2. Erosion of the land slope or backing of a retaining wall by waves, or spray falling thereon and necessarily passing over the top of an embankment. 3. From abrasion and damage, caused by vessels or barges rubbing or colliding against a slope. 4. From erosion caused by wave action produced by the passage of boats or ships propelled by machinery; or by wind waves. 5. Variation in the water level, causing unequal pressure. 6. In the case of a reclamation embankment, it being closed from the ends, and not by raising in layers, from the ground level. 7. Variation of the submerged area, and consequent change in the degree of exposure to deteriorating influences. * * * * * To sum up, the principal causes of slips in earthwork may be stated to be air, water, frost and thaw, over-pressure, and vibration; the chief agent both in cuttings and embankments being water, which forces forward the surface of the slopes and destroys the cohesion of the soil, and impairs its frictional resistance until the earth is unable to sustain the weight upon it; vibration aiding and completing the movement, as it not only tends to loosen the soil, but may disturb the equilibrium of earthwork which is almost moving, and only requires a slight shock to set it in motion: in fact, vibration is frequently the complementary agency that causes a slip, and is obviously felt most in loose soils; but if there should be fissures in earth of a tenacious character, or boulders in clay to disconnect it, the effect of vibration will be more serious in the latter case, as whole masses of earth may become detached, instead of an equal settlement proceeding as with soils, such as sand and gravel, which may become consolidated by shaking, owing to wedging of the particles, should the slopes be sufficiently flat to prevent lateral movement; but it may detach portions of the slopes in soil having little or no cohesion, and thus initiate a slip. In the following chapters the chief causes of slips in cuttings and embankments are considered, together with others bearing upon a solution of the subject, which is so interwoven that it is impracticable to preserve a successive order, but an endeavour has been made to separately indicate the cause and some remedies that may be adopted: but before proceeding to particularise, it may be well to name a few dominant principles of the alignment of public works, which if duly regarded may tend to prevent slips of serious importance.