In the first place, it may be desirable to describe briefly the system of sinking hitherto pursued in passing through strata yielding large quantities of water. The most important sinkings of this character have been carried out in the county of Durham, to the east of the point at which the Permian overlie the carboniferous rocks. In this district there is a thin bed of sand between the Permian rock and the coal measures. Towards this bed the feeders of water are generally found to increase, and in the sand there is usually a large reservoir of water. The mode of sinking will be understood by reference to Fig. 110. Whilst sinking in hard rock, it has ordinarily been the custom to place iron curbs, or cribs, wherever a bed of stone appeared to form a natural barrier between two distinct feeders of water. Thus it has frequently happened that important feeders have been tubbed back, rendering much less pumping power necessary than would have been required had all the feeders been allowed to accumulate in the shaft. As will be seen by Fig. 110, the number of wedging cribs employed is no less than thirteen in 250 feet. The cribs forming the foundation of each set of tubbing are generally much more massive and costly than the segments of tubbing. [Illustration: Cast-iron Tubbing. Fig. 111.] [Illustration: Wooden Tubbing. Fig. 112.] [Illustration: End Elevation showing Arrangement at Extensive Sinking. Fig. 113.] [Illustration: Side Elevation showing Arrangement at Extensive Sinking. Fig. 114.] [Illustration: Pass Pipe. Fig. 115.] [Illustration: Pass Pipe. Fig. 116.] [Illustration: Ball Type of Pressure Equalizer. Fig. 118.] The process of fixing the crib is as follows;--The diameter of the shaft is made about 30 inches larger than that of the inside of the tubbing. When a bed of rock, which may be considered sufficiently hard and close to separate the feeders above and below it, is reached, the shaft is contracted to the diameter of the tubbing, and a smooth horizontal face is made on which to place the wedging crib. The wedging crib, which usually consists of segments about 4 feet long by 6 inches high by 14 inches wide, is then placed on the bed. To give the crib a firm and secure position, it is tightly wedged with wood, both behind and between the joints; the tubbing is then built upon it to the next wedging crib, which rests upon a bell-shaped section of rock. When the tubbing nearly reaches this crib, the rock is removed piece by piece, and the top ring of tubbing is placed close up against the crib. It will thus be seen that the fixing of each crib is a costly process, often causing considerable delay.