The selection of the best available earth is one requiring careful consideration, and it may be necessary to experiment to test the capability of the soil to be made watertight by compression or other comparatively inexpensive means. In some cases none may be obtainable; if so, the only course to pursue may be to consolidate the earth as much as possible, and protect it with an impermeable homogeneous and durable covering, and one that experience has shown can be trusted to equally resist percolation. The whole practice of stable earth dam construction is comprised in the employment of homogeneous, fine, and tenacious earth uniformly deposited in thin layers, gently watered sufficiently to aid ramming and consolidation, rolled, pressed, or trodden down by the passage of carts, men, or animals, and in its due surface protection. Heavy rolling is to be preferred, for it is more effective than ramming, as may be judged by the greater compression; the thickness of the layers and the weight being so regulated that it compresses the earth to its state of maximum solidity, and does not pulverize it, as the compression is more uniform, and irregularly compact masses are not created, which not only destroy uniformity of condition of the mass, but cause seams and veins and destroy homogeneity. After rolling, the compressed layer should be gently watered, as the weight of the roller will have made the top crust drier than the lower portion, unless it has been made into too moist a state. The thickness of the layers should not exceed, when the soil is to be heavy steam-rolled, about 6 inches in earth, and 4 inches in clay soils; they may be compressed to about four-fifths to two-thirds of their normal thickness, the degree of compression to ensure maximum solidity indicating the openness of the original earth. In sand, the layers when simply wetted and rammed with an ordinary rammer, should be about 4 to 5 inches in thickness, and ordinary earth about 2 inches. The volume of the sand will be reduced from 10 to 15 per cent. Simple ramming of sand will only reduce it about 6 to about 9 per cent., and water about 4 to 6 per cent., making the total compression as before stated, the quantity of water used being some 20 per cent. of the volume of the sand. Embankments of moderate height have been made of sand when no other earth was available, except clay loamy soil in comparatively small quantities, the surface of the embankment being worked by thin bars, and the loam being incorporated with the sand and made to fill its interstices, surface protection being thus afforded, and all the usual means to promote consolidation being adopted. Wet earth well mixed with a grout of quicklime has also been used to make a firm embankment of little height. Clay is not a good material to use unless it is incorporated with a considerable percentage of sand to prevent fissures and to lessen expansion. Many prefer the sand to be in the proportion of about two-thirds of the mass, only one-third being clay; the soil being then a loam, and cannot be classed as a clay.