It is an advantage to prevent the percolation of water into soil that will not readily part with it, such as the clay earths, as it may be economically impossible to drain or restore the earth to its normal condition, and should the strata be upheaved, intermixed, and of a permeable and impermeable character, a scientific application of drainage can alone succeed. When the source of the water is ascertained, it can be seen whether a complete system of drainage is necessary over the whole of a cutting or only a portion of the slopes. Dampness and the egression of water may be merely local; if so, by boring a hole and inserting a drain into a slope to tap the flow it may be cured, the surface being made dry by a layer of ashes or other absorbent material. Pipes or tile drains may be sufficient when springs exist, or the flow of water is local, and in loose soils it is especially advisable to provide openings for cleaning any covered drains. Brickwork, masonry, concrete, pipes, or other rigid drains, may not be suitable for ground likely to unequally subside, as they will probably crack or leak, and loose-jointed pipes or over-lapping tile drains may be required. In treacherous clay soils surface longitudinal and transverse drains will most probably be insufficient, and deep draining of the mass be necessary, also the ground upon which earth is deposited will require to be drained and a layer of rubble stone placed upon it, a cutting or embankment being divided into small drainage areas by deep open dry stone trenches. Water may not sufficiently percolate into a hill, either because of its surface being covered with vegetation, or the soil being of an impervious nature. It may then flow down the slope of a cutting which will probably be bare and unprotected. To prevent slips the discharge must be carefully controlled and led away, particularly when the formation is drift soil upon rock, or the earth will be liable to saturation and degradation; also to prevent a flow of water under its seat, and upon the natural ground, should an embankment be deposited upon it. When a slope upon the hill-side of a cutting is of considerable length and steepness, it is advisable to bench it and divide it into a series of terraces and short slopes, and to provide catchwater drains, with impermeable surfaces so as to prevent any surface water attaining a high velocity, and scouring power. All surface water upon the side of a hill should be controlled, and catchwater drains may suffice to do this.