CONSTRUCTION OF DIKES. 401 



regular enclosure is commenced. TJsuallj, however, the first 

 step is to build low and cheap embankments, extending from an 

 older dike or from high ground, around the parcel of flat in- 

 tended to be secured. These are called summer dikes. They 

 are erected when a sufficient extent of ground to repay the cost 

 has been elevated enough to be covered with coai-se vegetation fit 

 for pasturage. They serve both to secure the ground from over- 

 flow by the ordinary flood-tides of mild weather, and to retain 

 the slime deposited by very high water, which would otherwise 

 be partly carried off by the retreating ebb. The elevation of the 

 soil goes on slowly after this ; but when it has at last been suffi- 

 ciently enriched, and raised high enough to justify the necessary 

 outlay, permanent dikes are constructed by which the water is 

 excluded at all seasons. These embankments are constructed of 

 sand from the coast-dunes or from sand-banks, and of earth from 

 the mainland or from flats outside the dikes, bound and strength- 

 ened by fascines, and provided with sluices, which are generally 

 founded on piles and of very expensive construction, for drain- 

 age at low water. The outward slope of the sea-dikes is gentle, 

 experience having shown that this form is least exposed to injury 

 both from the waves and from floating ice, and the most modern 

 dikes are even more moderate in the inchnation of the seaward 

 scarp than the older ones.* The crown of the dike, however, for 

 the last three or four feet of its height, is much steeper, being 

 intended rather as a protection against the spray than against the 

 waves, and the inner slope is always comparatively abrupt. 



The height and thickness of dikes vary according to the ele- 

 vation of the ground they enclose, the rise of the tides, the dii-ec- 

 tion of the prevaihng winds, and other special causes of ex]iosure, 

 but it may be said that they are, in general, raised from fifteen 

 to twenty feet above ordinary high- water mark. The water- 

 slopes of river-dikes are protected by plantations of willows or 

 strong semi-aquatic shrubs or grasses, but as these will not grow 

 upon banks exposed to salt-water, sea-dikes must be faced with 

 stone, fascines, or some other revetement.\ Upon the coast of 



* The inclination varies from one foot rise in four of base to one foot in 

 fourteen. — Kohl, iii., p. 210. 



f The dikes are sometimes founded upon piles, and sometimes protected by 

 one or more rows of piles driven deeply down into the bed of the sea in front 



