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well. If over-pumping has taken place, the pumps have to be lowered; the cone increases in vertical height, and a larger concentric circle is added to the central one.
In porous rocks of great thickness the plane of saturation is often at a considerable distance from the surface, the annual rainfall absorbed being balanced by the springs run off at low level, and the floods passing across the outcrop too quickly to sink into the strata. In these cases it would be possible to raise the height of the saturation-level, and thus increase the storage powers of the rocks by sinking “dumb-wells” in the porous strata, and draining the stormwater channels into them, which would have the effect of increasing the summer discharge of the springs.
From an investigation I made in 1878, as to the area of each of the various rocks in each river basin, which forms the basis of the paper I laid before the Society's Water Congress, convened by his Royal Highness the President, I estimate the area in England and Wales, occupied by porous rocks, to be not less than 26,633 square miles; while the tertiary, gault, weald, oolitic, liassic clays, the triassic and permian marls, and the shales of the carboniferous occupy a further 19,308 miles, in nearly the whole of which occur pervious rocks, a larger portion of which might be rendered available for storage purposes by sinking dumb-wells through the overlying strata, and discharging the land drains into them, which would prevent the water passing away in devastating floods.
Experiments were made simultaneously on the percolation of rainfall by Dr. Dalton, at Manchester, and by M. Maurice, at Geneva. Dalton's gauge consisted of a cylinder, 10 inches in diameter, and 3 feet deep, open at the top, closed at the bottom, and filled with earth, and sunk to the ground level ; his experiments were carried on from 1796 to 1798, 25 per cent of the rainfall being absorbed. M. Maurice's observations, from 1796 to 1797, gave 39 per cent. of percolation. Mr. Dickenson's, from 1836 to 1843, gave 42 per cent. Mr. Greaves' observations, extending from 1852 to 1873, with an average rainfall of 25.8 inches, gave the percolation of 26-6 per cent., his gauge being a slate box with an area of one square yard, a yard in depth filled with soft earth, loam and gravel mixed, trodden in, and turfed over. Mr. Greaves states, “the gauge stands at Lea Bridge, 1} miles west of, and 6 miles north from the meridian of Greenwich.” He finds the abundance of water in a river to be more closely dependent on percolation than on mere rainfall; for consecutive months there is no percolation whatever ; five times there was no percolation for six months; and only in one year (1860) was there percolation every month. The greatest percolation is after thaws of snow, especially after frequent thaws of small falls of snow. A wet winter gives abundant springs in the following autumn ; but if that be followed by a dry winter, it will obliterate the effect of the previous wet winter ; this is the case in the present season, 1884, when the springs are remarkably low, as has been pointed out in the Times of June 7, by Mr. Baldwin Latham.
Mr. Greaves's experiments show the smallness of percotation through earth on the whole, and its entire absence during warm summer weather; and they also show the small thickness of earth under which water may be safe from evaporation, which he places at a depth of 36 inches; and even at 24 inches he considers it doubtful " whether, in the latitude and temperature of London, capillarity has more than a negative action beyond 12 inches in depth." Probably, in a moderately open soil, capillarity extends only a few inches, but the higher capillary power of clay soil causes a constant summer exhalation. From these facts it is evident that the more thoroughly a soil is underdrained, the nearer it resembles the “percolation gauge,” and the less likely is water to pass off as flood in an open watercourse. The more free drainage is promoted, and the more "dumb wells," or " inlet drains," are constructed, the greater will be the quantity of water stored; in other words, the percolation period, ending naturally in February or March, will be artificially extended so as to catch a large proportion of the summer rains; and “intermittent springs," which are dependent not on surface-present rain, will be increased in volume, and to some extent rendered more permanent, at times when the heavy rains of summer, through excessive evaporation, are adding nothing by percolation to the underground stores.
The investigation of individual observers, and the ten Reports of the Underground Water Committee of the British Association, which I have drawn up, as Secretary of the Committee, have made well known the large quantities of water now pumped from the millstone grit, the permian sandstone and triassic sandstones, the polites, the greensands, and the chalk ; towns like Liverpool, Birmingham, Birkenhead, and Nottingham, receiving from the triassic sandstones quantities reaching, in the case of the city of Liverpool, 6 million gallons per day; while the analyses of the Royal Rivers Pollution Commission, show us the pure quality and great value of these waters from a sanitary point of view.
In England and Wales, the pervious portions of the carboniferous, the secondary, and tertiary rocks, occupy an area of not less than 26,600 square miles, while an additional area of 19,000 square miles exists of rocks of this age, which, though impermeable and carrying off the rainfall in floods, yet overlie pervious strata that might, to a great extent, be made available for storage purposes, were dumb-wells carried into them from the surface; such dumb-wells would artificially act as do the swallow holes which feed the chalk and carboniferous limestone waters.
In the chalk, though water is absorbed with great rapidity, and retains a quantity, according to the experiments of Professors Ansted and Miller, equal to one-third of its bulk, it parts with it with excessive slowness; and the water available to feed springs, and to supply wellborings, is mainly due to free water passing down the cracks and fissures which traverse the chalk surface in all directions, the larger fissures allowing the passage of rain water down to the lower portions of the chalk. That the deep-seated impermeable beds occurring at the base of the chalk formed the surface on which the passage of water chiefly took place, was pointed out by Professor Prestwich, in 1851 ; and the gradient on which water stands in the chalk was described in the Sittingbourne district by Mr. W. Bland, in 1852, which gave an inclination of 47 feet per mile of fall in one direction, and 45 feet per mile in the other; in Hertfordshire, the average inclination between Dunstable and Watford was found by the Rev. J. Clutterbuck, in observations extending from 1842 to 1850, to be only 14 feet per mile. The later observations of Mr. Baldwin Latham on this subject show the remarkable increase of gradient, produced by the temporary rise in the water level of the hilly districts of the chalk, after heavy and continued rains, which becomes gradually depressed after their cessation, before the springs in the low ground have ceased to give their maximum yield.
Over the whole of the area of 26,600 square miles, drawing wells may be sunk with advantage, and their supply increased by sinking inlet or drainage wells to carry into the strata water now almost entirely lost, and rendered not only unproductive of good, but the agent of actual harm, in the destruction of property and agricultural produce by floods. Care will have to be taken that, in constructing such dumb-wells, communication is not set up between drains carrying objectionable matter and the underground sheet of water, and that in draining wells and bore-holes the point at which the water is abstracted is sufficiently removed from the surface to insure the water having naturally filtered through the superincumbent strata.
In the case of the 19,000 square miles of impermeable strata overlying pervious strata, for the most part already containing water absorbed, in the area of outcrop of the pervious, Artesian borings will give large supplies, in numerous districts where no attempt in this direction has been made ; and where the Artesian gradient, or water level, is not as high as the base of the impermeable stratas overlying the permeable bed, dumb-wells may also be sunk with advantage, and in relieving the clay beds of their
floods in winter, will help to increase the summer discharge of the rivers, and render the intermittent springs of more permanent value.
WATER FROM THE CHALK.
By JOSEPH LUCAS. At the first Congress on National Water Supply, convened by the Society of Arts in 1878, I had the honour to read a paper indicating an opinion “that the real stumbling block to the practical solution of the water question of the country in general is not, as has been alleged, its cost, but the general absence of data respecting its sources,” and advocating a survey of the water-bearing strata on principles described in the paper, and which I now reiterate.
The proposition met with the support of water engineers, and was endorsed in a leading article in the Times, summing up the results of the Congress.
At the next meeting in 1879, at which the Council of the Society of Arts did me the honour to confer upon me their silver medal for a paper on “ National Water Supply,” in a second paper, entitled “Watershed Lines," I gave the measurements in square miles of the apparent and real areas of chalk country lying within the basins of thirteen rivers between and inclusive of the Medway and the Meon, in South Hants, as determined by my hydrogeological surveys.
In the following year, the Institution of Civil Engineers, who had previously published a map and paper giving the results of a somewhat crude survey of the chalk water system (1872-6), did me the honour to publish a map and paper descriptive of the “ Lower Greensand Water System of Surrey and Hants." For this survey the Council of that institution awarded a Telford medal and premium. In its application to the subject of London water supply, the hydrogeology of the question has frequently been called