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that throws it down, in this way extending the field. Again, whindykes serve the purpose of dams, and prevent water passing fromone “waste” or worked-out space to another.
It is also of importance to fix on the best position and form forthe pits. Where much water may be expected, the best form forthe pit-shaft is circular, so that the water met with in sinking maybe kept back by tubbing; that is, lining the shaft with suitablematerial, such as stone, timber, or cast-iron, the latter being pre-ferred. When the pressure of water is great, sometimes the tub-bing is formed of half rings, so as to fit the shaft; but where pumpsand brattices interfere, Segments of cast-iron are used, about 4 feetin length and 2 feet in height, and from ^ of an inch to 1 inch inthickness. The Segments are made to form a smooth surface inthe shaft, and they are fitted to each other by means of flanges,3 to 4 inches at each end, and the spaces between the Segmentsare filled up with thin deal. Stone tubbing is merely commonwalling, with the foundation made tight by means of grooves cutin the stone, the joints and backing being filled up with cement,which, if carefully executed, will answer for light purposes; but thesuccess of this method of tubbing is of too precarious a nature tomeet with general application for important works, and wood oriron is preferred. It sometimes happens in sinking pits that all thewells and springs in the neighbourhood are drained off, but this evilmay be prevented by tubbing the shaft.
Some pits are sunk at great expense, owing to the nature of thestrata which have to be passed through, and other difficulties, as,for example, a heavy flow of water. Such instances occur in thenorth of England, as at Pemberton’s Pit, Monkwearmouth, nearSunderland , and a pit at Seaham near Durham, which is 300fathoms deep and cost the enormous sum of .£100,000. Before thesteam-engine was introduced, the coal-pits capable of drainage withhydraulic machinery or water-engines were comparatively few innumber; and when drained by wind-mills, as was sometimes thecase, the pits were drowned in calm weather. The driving of daylevels was thus a primary object with the early miner; and thisSystem of draining is the cheapest where circumstances allow of itsadoption. The day levels were often of sufficient dimensions toadmit of roads, and even in some cases of canals, being formed inthem, so that machinery was not required. In modern times, how-ever, the water is pumped from great depths by steam power, the