As drift soil upon the inclined surface of a rock may be held by friction, the least change may impair it sufficiently to cause movement, therefore it is advisable to have a narrow gullet or heading excavated in short lengths, followed _pari passu_ with protective works, whether a retaining wall or not, so that the earth is weakened as little as possible by exposure and deprivation of its usual support; and there are many places in which, unless the system of excavating in short lengths be adopted, failure and slips must result. In sidelong ground the formation is frequently partly in cutting and partly in embankment, if so, and the drain is impermeable and can be thoroughly relied upon, it may be advisable to make the formation incline towards the hill, so as to drain it, lessen, and perhaps prevent percolation into the semi-embankment or under its seat, for the soil being loosened by the process of excavation and deposition is in the mass more porous than in a cutting. The chief aim of all drainage operations for the preservation of earthworks in sidelong ground is to prevent the hill waters guidelessly flowing into, upon, or under them, their accumulation, or any obstruction to their easy discharge; and to attain this object it is necessary to know the location of the sources of the supply, the quantity, and other particulars, _vide_ Chap. IV. A proper system of drains must therefore be made upon the hill-side to gather and gently guide the waters, to lead them by channels across the formation and discharge them outside it, care being taken to protect and line the surface of the drains so that no leakage occurs or any erosion of the face. In countries where the rainfall is excessive or sudden at certain places or entirely, it may be sufficient to protect the slopes so as to allow of a flow over them, and particularly upon the valley slope of an embankment, and in a lesser degree upon the hill-slope of a cutting.