but it will be advisable to consider the sources of pollution of such well-water somewhat more in detail.

Where the level of the underground water is but a few feet from the surface, it is obvious that the surfacewater, which may contain impurities, has but little chance of being purified by oxidation in its passage through the soil to the well. But the grosser pollutions that shallow well-waters suffer from, come, not from this source, but from leaking drains and cesspools in the vicinity.

Cesspools are but rarely made watertight, as they would then require to be frequently emptied. When sunk in a porous soil and merely lined with bricks without mortar or cement, the liquids soak away, and the solids-small in volume-so gradually accumulate that the cesspool can be closed over and need not be opened for years. The liquid sewage percolates through the soil and joins the undergound water below. As the underground water is-as before explained-slowly but steadily moving along in the direction of its natural outlet, the position of the well, in regard to the cesspool, is all important. Should the well be above the cesspool, the underground water flowing from the well to the cesspool, the risk of pollution is greatly diminished, so long as but little water is drawn from the well. If the well is below the cesspool, it must infallibly be polluted with the cesspool soakage. The direction of flow of the underground water can be usually determined from the contour of the surrounding country; and this evidence can be confirmed by observations on the height of the underground water at different places, as determined by the height of the water (above sea level or ordnance datum) in different wells, for the level of the underground water falls as it approaches its outlet,

giving rise to a curve which has been already considered (see p. 21).

When, however, the amount of water abstracted is sufficiently great to cause a considerable depression of the water in the well, the conditions are altered; for the well then drains an area all around it in the form of a circle, that is to say, the water in the well is renewed not only from above-as regards the flow of underground water-but from below; and in such a case it would not matter what position the well had to the cesspool, if the cesspool was included in the area drained by the well, for pollution must inevitably occur. The distance within which a well draws water to itself, when its own water level has been depressed by pumping, depends on the amount of the depression and on the nature of the soil.

This distance the radius of the circle drained by the well-is best expressed in terms of the depression. In fine sands and gravels, which offer considerable resistance to the passage of water, the distance varies from 15 to 39 times the depression. In the chalk, where fissures facilitate the passage of water, the distance is 57 times the depression. In very coarse gravel, which allows free passage of water, the distance is from 68 to 160 times the depression; and in the new red sandstone, where extensive fissures exist, the distance is 143 times the depression. These results are founded on observations made, chiefly abroad, by sinking borings at different distances around a well. They are very instructive, but require confirmation by extended observation.❤

The surface of the underground water in the area of • See Article on "Water," in Our Homes, by Rogers Field and J. W. Peggs.

the circle drained by a well depressed by pumping, has the form of a curve, analogous to the natural curve of the underground water, with steep vertical gradient near the well, but rapidly becoming more nearly horizontal as the distance from the well increases (fig. 2).

We have thus seen, that the conditions which determine the freedom or otherwise of a shallow well from cesspool or sewage pollution are:-(1) Its position as to cesspools or other sources of pollution, with regard to the flow of underground water; (2) the amount of depression of water level in the well which may be produced at any time by pumping; (3) the nature of the soil in which the well is sunk, as regards porosity and the easy passage of water. It is quite possible, if these conditions are attended to, to sink a well that shall be uncontaminable in or near a village, in which the shallow wells generally are horribly polluted with cesspool soakage.

The well must be sunk in such a position as regards possible sources of pollution, that the underground water flows from the well to the sources of pollution. The distance of the well from such possible polluting sources should be from 100 to 160 times the depression of the water in the well that is ever likely to be produced by pumping, this distance varying with the nature of the soil. The mouth of the well should be closed over and the water raised by an iron pump; draw wells where the water is raised by a windlass, chain, and bucket through an open mouth are liable to accidental contamination from refuse being thrown in, or animals falling in. To prevent contamination from impure surface washings, the mouth of the well should be protected by a coping, and the drainage water from the pump conducted away to a safe distance.

If the porous stratum in which the well is sunk is of

FIG. 2.-Depression of water in shallow-well by pumping. A. Well. B. Cesspool. C. Underground water curve (after Field and Peggs).

considerable thickness, the underground water being 30

feet or more from the surface, the sides of the well for this depth should be imperviously steined with brickwork set in and lined with hydraulic cement. If this is done, water percolating from the surface must pass through at least 30 feet of soil before entering the well. In its passage through the soil, the organic impurities in the water will be, to a certain extent, removed by oxidation.

It is a curious circumstance in regard to the grossly polluted waters of many shallow wells, that they are as a rule clear, sparkling, and very palatable. The organic filth from cesspools and drains, in its passage through even a few feet of porous soil, is filtered and deprived of suspended matters, but without losing its dangerous properties. The shallow well into which the filth percolates is found to furnish a water loaded with ammonia and chlorides-evidences of sewage (urine) contamination-with organic matters in solution, and with nitrates and nitrites, the oxidised residues of ammonia and organic matters, but yet, from its containing abundance of carbonic acid gas, sparkling and palatable. If such a water, however, is put in a bottle and kept in a warm place, it very soon becomes turbid, then putrid, and is found to swarm with bacterial life. Such wells too, after a heavy rainfall, are very liable to furnish a turbid and foul smelling water which nobody. would think of drinking. The heavy rain washes foul substances in the soil, derived by soakage from manure heaps, middens, privies, or cesspools, straight into the well, no time being allowed for that filtering and partial purification by oxidation which does so much to give the well-water at ordinary times its pure but deceptive appearance.

Polluted shallow well-waters are usually excessively