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DIKES DRAINAGE] hard crust. Many of the lakes are nothing more than deep pits or marshes from which the peat has been extracted.

Dikes.—The circumstance that so much of Holland is below the sea-level necessarily exercises a very important influence on the drainage, the climate and the sanitary conditions of the country, as well as on its defence by means of inundation. The endiking of low lands against the sea which had been quietly proceeding during the first eleven centuries of the Christian era, received a fresh impetus in the 12th and 13th centuries from the fact that the level of the sea then became higher in relation to that of the land. This fact is illustrated by the broadening of river mouths and estuaries at this time, and the beginning of the formation of the Zuider Zee. A new feature in diking was the construction of dams or sluices across the mouths of rivers, sometimes with important consequences for the villages situated on the spot. Thus the dam on the Amstel (1257) was the origin of Amsterdam, and the dam on the Ye gave rise to Edam. But Holland’s chief protection against inundation is its long line of sand dunes, in which only two real breaches have been effected during the centuries of erosion. These are represented by the famous sea dikes called the Westkapelle dike and the Hondsbossche Zeewering, or sea-defence, which were begun respectively in the first and second halves of the 15th century. The first extends for a distance of over 4000 yds. between the villages of Westkapelle and Domburg in the island of Walcheren; the second is about 4900 yds. long, and extends from Kamperduin to near Petten, whence it is continued for another 1100 yds. by the Pettemer dike. These two sea dikes were reconstructed by the state at great expense between the year 1860 and 1884, having consisted before that time of little more than a protected sand dike. The earthen dikes are protected by stone-slopes and by piles, and at the more dangerous points also by zinkstukken (sinking pieces), artificial structures of brushwood laden with stones, and measuring some 400 yds. in circuit, by means of which the current is to some extent turned aside. The Westkapelle dike, 12,468 ft. long, has a seaward slope of 300 ft., and is protected by rows of piles and basalt blocks. On its ridge, 39 ft. broad, there is not only a roadway but a service railway. The cost of its upkeep is more than £6000 a year, and of the Hondsbossche Zeewering £2000 a year. When it is remembered that the woodwork is infested by the pile worm (Teredo navalis), the ravages of which were discovered in 1731, the labour and expense incurred in the construction and maintenance of the sea dikes now existing may be imagined. In other parts of the coast the dunes, though not pierced through, have become so wasted by erosion as to require artificial strengthening. This is afforded, either by means of a so-called sleeping dike (slaperdyk) behind the weak spot, as, for instance, between Kadzand and Breskens in Zeeland-Flanders, and again between ’s Gravenzande and Loosduinen; or by means of piers or breakwaters (hoofden, heads) projecting at intervals into the sea and composed of piles, or brushwood and stones. The first of such breakwaters was that constructed in 1857 at the north end of the island of Goeree, and extends over 100 yds. into the sea at low water. Similar constructions are to be found on the seaward side of the islands of Walcheren, Schouwen and Voorne, and between ’s Gravenzande and Scheveningen, and Katwijk and Noordwijk. Owing to the obstruction which they offer to drifting sands, artificial dunes are in course of time formed about them, and in this way they become at once more effective and less costly to maintain. The firm and regular dunes which now run from Petten to Kallantsoog (formerly an island), and thence northwards to Huisduinen, were thus formed about the Zyper (1617) and Koegras (1610) dikes respectively. From Huisduinen to Nieuwediep the dunes are replaced by the famous Helder sea-wall. The shores of the Zuider Zee and the Wadden, and the Frisian and Zuider Zee islands, are also partially protected by dikes. In more than one quarter the dikes have been repeatedly extended so as to enclose land conquered from the sea, the work of reclamation being aided by a natural process. Layer upon layer of clay is deposited by the sea in front of the dikes, until a new fringe has been added to the coast-line on which sea-grasses grasses begin to grow. Upon these clay-lands (kwelders) horses, cattle and sheep are at last able to pasture at low tide, and in course of time they are in turn endiked.

River dikes are as necessary as sea dikes, elevated banks being found only in a few places, as on the Lower Rhine. Owing to the unsuitability of the foundations, Dutch dikes are usually marked by a great width, which at the crown varies between 13 and 26 ft. The height of the dike ranges to 40 in. above high water-level. Between the dikes and the stream lie “forelands” (interwaarden), which are usually submerged in winter, and frequently lie 1 or 2 yds, higher than the country within the dikes. These forelands also offer in course of time an opportunity for endiking and reclamation. In this way the towns of Rotterdam, Schiedam, Vlaardingen and Maasluis have all gradually extended over the Maas dike in order to keep in touch with the river, and the small town of Delftshaven is built altogether on the outer side of the same dike.

Impoldering.—The first step in the reclamation of land is to “impolder” it, or convert it into a “polder” (i.e. a section of artificially drained land), by surrounding it with dikes or quays for the two-fold purpose of protecting it from all further inundation from outside and of controlling the amount of water inside. Impoldering for its own sake or on a large scale was impossible as long as the means of drainage were restricted. But in the beginning of the 15th century new possibilities were revealed by the adaptation of the windmill to the purpose of pumping water. It was gradually recognized that the masses of water which collected wherever peat-digging had been carried on were an unnecessary menace to the neighbouring lands, and also that a more enduring source of profit lay in the bed of the fertile sea-clay under the peat. It became usual, therefore, to make the subsequent drainage of the land a condition of the extraction of peat from it, this condition being established by proclamation in 1595.

Drainage.—It has been shown that the western provinces of Holland may be broadly defined as lying below sea-level. In fact the surface of the sea-clay in these provinces is from 11 to 16 ft. below the Amsterdam zero. The ground-water is, therefore, relatively very high and the capacity of the soil for further absorption proportionately low. To increase the reservoir capacity of the polder, as well as to conduct the water to the windmills or engines, it is intersected by a network of ditches cut at right angles to each other, the amount of ditching required being usually one-twelfth of the area to be drained. In modern times pumping engines have replaced windmills, and the typical old Dutch landscape with its countless hooded heads and swinging arms has been greatly transformed by the advent of the chimney stacks of the pumping-stations. The power of the pumping-engines is taken on the basis of 12 h.p. per 1000 hectares for every metre that the water has to be raised, or stated in another form, the engines must be capable of raising nearly 9 &#8468; of water through 1 yd. per acre per minute. The main ditches, or canals, afterwards also serve as a means of navigation. The level at which it is desired to keep the water in these ditches constitutes the unit of water measurement for the polder, and is called the polder’s zomer peil (Z.P.) or summer water-level. In pasture-polders (koepolders) Z.P. is 1 to 1 ft. below the level of the polder, and in agricultural polders 2 to 3 ft. below. Owing to the shrinkage of the soil in reclaimed lands, however, that is, lands which have been drained after fen or other reclamation, the sides of the polder are often higher than the middle, and it is necessary by means of small dams or sluices to make separate water-tight compartments (afpolderingen), each having its own unit of measurement. Some polders also have a winter peil as a precaution against the increased fall of water in that season. The summer water-level of the pasture polders south of the former Y is about 4 to 8 ft. below the Amsterdam zero, but in the Noorderkwartier to the north, it reaches 10 ft. below A. P. in the Beschotel polder, and in reclaimed lands (droogmakerijen) may be still lower, thus in the Reeuwyk polder north of Gouda it is 21 ft. below.

The drainage of the country is effected by natural or artificial means, according to the slope of the ground. Nearly all the polders of Zeeland and South Holland are able to discharge naturally into the sea at average low water, self-regulating sluices being used. But in North Holland and Utrecht on the contrary the polder water has generally to be raised. In some deep polders and drained lands where the water cannot be brought to the required height at once, windmills are found at two or even three different levels. The final removal of polder water, however, is only truly effected upon its discharge into the “outer waters” of the country, that is, the sea itself or the large rivers freely communicating with it; and this happens with but a small proportion of Dutch polders, such as those of Zeeland, the Holland Ysel and the Noorderkwartier.

As the system of impoldering extended, the small sluggish rivers were gradually cut off by dikes from the marshy lands through which they flowed, and by sluices from the waters with which they communicated. Their level ranges from about 1 to 4 ft. above that of the pasture polders. In addition, various kinds of canals