The earth and its inhabitants The earth and its inhabitants .. earthitsinhabita386recl Year: 1883 HYDEOGEAPHY. 467 covered with a thick layer of clay, whilst eleven parallel rows of piles, packed with stones and blocks of basalt, defend its base. Dykes, however, would be of little service if the land they defend could not be drained. Nearly all the polders lie below the mean level of the sea, the bed of one of them, the Zuidplas, near Gouda, being 18-4 feet below it. Drains are only available as long as the bed of the polder is higher than the sea at low water. In all other cases mechanical
The earth and its inhabitants The earth and its inhabitants .. earthitsinhabita386recl Year: 1883 HYDEOGEAPHY. 467 covered with a thick layer of clay, whilst eleven parallel rows of piles, packed with stones and blocks of basalt, defend its base. Dykes, however, would be of little service if the land they defend could not be drained. Nearly all the polders lie below the mean level of the sea, the bed of one of them, the Zuidplas, near Gouda, being 18-4 feet below it. Drains are only available as long as the bed of the polder is higher than the sea at low water. In all other cases mechanical means must be used to pump out the water. Windmills have been employed for that purpose since the thirteenth century, but within the last thirty years the use of steam has become general. A single engine suffices for the drainage of hundreds of acres, for the quantity of rain (25-1 inches on an average) is but little in excess of the amount of evaporation (22-5 inches). Some of the polders, however, remain under water for two or three months during winter, for they cannot be emptied as long as the rivers are in flood. Their sanitary condition, under these circumstances, leaves much to be desired. Fig. 262.—Ameland. 5 Miles. The word polder is derived from ^joe/ (pool), and actually the lowest part of each basin is often occupied by a sheet of water long after its higher portions have been brought under cultivation. Formerly the work of drainage proceeded very slowly, but since steam has been called into requisition it is effected almost instan- taneously. In the more ancient polders the drains converge towards the centre, intersecting the contours of the ground at right angles. Yery different is the appearance of a polder drained by steam-power, for drains and roads divide it into rectangles. The Zuiderpolder, near Amsterdam (Figs. 259 and 260), exhibits both methods in combination. Warping is hardly ever practicable in the Netherlands, owing to the horizontality of the co
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