In a drained district, water will not usually be encountered in large quantities until a depth is reached below the level of general drainage, _i.e._, about the invert of the nearest drains or sewers, especially in porous soils, but in the case of a pervious subsoil, such as sand or gravel, or if the tides rise in an adjacent river or the sea to the level of any foundations or above it, more water may percolate over the site or drainage area than has time to flow away between tides, the water will then rise, and systematic pumping becomes a necessity, unless the volume of the ingressing waters can be sufficiently reduced by the deposition of an impervious layer upon or in the river-bed or sea-shore, or by sheet-piling or other means, or the flow confined, which may be a risky operation in loose soil, depending upon its resistance to scour. In clay soils, so far as slips are concerned, the action especially to be feared is the trickling of water down fissures which may extend to depths below the bottom in cuttings and create slimy surfaces, disconnecting masses of earth, and finally offering a ready means of movement, which swelling of the clay or vibration may complete. The experiments of Mr. Baldwin Latham, M. Inst. C.E., on the absorption and retention of water by clay soils, gave the following results:—The stiffest clays retained the greatest quantity of water. Clay soils can absorb and retain from 40 to 60 per cent. of water by weight. Marly clays hold less water than the pure clays. In the case of loamy soils, the percentage of water retained varied from 35 to 60 per cent. by weight, the mixture of sand and clay, therefore, limited the amount of water which it would naturally hold. As in chalk soils fissures occur, the percolation of water and the effects of the atmosphere through the pores causes movement, and even crevices and breaks in rocks are not to be disregarded with impunity, as they are channels of disintegration. Most earths when dry attract water, but if they are regularly irrigated, they require less moisture, depending upon the nature of the earth, and slope and relative level of the land. All soils when broken, as in embankments, absorb more water than when in an unbroken state, as in cuttings; for instance, it has been found by experiment that clayey and retentive earths will absorb about 7 per cent. more water, and light porous soils about 6 per centum. Of course, the increment varies. Even wet retentive soil, if handled, becomes considerably less impervious to moisture. Mr. Evans, F.R.S., has proved by experiment that percolation through pure chalk is much greater than through ordinary top soil consisting of gravel, loam, and mould, both being covered with turf, and that in winter the average proportions of percolation are as about 1 for soil to 1·5 for chalk; in summer 1 for soil to 2·6 for chalk. The depth also to which chalk will allow a passage of water is some 60 per cent. more than ordinary top soil.