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THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



191 



which were laid before me by Mr. Bold. I think no one can ex- 

 amine the instances he records, of wells at great distances heina; 

 immediately drained by pumping at Green Lane, without being 

 struck by the remarkable facility with which the influence of the 

 pumping is transmitted. If the cases adduced had been tew and 

 partial, one might have hesitated in admitting such easy permea- 

 bility as I believe to exist; but the sympathy here evinced is at 

 once so extensive, and the evidence so authentic, as to free my 

 mind from all doubt. 



It was urged, that the instances alluded to by Mr. Bold were 

 only from shallow wells, and that the effect would not have been 

 produced if they had been deep ones; but these wells cannot be 

 truly stated as all shallow, nearly one half of them being from 

 twenty to thirty-nine yards deep. This, however, does not strike 

 me as of much importance, for if in both cases the easy diffusion, or 

 the migration of water from one part of the formation to the otlier 

 be equally well established, it matters not whether the wells be 

 shallow or deep. 



My opinion is that, in considering the question of the supply of 

 water, the rock may be looked upon as almost equally permeable 

 in every direction, and the whole mass regarded as a reservoir up 

 to a certain level, to which, whenever wells are sunk, water will 

 always be obtained, more or less abundantly; and a very careful 

 consideration of the facts that have come to my knowledge in the 

 present investigation leads me to consider this view as the simplest 

 and the only one capable of general application. 



Quantity of Water to be got from Wells. 



By thus recognising the permeability of the sandstone to a great 

 extent, the question is relieved from many technical difficulties, 

 which have caused much discussion without leading to any practi- 

 cal result. I shall, therefore, now assume that wells are sunk into 

 that portion of the rock which is charged with water, and en- 

 deavour to ascertain what amount of water can be drawn from in- 

 dividual wells so circumstanced. 



The effect of pumping from a well under such conditions will 

 be to drain the adjacent rock, producing a comparative dryness 

 on all sides, in such a manner as would be represented by an in- 

 verted cone; the bottom of the well being the apex of that cone, 

 the sloping sides would represent the inclined surface of the water, 

 Sowing towards the well in all directions; and, as the pumping is 

 continued, the sides of the cone will become more and more ob- 

 tuse, or, in other words, more nearly horizontal, until an inclina- 

 tion is established where the friction of the water, in moving 

 through the pores and fissures of the rock, is in equilibrium with 

 the gravity upon the plane. And this condition of equilibrium 

 once established, any further pumping power would be useless, as 

 the water must gradually lower in the well until it is exhausted; 

 and no additional power of pumping could then avail in increasing 

 the quantity of water drawn from it. 



The natural alternative under these circumstances is, to deepen 

 the well by sinking a bore-hole, or to extend the filtering surface 

 at its bottom by means of tunnels; and which of these methods 

 is preferable has given rise to much difference of opinion. Where 

 the pumping is periodical, the advantage of the tunnel or lodgment 

 is unquestionably considerable, for it admits of the collection of a 

 large body of water into these tunnels, as reservoirs, without 

 causing much difference of level in the well itself; thus storing a 

 quantity of water, in addition to what percolates gradually through 

 the rocic, which may be pumped out of the reservoirs, and with 

 any rapidity. 



Taking the view to be correct that the pumping from a well 

 drains a conical mass of sandstone, until an equilibrium is estab- 

 lished between the supply into the well and the draught from it, it 

 would appear that the advantage of tunnelling is almost confined to 

 its operation as a reservoir, for as the tunnel is extended it can only 

 increase the drainage power of the well to the extent of a narrow 

 band on each side of it, the slopes still corresponding with those of 

 the side of the cone. Hence it is, I believe, demonstrable, that 

 every attempt at increasing the yield of a well by tunnelling in the 

 immediate vicinity of the well, can produce but limited permanent 

 advantages. 



In illustration of this, let us suppose that in the adjoining figure, 

 a well is sunk at a, and that it drains an area represented by the 

 circle b c d e, and that a tunnel is driven from a towards d, say 

 one mile in length, and that another well is sunk at d upon the 

 extremity or upon the terminus of this tunnel. The only effect of 

 this would be to increase the drainage area of the well a by the 

 area / g h, together with the small triangular spaces shown on 

 the figure; whereas instead of the tunnel being driven from a to 



rf, if the well at d had been sunk at h, the area drained would 

 have been double that which was originally drained by a. 



This method of looking upon the area drained by a well as re- 

 presented on the surface by a circle is not strictly correct, because 

 its form will be of course modified by the relative sizes, characters, 

 and directions of the fissures through which the water finds its 

 way to the well. The area represented by the circle in the figure 

 will, therefore, most probably be very irregular in outline, but the 

 way described by which the supply of water is transmitted to the 

 weU remains unchanged, and the conclusions to be derived from 

 this reasoning may practically be depended on. 



There is another defect in the system of tunnelling for the pur- 

 pose of enlarging the supply of water at any great depth. Experi- 

 ence in Liverpool has clearly pointed out the necessity of, from 

 time to time, deepening the wells in consequence of the increased 

 demand upon them, and, with a system of tunnelling, the result 

 would be similar, involving very great expense, and a most incon- 

 venient operation. This inconvenience and expense has not 

 hitherto operated with the public wells to any injurious e.\tent, 

 because the increase that has been made in them has not been con- 

 siderable, but where we have to look forward to the utmost efforts 

 being used to obtain the greatest possible supply of water from the 

 sandstone at the fewest points, arrangements certainly ought to be 

 contemplated for augmenting from time to time the supply at those 

 points. 



This view of the subject leads me to the opinion, that increasing 

 the number of wells is likely to be a more permanent source of 

 supply than extensive tunnelling, although the latter certainly 

 admits of an easy mode of connecting the various sources of supply, 

 and consequently of concentrating the whole of the pumping estab- 

 lishments. 



Increase of Snpply by Boring. 



The other alternative for increasing the supply, namely, that of 

 deepening the well, or of boring under it, has invariably been at- 

 tended by an addition to the quantity of water yielded for at least 

 some length of time; all the evidence which was given before me 

 testifies that this may be safely reckoned upon. Let us now ex- 

 amine what would be the effect upon a well so deepened, with its 

 increased amount of pumping. 



It is clear that the space drained by the well before being deep- 

 ened will now be increased in extent, and that an additional area 

 will contribute water to the supply of the well; and this extension 

 and consequent increase of the supply of water will proceed until 

 the plane down which the water flows towards the weU shall have 

 attained nearly the same angle as was originally maintained by the 

 shallower well — that is, the angle of equilibrium between the force 

 of gravity and the friction in passing through the fissures. But 

 tlie increase of supply to a well by deepening it, is by no means in 

 proportion to the depth gained, as the supply is in all cases limiteil 

 by the resistance which the water experiences in flowing towards 

 the well through the fissures. It is, however, probable that a more 

 permanent increase will be produced by deepening the wells than 

 by constructing tunnels; a suggestion of course chiefly applicable 

 to wells situated at a considerable distance from the River Mersey. 



But all the public wells have already been sunk to the level of 

 low-water mark, and from their proximity to the river it would, in 

 my opinion, be inexpedient to increase the depth, either by sinkmg 

 or boring, for if the sandstone be as pervious as I lliink it has been 

 proved fo be, a considerable amount of the supply to them would 

 be derived from the river itself, and consequently the quality of 

 the water much impaired. 



There are numerous instances of wells having become brackish 

 in the vicinity of the river; and, even at the distance of twelve 



