424 Chronicles of Science. \_3v\y, 



during the remaining sis months of the year, so that six months is 

 required for percolation. The same fact has been stated to the 

 writer in the case of a well in South Lancashire, and it is owing to 

 this remarkable property of admitting of slow percolation that the 

 rock becomes in effect both a natural filter and a reservoir. During 

 the slow percolation of the surface-water — noxious impurities are 

 separated from the pure element, and a store is also laid up in seasons 

 of plenty to be dealt out in times of dearth. On the other hand, water 

 derived from surface drainage, in order to be made permanently 

 useful, requires the construction of " lodges " or basins for the 

 storage of the surplus waters of wet seasons. 



It may here be proper to describe shortly the process of filtration 

 as well as the general manner of constructing a deep well in this 

 New Ked Sandstone. (See Fig. 2.) 



The site having been chosen, a well of a diameter sufficient for 

 the play of a double-pumping engine, is commenced and carried down 

 till the natural water-level (L) is reached. A temporary (or per- 

 manent) engine is then put up, and the water is kept down, until it 

 threatens to become too plenteous for the engines to deal with. 

 Large chambers or tunnels (0 G) are then generally driven in op- 

 posite directions for lodgment, and from the floor, a bore-hole (H), 

 of 6 to 12 inches in diameter, is driven to any required depth. Now 

 the manner in which the water is drawn from the surrounding sand- 

 stone rock is somewhat as follows. Before the well is sunk, and sup- 

 posing the rock to be homogeneous (which is, of course, not strictly 

 the case), the water will be found standing at a certain level depend- 

 ing on the position of the springs and other natural or artificial 

 sluices. This water-level (L) is only approximately horizontal, as it 

 tends to vary with the surface, and rises and falls according to the 

 season. The water, however, is at rest, or in equilibrio. But when a 

 well is sunk below this level, and this water is extracted, the equi- 

 librium is disturbed. A vacuum is created into which the surround- 

 ing waters will flow, in order to fill it up. Two forces will then be 

 brought into play — gravity and lateral pressure acting vertically and 

 horizontally — both of which are resisted by the friction of the rock — 

 which will be proportional to its density — and the resulting motion 

 will be a diagonal series of curved lines (I, I, Z,) converging towards the 

 axis of the well. From the above description it will be apparent that 

 in order to increase the area of supply, it is necessary to deepen the 

 well — and that the limit to the distance will be reached when the 

 friction of the rock is equal to the lateral pressure. 



The above general principles are subject to modification, arising 

 from the unequal density of different beds, the stratification of the 

 rock itself, bands of marl faults and fissures. As the effect of these 

 latter is not unfrequently a matter of controversy, I shall say a few 

 words on the subject here. 



Faults (or vertical dislocations) in the New Bed Sandstone differ 

 from those in the Coal-measures in being filled in with broken sili- 

 cious material (often indeed ground and hardened, but full of frac- 

 tures), instead of clay or other impervious matter, which in the working 

 of a coal mine often renders the presence of one of these faults a bar- 



