DRAINAGE OF LANDS DIKED IN. 
345 
Notwithstanding this slow sinking, most of the land enclosed 
by dikes is still above low-water mark, and can, therefore, be 
wholly or partially freed from rain water, and from that re¬ 
ceived by infiltration from higher ground, by sluices opened 
at the ebb of the tide. For this purpose, the land is carefully 
ditched, and advantage is taken of every favorable occasion for 
discharging the water through the sluices. But the ground 
cannot be effectually drained by this means, unless it is ele¬ 
vated four or five feet, at least, above the level of the ebb tide, 
because the ditches would not otherwise have a sufficient 
descent to carry the water off in the short interval between 
ebb and flow, and because the moisture of the saturated sub¬ 
soil is always rising by capillary attraction. Whenever, there¬ 
fore, the soil has sunk below the level I have mentioned, and 
in cases where its surface has never been raised above it, 
pumps, worked by wind or some other mechanical power, 
must be very frequently employed to keep the land dry 
enough for pasturage and cultivation.* 
has fallen under them. The foot that the elevation spoken of is observed 
only in the spring, gives countenance to this theory, which is perhaps 
applicable also to the cases stated by Staring, and it is probable that the 
two causes above assigned, concur in producing the effect. 
The question of the subsidence of the Netherlandish coast has been 
much discussed. Not to mention earlier geologists, Venema, in several 
essays, and particularly in Net Dalen van de NoordelijJce Kuststreken van 
ons Land , 1854, adduces many facts and arguments to prove a slow sinking 
of the northern provinces of Holland. Laveleye (. Affaissement du sol et 
envasement des fieuves survenus dans les temps historiques , 1859), upon a 
still fuller investigation, arrives at the same conclusion. The eminent 
geologist Staring, however, who briefly refers to the subject in De Bodem 
van Nederland , i, p. 356 et seqq ., does not consider the evidence sufficient 
to prove anything more than the sinking of the surface of the polders 
from drying and consolidation. 
* The elevation of the lands enclosed by dikes—or polders , as they are 
called in Holland—above low water mark, depends upon the height of the 
tides, or, in other words, upon the difference between ebb and flood. The 
tide cannot deposit earth higher than it flows, and after the ground is once 
enclosed, the decay of the vegetables grown upon it and the addition of 
manures do not compensate the depression occasioned by drying and con- 
