Deep water is generally required close to the work in railway piers or jetties to enable vessels to get alongside; a heavy and monolithic wall must consequently be erected; however, in the case of loose soil, the vertical system simply should not be used unless the foundation is thoroughly protected and below the reach or effect of wave action, and no re-entering or right angles should exist, as they increase the action of the waves. When a rubble mound is cast in and a vertical structure placed thereon, great care must be taken that there are no holes, except the natural interstices between the stones, and that they have a firm foundation and sink equally, or the random mound may give way and the superstructure will then necessarily follow. [Illustration: DATA.—Sand embankment: Width of formation, 30 ft.; inclination of the slopes, 3 to 1; height of embankment from the surface of the ground, 30 ft.; depth of water, 20 ft.; height of wave, 5 ft.] As an illustration of the deleterious effects of the recoil of waves may be mentioned that a high vertical wall with a parapet has been found to endanger the toe, but when the parapet has been removed in order to allow the head of the waves to leap over the top of the work, the structure remained stable. To prevent the recoil of the sea and the scooping away of the base and the foreshore in much exposed situations, and consequent slips and subsidences, breakwaters which simply act as wave screens, and not as wind screens, are sometimes kept a little below high-water mark so that the heads of the waves may have free action, although their onward motion is prevented. On the contrary, light open work, such as a lattice screen, although it may somewhat lessen wave action, does not prevent it passing through. The principal causes of the failure of vertical walls when placed upon an easily eroded foundation are by the scouring action of the recoiling waves, therefore their magnitude should be reduced to the lowest limit: by the impounded air driving out particles of the structure: by waves travelling upon the top, therefore their forward motion should be deflected and rendered vertical: by the hammering action of the mass caused by its being alternately quickly submerged and unsubmerged; the practical effect being that the foundation is intermittingly released of a portion of the load and then fully strained, therefore the height of the waves should be reduced as much as possible. Respecting the action of falling water, experiments were recently made in India, which proved that “the greatest intensity of pressure does not exceed that due to a column of water of a height equal to the fall;” the greatest intensity of pressure being always fractionally under the hydrostatic head. As an illustration of the constant change of the load upon the foundations of an embankment in tidal waters, the following calculations have been made. The weight of a cubic foot of sea water is taken as 0·028 ton.