As an example, let Fig. 30 represent a sectional plan of a portion of the water-bearing stratum at the bottom of the shaft. This stratum is underlaid by an impervious stratum, and, consequently, the water will flow continuously through the former in the direction of the dip, as shown by the arrow and the dotted lines. That portion of the stratum to the rise of the shaft, S, which is included within vertical lines tangent to the circle at the points _m_ and _n_, will be drained by the shaft. The breadth of this portion will, however, be extended beyond these lines by the relief to the lateral pressure afforded by the shaft, which relief will cause the fillets of water to diverge from their original course towards the shaft, as shown in the figure. Hence the breadth of drainage ground will be _a b_, and it is evident that the shaft, S, can receive only that water which descends towards it through this space. But if tunnels be driven from the shaft along the strike of the stratum, as at _m c_, _n d_, these tunnels will obviously intercept the water which flows past the shaft. By this means the drainage ground is extended from _a b_ to _a´ b´_, and the yield of the well proportionately increased. It should be remarked that when the strata is horizontal or depressed in the form of a basin, that is, when it partakes more of the character of a _reservoir_ than a _stream_, the only use of tunnels is to facilitate the ingress of water into the shaft, and in such case they should radiate from the shaft in all directions. They are also of service in case of accident to the pumps, as the time they take to fill up allows of examination and repairs being made in that time to the pumps, which could not be got at if the engines stopped pumping and the water rose rapidly up the shaft. The size of the headings is usually limited by the least dimensions of the space in which miners can work efficiently, that is about 4-1/2 feet high and 3 feet wide. The horse-shoe form is generally adopted for the sides and top, the floor being level, for the drawing off of the water by the pumps is quite sufficient to cause a flow, unless of course the dip of the stratum in which the tunnels are driven is such as to warrant an inclination. Where there is any water it is not possible to drive them with a fall, for the men would be drowned out. The cost of some headings in the new red sandstone which the writer recently inspected, varied from 30_s._ a yard in ordinary stone, to 4_l._ 10_s._ a yard in very hard stone. The foregoing remarks do not apply to headings driven in the chalk, where it is the usual practice to select the largest feeder issuing from a fissure and follow that fissure up, unless the heading is merely to serve as a reservoir, when the direction is immaterial.