[Water supply of towns].

Date:
[1856]
Catalogue details

Licence: Public Domain Mark

Credit: [Water supply of towns]. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

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    is as completely checked by a bed of clay as by the solid rock. Hence the course of the percolation follows the direction of the porous beds. It may happen that water, in pursuing a lateral direction in the pervious strata, passes beneath as well as above an impervious layer; and between the two it will be hemmed in ^vith the whole force of the downward pressure at the spot wliere it enters the soil. If there be no outlet, the pervious stratum thus enclosed will be saturated and choked, and the water must either find some other course, or lie stagnant on the surface; but if an opening occurs anywhere in the upper enclosing bed, through it will the Avater rush up with a force determined by the difference of level between the mouth of the opening and the surfoce where the rain first enters. Springs are often formed artificially by boring in this way thi-ough a bed of clay or rock, in order to tap a deep porous layer charged with water, tlie result of steady percolation from some distant surface. Sometimes the passage of the liquid is by fissures and crevices in the rocks—at other times by compact beds of shingle, sand, or gravel. The gravitating force carries it doAvnward to the lowest depths that it can reach, and again upward so as to find one level in every direction. Hence water falling on high groimds, and entering the soil, is sure to emei'ge somewhere on the low grounds, perhaps with a considerable pressure. If we could conceive a mountain formed of a porous top and impervious side, so as to retain the water while it sinks into the mass—and if this impervious layer were to cease at the foot, the water would pass down as in a siphon, and bm-st out vfith energy at the tei-mination of the layer in question. But this upper stratum might be conceived to extend for miles along the plain in unbroken contiimance, m •wliich case the mountain water would have no opportunity of sho-n-ing itself; till at last some interruption or cleft, or some excavation by the hand of man, occurs to relieve the pent-up waters; and out they flow with the high pressure still upon them, nobody being able to guess where they had their origin. The conditions necessary to the occurrence of sprhigs are therefore: —ls«. The rain falling on tlie higher grounds must find admission into the interior to some considerable depth; 2d, By the force of the pressure from above it must pass in lateral dhections—m other words, it must find pervious beds or openings right and left; 3d, It must become hemmed in above by some stratum that does not give an easy passage, and therefore concentrates the pressure on the places where open- ings occur; and ith. In the lower grounds where it has descended beneath an obstructing bed, there must be interruptions, fissures, or pervious strata, whereby it can rise to the surface again. iMountain districts, and a varied and irregular stratification of alternating pervious and impervious layers, are favourable to the concentration of water and its discharge m the fomi of springs; while, on the other hand, flat regions and uniform coverings of sand and gi-avel render springs impossible. In this last case there would be a uniform soakago of the soil, varied only by such shallow pools, lakes, and rivulets as the inequality of the surface could give rise to. A compact rocky formation, without fissures or other communication with the subter- ranean depths, would likewise be devoid of springs; the water prohibited from filtering its way. into the interior would pass down at once over the surface to seek the lowest level that it could find. The slow percolation through the interstices of a gravelly layer, or by