In such formations as the gravels when they hold much water, it usually is sufficient to sink a well in a vertical direction so far as to maintain a good depth of water at all times of the year, provided there be not many wells drawing their supply from the same bed. It rarely happens that the permeable gravel strata of largo towns aro able to supply the demand made upon them, and it is therefore almost always the case that after the wells have been gradually deepened, their use is forcedly abandoned in such places : the area of supply is in fact limited ; the demands of a large population practically are unbounded. .
But there is another reason of far mom serious importance why the ordinary wells of towns are at the present day being gradually abandoned, iu the contamination to which they are exposed from the infiltrations from drains, sewers, cesspools, and dead wells, which eventually introduce so large a proportion of organic matters (in the form of the nitrates) into the waters, as to render them utifit for human consumption. The infiltrations from the sewers, or the leakage from the water pipes of a town distribution may, it is true, provide an unexpected quantity of water in the superficial wells ; but the earth traversed by it. is so charged with organic matters, under all towns, that the quality of the water must be of the most objectionable nature. In towns situated upon the chalk even it Liss been found that unless the wells are sunk to a great depth, they are subject to con tamination from the causes referred to ; and it therefore becomes more than usually important to form an impermeable brick steining round those portions of the wells which are within the range of the effects of surface'waters.
The gravel, or sand beds, hitherto considered, are penetrated by water in the whole of their thickness able to yield a supply ; and the chalk marl, the green sand, the new red sandstone in some of its beds, present the same hydroscopic conditions. But such formations as the ripper chalk, the oolites, the elate rocks, &c., do not allow the water to pervade the whole mass ; on the contrary, the water only passes through them freely upon their great planes of stratification, or by following the joints of cleavage, or the vertical fissures which so com monly occur in those rocks. If then, wells sunk in the former class of materials should not contain much water (always assuming them to be sunk of the ordinary dimensions, that is to say of about 4 feet clear internal diameter), there is little probability of obtaining an increased supply by domeing out the bottom of the well, or by driving headings or lateral edits. In the latter class of materials, however, it frequently happens that very beneficial results are obtained by thus increasing the contributing area ; for it is by no means rare to encounter springs of considerable volume by thus opening up succes sively the edges of the divisional planes. It is advisable in forming
headings, in the strata referred to, to drive them at a slight inclination horizontally to the axis of upheaval of the strata, and vertically to follow their dip, making the height of the heading as great as possible, in order to lay bare the edges of a great number of the beds.
The water obtained from superficial wells is raised to the surface by some one or other of the ordinary machines. Where the consumption is small and the depth inconsiderable, the bucket and windlass will be found generally speaking to be sufficient ; where the consumption is large, pumps must be used ; common suction pumps will suffice when the lift does not exceed 28 feet ; beyond that depth force pumps must he used, and they may be driven by hand, by wind, animal, or steam-power, as may be most advisable. All wells are improved by frequent and hard pumping ; for not only does the water they contain undergo a species of unsatisfactory chemical action whilst standing in them, but it is also found that by relieving the pressure upon the edges of the water-bearing strata the water passages become increased.
Dead wells and absorbing wells depend for their action upon the hydrostatical law that water will, when left to itself, attain a uniform level over the whole of the surface. If then a column of water be made to stand upon a water-bearing stratum, the level of the former will subside to that of the normal water-line in the latter ; and the excess of the column will be dispersed throughout the stratum. Advantage was formerly taken of this law, in such towns as Southamp ton, to get rid of the liquid matters of cesspools by sinking them into the permeable water stratum of the district, and leaving the bottom of the excavation open ; the liquid matters thus diffused themselves throughout the superficial water-bearing stratum to the ultimate destruction of the wells fed by the latter. Of late years an attempt has also been made at Paris to get rid of the foul waters of the Voirie of that town, by sinking a boring of large diameter to the permeable strata between the chalk and the upper tertiary beds of that locality ; but not only has it been found that serious injury was thus done to the neighbouring wells, but that the bore holes very soon became choked up. The fact is that dead wells and absorbing wells are public nuisances of a very serious nature, and their formation ought to be rigorously forbidden and carefully prevented. Unfortunately there is no legislation on any part of the natural conditions of flow of underground waters; nor do any legal means exist by which a landed proprietor could he prevented from contaminating the wells of a large district around him.
(See Swindell, On Well Boring and Sinking ; DegoussEe, Guide du Sondrur ; Burat, La Geologic appliquee aux Arts.)