following. If the reverse order were followed, the bed would buckle and break up, would not make a good joint with the soil, and would give rise to leakage through the dam, which would be difficult to remedy. The dam is built up in this way, layer by layer, each successive layer narrower than the one below it, until the top is above low-water level. The rest of the dam is then built by hand, and the outside is covered with stone.

To cover the submerged slope of an existing dam or an eroded bank, the bed is moored alongside, and the side next to the bank is loaded first till it touches the ground; the loading is then continued outwards until the whole bed is sunk and lies flat against the soil, Fig. 4. For this work small riprap is used, of sizes such that it can be easily thrown by hand. Piles are then driven where required,

and between these are laid larger stones so as to form as smooth a surface as circumstances will allow.

The construction of a groyne on a steep submerged slope is carried out in a similar manner, beds being laid in successive layers until the required length is reached, when the end of the groyne is surrounded by an apron of smaller beds, to defend the body of the work against undermining by the current; such groynes are chiefly used in estuaries. On the sandy coast of the North Sea, where the range of the tides is seldom more than 6 feet and the beach has a slope of about 1 in 40, the groynes are differently constructed. The outer ends of these groynes do not go farther out than about 100 feet beyond the low-water mark of spring-tides, in a depth of 2 or 3 feet of water. Over this distance a bed of two tiers of fascines is sunk in the usual manner. Above this, on the dry, the bed is constructed in situ, and the whole is covered with riprap overlaid with large blocks carefully

packed with stone shards rammed into the interstices. The middle lies about a foot higher than the sides. Round the edges, round oak piles are driven to keep the whole together, and these again are supported by heavy stones resting on the sand, if it is sufficiently compact, or on a layer of fascines. Along the centre-line is driven an open row of piles 6 to 8 inches in diameter and 1 foot or 1 foot 6 inches apart. These groynes (Figs. 5) are 18 to 24 feet in width at the outer end, narrowing as they approach the high-water line. Their longitudinal slope is parallel to the average slope of the beach. Their average length is about 1,000 feet, and they are laid at distances apart equal to about one-and-a-half times their length. The object of these groynes is not so much to deflect the littoral currents as to defend the beach against scour, which would scoop out deep channels close to the toe of the sand-dunes. Clay, loam or sods are not generally used, as they could not be brought to the work at a reasonable cost, and impermeability is not required. Fascines and other arrangements of soft woods soon decay, but by the time this occurs they are generally replaced by sand and stones firmly bound together by shells and seaweed. The oak piles last longer, but must eventually be strengthened unless the work is surrounded by sand to a sufficient depth to protect them, which, however, is never the case at the outer ends.

Where sea-dams are exposed to a high sea, the waves rush up the slope to heights above the level of the highest crests. The top of the dam is therefore laid some feet above this level. The destructive force of the waves practically ceases at a level slightly above the mean level of the sea, but unless the slope be protected above this level, the rush of water soon causes considerable damage, especially if the slope be sandy, in which case the wind also plays an important part. Very heavy or costly work is not required for this part of the slope, and a straw matting is sufficient. A layer of straw, sedge, rushes or reeds is first laid, to a thickness of about 2 inches; this is held down by wisps of tough rye-straw, 2 feet in length, 2 inches in width, and inch in thickness, both ends of which are planted down into the soil, in holes 4 inches apart, and at rightangles to the direction in which the lower layer of the covering is laid, the ends of two contiguous wisps being inserted in one hole. In this way a matting is formed of sufficient size to keep the flow of water from the soil and from the slope of the dam. To keep the stone pitching in position piles are driven, in some places, in great numbers. Diversity of opinion exists as to whether the tops of these piles should be flush with the surface of the stone-pitching or should project above it. If they project

above it, it is held, on the one hand, that the sea, continually striking against them, causes them to shake the stone covering and loosen the joints; and, on the other hand, that the piles break the force of the surf and prevent the sea rushing up the slope.

The very important sea-dam at Westkappel, in Walcheren, is exposed to the full force of south-west gales, which sweep over the deep channel of the Wielingen and raise waves 12 feet to 16 feet in height. The body of the dam, and also the beach and sea-bottom, are of fine sea-sand. The dam has a heavy stone covering, and long low groynes in front, the whole being further strengthened by numerous piles, braced together at the tops. The stone is laid up to high-water level, or a foot or two above it; and the upper end of the slope is covered by a straw matting, which of course requires constant repair. The maintenance of the sea-dams in Walcheren and the adjoining islands is a heavy burden; indeed, it may be said that as much labour is expended on maintenance of the dikes as on cultivation of the land. Only very fertile land can support this continual outlay, and in many cases not only does all the rent paid by the farmers go to maintain the dams, but the province and the country at large have to come to the rescue. Such districts are called "calamiteuse polders," and they certainly are a calamity to the landowner, but someone must benefit by them or they would be given back to the sea.

It is of course essential, in order to obtain the best results, that not only should the workman know his trade, but the materials employed must be well suited to the purpose. The choice of the kind of wood to be used in fascines or hurdles is not unimportant; all sorts of willow are suitable, but for the fascine-bands tough willow or osier shoots are indispensable. Willow has the additional advantage that, in places where salt water cannot reach, it sprouts again and takes root, in this way binding the soil and rendering the whole work more permanent. Other woods which may be used are hazel, elm, ash and oak, if the boughs be straight, sound and tough, and 10 to 12 feet in length, including the green tops. Young shoots of these trees, of 3 to 4 years' growth, are excellent, and are largely used. In some localities the branches of some conifera are occasionally employed for spread matting, but these are usually too brittle if they have been stored for any length of time. Oak is preferred for the larger pegs, stakes and piles, and is used round, being only barked. Piles of 10 inches to 14 inches square are mostly of Norwegian yellow pine, in some cases creosoted; but Demerara greenheart, although nearly imperishable in sea-water and not liable to attack by the Teredo Navalis is too costly for work in which huge quantities

of timber are required, and is used only in exceptional cases, and then often not for the sea-defence work proper. The reeds, rushes and straw used should be preferably of the tougher varieties, and must be brought on to the work in clean untangled sheaves, free from weeds, and still fresh and pliant; if dead and dry they are too brittle for handling. The best brick riprap is the clinkered refuse of the kilns, in pieces not smaller than half-a-brick, so as to be easily hand-packed into the spaces between the hurdles. For this purpose natural stone broken to the same sizes may also be used, if heavy and hard. Heavy stone covering should be preferably of one kind of stone, the heavier the better, hard and not porous, so that it may not be disintegrated by frost. Basalt or trap-rock, in columns or boulders, compact limestone, granite, gneiss, greywacke and greenstone are preferable to rocks of less specific gravity. Blocks weighing 6 to 10 cwt., if properly laid and well rammed in, are seldom displaced, even by heavy seas; it is only where the joints between the stones are open, but stopped up at their inner ends, that the sea is able to exert its full lifting-power, the blows dealt by the waves causing a water-hammer action in the joint and lifting the block off its bearing. Soft limestone, conglomerates or sandstone should be avoided, being too friable and unable to withstand atmospheric influences, particularly frost. Blocks of well-baked clay, 1 foot square and 8 inches in thickness, laid on flat layers of bricks, afford a good protection against a moderate sea. Blocks of concrete or slag also, where readily procurable, may be used with success. Grouting is to be avoided, as it makes the pitching too stiff and unyielding. The covering should be sufficiently yielding to drop down into any hollow or depression that may be formed under the stones, as otherwise there is danger that the sea may, unobserved, scoop out a large cavern, which may give rise to considerable damage.

The choice of materials is governed to a large extent by their cost. A fasciné-bed of three tiers, with the hurdles on the top, costs about 28. Od. per cubic yard, or 8d. to 10d. per square yard of surface. Even with a well-arranged scheme and excellent materials, experienced craftsmen are necessary. In the Netherlands, the inhabitants have been trained for generations in this trade. In other countries, with different climates and different materials, the Dutch rules and customs cannot always be observed; the quality of the labour available is often inferior, but the principles underlying the Dutch methods are followed wherever the circumstances are similar. The United States engineers used rafts to support the riprap in the construction of the jetties for the port of Charleston, in South Carolina.