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 country ; yet it would be worth while to devise 

 some means for preventing 63,570 million cubic feet of fertilising alluvium being 

 annually swept into the ocean. 



One of the greatest engineering works achieved in modern times was the 



