, and thus the work goes on. It was in this way that Brunel constructed the shafts which formed the descent into the Thames Tunnel. Sometimes shafts are sunk under water, and in such case they are lowered in a perpendicular position until the ends strike the bottom, and then the water is pumped out. An ingenious apparatus raises the mud from the bottom, and a pump is kept at work to remove the water. Sometimes, in sinking a shaft through quicksand, the water runs in faster than any ordinary mode of drainage will remove it. M. Triger, an ingenious Frenchman, invented a machine by which the water could be pumped out. The cylinders of iron were five or six feet in diameter, and he divided them into three compartments, as nearly air-tight as possible. He forced compressed air into the lower one, and enclosed the workman inside. The man was thus in a sort of diving-bell. The compressed air, being forced against the bottom of the shaft, prevented the great mass of water from filtering through the sand. The small quantity which filtered in was, by the force of the compressed air, driven through the sand pipe communicating with the surface. “Imagine an army of mice,” the inventor graphically said to M. Simonin, “and a cat suddenly to make her appearance, and you would have the picture of water reaching the bottom of our shafts through a thousand holes in the ground, if the presence of the air is lowered, and returning suddenly to the surface as soon as the air recovers its tension.”