But in North Holland and Utrecht on the contrary the polder water has generally to be raised.
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.
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.
It sometimes happens that a polder is not in direct contact with the boezem to which it belongs, but first drains into an adjacent polder, from which the water is afterwards removed.
In the Frederiks Park is a pump-room supplied with a powerful chalybeate water from a spring, the Wilhelminabron, in the Haarlemmer Polder not far distant, and in connexion with this there is an orthopaedic institution adjoining.
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.
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 214 ft.
These waters are utilized as the temporary reservoirs of the superfluous polder water, each system of reservoirs being termed a boezem (bosom or basin), and all lands watering into the same boezem being considered as belonging to it.
Boezems, like polders, have a standard water-level which may not be exceeded, and as in the polder this level may vary in the different parts of an extended boezem.
The roads are divided into national or royal roads, placed directly under the control of the waterstaat and sup- 'ported by the state; provincial roads, under the direct control of the states of the provinces, and almost all supported by the provincial treasuries; communal and polder roads, maintained by the communal authorities and the polder boards; and finally, private roads.
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.
Below the level of the polder, and in agricultural polders 21 to 31 ft.
The four polders with their areas of fertile soil would be: (I) North-west polder, area 53,599 acres; fertile soil, 46,189 acres.