With regard to the protection of the toe or lower portion, of a slope by means of a retaining wall, it may be the only effectual support in one case, and not succeed in an exposed country without a complete system of open or closed drains, and then its adoption may be superfluous. In the following few paragraphs an endeavour is made to indicate some situations in which retaining walls have or have not been completely successful when erected for such purpose. First, it is most important that ample provision be made for draining the back of the walls, for if the egression of the surface waters be obstructed, they must accumulate and cause hydrostatic pressure, soften the lower portion of the earth, thereby failing to partly effect one of the objects of their erection, and cease to protect the surface; the probable result being that the wall is pushed forward, broken up, or overturned. The drainage must be regulated by the quantity and velocity of the flow, and ample weep-holes should be provided to prevent an accumulation of water at the back. In damp soil there should be one to about every three or four superficial yards, and an outlet at each wet place, or a wall may not stand. The wall should be backed with a filtering medium such as coarse gravel, hard ashes, ballast, and no retentive earth should be used. In clay earths a retaining wall should always have a dry porous backing, as it not only reduces any pressure due to a head of water, but also allows of the earth swelling without affecting the stability of the wall, as it probably would if the clay rested against the back of the wall. Retaining or breast walls are particularly useful in loose soil having no cohesion, _i.e._, those of a sandy character or consisting of very small grains, and which upon saturation by water or by the action of its flow become in an unstable state or one of actual movement; in such, a case not only is it requisite to protect the surface of a slope, but support at the toe is indispensable. In cohesive soil, such as clay or clay marls, surface protection, combined with systematic and thorough drainage, may be all that is required to make a slope stable, and the erection of an impervious or solid high wall be unnecessary, always provided the earth cannot slide from being superimposed upon an inclined stratum. In fact, retaining walls in retentive soil have been found to induce a slip, because they neither drain the earth nor prevent the additional impregnation of water, and they have consequently been destroyed.