416 DRAINAGE OF LANDS DIKED IN. 
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 
pebbles are cleared and cultivated, and the stones removed from the surface, 
new pebbles, and even bowlders of many pounds weight, continue to show 
themselves above the ground, every spring, for a long series of years. In 
clayey soils the fence-posts are thrown up in a similar way, and it is not un- 
common to see the lower rail of a fence thus gradually raised a foot or even 
two feet above the ground. This rising of stones and fences is popularly 
ascribed to the action of the severe frosts of that climate. The expansion 
of the ground, in freezing, it is said, raises its surface, and, with the surface, 
objects lying near or connected with it. When the soil thaws in the spring, 
it settles back again to its former level, while the pebbles and posts are pre- 
vented from sinking as low as before by loose earth which has fallen under 
them. The fact 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 Het Dalen van de Noordelijke 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 fleuves 
survenus dans les temps historiques, 1859), upon a still fuller investigation, ar- 
rives at the same conclusion. The eminent geologist Staring, however, who 
briefly refers to the subject in De Bodem van Nederland, i., p. 356 et segg., 
does not consider the evidence sufficient to prove anything more than the sink- 
ing of the surface of the polders from drying and consolidation.—See HLIsiE 
Recuuvs, La. Terre, vol. i., pp. 780, 732. 
