Mr. Downing on the Drainage of Haarlem Lake. 453 



did not exist. The Poldeks are the lands lying below the level of the water of 

 the basin, and must therefore be kept dry by artificial means, each separate 

 polder being surrounded by its own dyke to keep out the waters of the basin ; 

 over which dyke the waters of the polder must be raised mechanically. The 

 depth of the land in the polders, below this level, varies from a few inches to 

 20 feet. 



In the Rynland Hydraulic Administration the relative areas of these three 

 descriptions of surface are: — 



Natural Lands (two- thirds of which are Dunes or Sand-hills), 76,000 acres. 



Basin, 56,000 „ 



Polder Lands, 173,000 „ 



305,000 acres. 



From which we have this surprising result, that but one-fourth of the total area 

 is land situated above the level of the water; let the boundary dykes fail, or the 

 mechanical raising of the waters cease, and the vast area of 173,000 acres re- 

 verts to its original condition, becoming again covered with water. 



It will be understood now, from what has been said, that when the exterior 

 are lower than the interior waters, the basin acts as a channel to conduct the 

 waters flowing off the natural lands, and raised mechanically from the polders 

 to the sluices in the boundary dyke, and so to the sea ; but if, on the contrary, 

 the exterior waters are at any time higher than the interior, the basin acts as a 

 reservoir to contain these waters derived from the natural lands and polders, 

 until a favourable time for discharging them through the sluices arrives. Hence, 

 if for any lengthened period the external waters remain at a high level, closing 

 the self-acting sluices, and stopping the discharge of the internal waters, the 

 proportion of the area of the basin, relatively to that of the rest of the admi- 

 nistration, becomes of the highest importance, for at every polder there is a 

 scale fixed, like the scale of feet at a dock entrance ; the zero of these scales, 

 which is common to the whole country, is the top of a certain pile at Amster- 

 dam, and is briefly called the A. P. (Amsterdam pile), or O. A. (zero of Ams- 

 terdam). It is about the mean level of the tides in the Zuyder Zee, and also the 

 surface of the natural lands in the greater part of the two provinces of Holland. 

 It is most suitably, then, the point of reference in all hydraulic works and ob- 



VOL. XXIII. 3 o 



