With respect to the deteriorating influence of vibration as regards the slope of a cutting or embankment, it is well established that vibration will cause movement in a retaining wall which would otherwise be stable, and that soils possessing considerable powers of cohesion are not so easily affected by vibration as those of a looser character consisting of particles having more or less rounded surfaces; but the action of water may produce seams in such earths as clay, and create a smooth greasy sliding surface upon which any reposing mass may but require the least disturbing force, on the principle that the least impact is sufficient to impart motion to the largest body; such as a man walking upon it or the tread of an animal, to unbalance the delicate state of equilibrium. This action may be gradual, continuous, and increasing, as the earth will always be subject to changes of weather. On the contrary, vibration in soils having particles insoluble in water, provided water does not dissolve any cementing material between them, may cause them to equally settle and become firmer by being pressed together than if they were not subject to such operation, and should the particles wedge by shaking and the slopes be sufficiently flat, vibratory motion may, under these circumstances, tend to consolidate certain earths in an embankment; but not so in the slopes of a cutting which vibration must disturb by reason of it agitating and loosening the surface and making it less dense. For instance, in sandy soils the surface friction on a cylinder, when sinking operations are not being prosecuted and when the material is being raised from the interior, is different; the latter resistance being from 20 to 25 per cent. less than the former, consequent upon the disturbance, and although fine, soft drift sand usually presents greater frictional resistance than firm sand, it obviously cannot be taken as equal to that of firm sand, as it is quickly dissipated. The conditions of earth being so very diverse no rules can be deduced, for even the effects of such stupendous force as that of earthquake vibrations vary according to the nature of the ground, however, the weakening effects of vibration are undoubtedly very considerable. It is known that the lateral thrust of earth is thereby much augmented, and, therefore, that the strain upon the frictional resistance and the cohesion of the earth is increased; and experiments have shown that when a wall is nearly strained to the point of overturning, slight vibration will quickly destroy the equilibrium, thus demonstrating that it adds to the lateral pressure. An analysis of some reliable experiments proves this increase to usually range from 10 to 60 per cent., but it is evident that the practical effect of any increase may be very much greater than a mere computation of the increment, for it may supply the additional strain, however small, necessary to initiate a movement, hence the danger.