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. There is much diversity of opinion as to the relative value of a central puddle wall or protection of the slope and toe. Such a wall is placed in the centre, not only to give support to an earth embankment and prevent any through leakage, but also to keep the puddle in a uniformly moist condition, and therefore to prevent it fissuring and cracking from exposure to the sun or air. In an embankment erected upon an earth foundation it may be advantageous and warrant the expense, but when the embankment of earth is placed upon rock it should not be erected, for it is impossible to make a watertight joint between rock and clay puddle, and should it be placed in a rock trench leakage will not be prevented by it, and water will accumulate when it does not at once pass through the seat of an embankment until the surface of its base becomes softened, and the whole mass almost worthless either as a means of support or of prevention of through percolation; and, moreover, there is the danger that the clay will draw away from the sides of the rock. What is wanted is, as it were, to insert as far as the top of an embankment an artificial rock having a thorough connection with the foundation, consequently a Portland cement concrete wall should be adopted in such a case and not one consisting of clay puddle. Two great advantages of a Portland cement concrete central support over that of a clay puddle wall are that it does not settle and the weight is evenly distributed, whereas a puddle wall in subsiding may draw away from the earth of the embankment and cause cavities and cracks and consequent leakage. The chief consideration is to produce an embankment having impermeability, solidity, and homogeneity, and a thorough connection at the foundations; and as the central puddle wall system rarely does so, and usually only under circumstances which would probably hardly warrant the expense of its adoption, it is in a state of obsolescence; and should central wall support be necessary, which may not infrequently be the case, Portland cement concrete is being adopted in its place, a solid and durable material being used instead of one varying in shape and size, according to its state of dryness or wetness, liable to fissure, and incapable of being permanently and firmly joined to any material. This system may be described as one of arresting the percolation of water in the interior of an embankment; but the great aim is to prevent any percolation into the mass, as when once a passage is made the water will escape along the line of least resistance, and a fissure become a cavity, a cavity a breach.