Engineering Wonders of the World
Volume III

IN a previous article (Vol. I., p. 370 foil.) has been described the extremely ar- duous work of the lighthouse engineer and the nature of the terrific destructive forces with which he has to contend. Another branch of marine engineering, that of harbour construction, is beset with the same diffi- culties, though possibly in not so aggravated a form, as harbour works are not so isolated as the rocks on which lighthouses have to be raised. We are concerned here primarily with works carried out to oppose the violence of the waves, and to render safe for shipping areas of water which, but for some Breakwaters. . t . such protection, would be utterly unsuitable for anchorage in bad weather. The breakwater is a mere barrier, either reducing the size of a wave or checking its progress altogether. Its shape and char- acter depend partly on the conditions of the site, partly on the work for which it is de- signed. It may be either an artificial bank of rubble with long slopes paved on the top ; or a rubble mound brought up to within a few feet of low-water level at spring tides, and capped with, a built pier; or a more or less vertical wall based upon the sea bottom. The breakwaters of Plymouth, Portland, and Dover Harbours respectively are good ex- amples of the three types. We may add that different forms of construction are found in some cases in one breakwater at different depths of water. Thus, what begins at th© (1,408) Waves: their Motion and Force. shore end as a wall built on the bottom may be given a footing of rubble, the height of which increases with the declination of the ground, as it progresses seawards. Before going further into our subject, a few words on the nature of waves will be of value. There are two main orders of waves : (1) waves of translation, in which the bulk of water moves bodily in the direction of the wave, as when a wave breaks on the beach ; (2) waves of oscillation, in which the particles move vertically as well as horizon- tally, the motion being that of a mass rolling along a surface. Towards the top of the wave the particles move in the direction of the wave ; in the trough, in the opposite direction. The motion is greater at the crest and in the trough ; least at half height of the wave. The destructive power of a “ roller ” is proportionate to its height. A wave thirty feet high may produce a pressure of one ton on every square foot of a surface opposing it squarely. Even much, higher pressures have been recorded—nearly three tons per square foot at Skerry vore Lighthouse, and three and a half tons at Dunbar. On entering shallow water a roller becomes a wave of translation, and hurls itself horizon- tally against any obstacle. To rob a wave of its onward movement, two methods, used singly or in combination, are employed. The first is to offer a long incline to the wave, up which it must rush, 5 VOL. ill.