At the same time they did not imperil the security of the country. Roads
on causeways or reared on sunken piles would have opened the door to an
invader, but the canals provided an additional weapon of defence, for the
opening of the dykes sufficed to turn the country again into its primeval
state of marshland. The occasion on which this measure alone saved
Holland during the French invasion of 1670 is a well-known passage in
history, and the hopes of the Dutch in resisting the attack of any
powerful aggressor would centre in the same measure of defence, which is
the submerging of the country, practically speaking, under the waters of
the canals and rivers. There exists a popular belief that there is at
Amsterdam one master key, a turn of which would let loose the waters over
the land, but whether it is well founded or not no one except a very few
officials can say.

Pending any unfortunate necessity for breaking through the dykes and
letting loose the waters, it may be observed in passing that the effectual
maintenance of the dykes is a constant anxiety, and entails strenuous
exertions. They stand in need of repeated repairing, and it is computed
that they are completely reconstructed in the course of every four or five
years. A sum of nearly a million sterling is spent annually on the work.
A large and specially trained staff of engineers are in unceasing harness,
a numerous band of dyke watchers are constantly on the look-out, and when
they raise the shout, 'Come out! come out!' not a man, woman, or child
must hold back from the summons to strengthen the weak points through
which threatens to pass the flood that would overwhelm the land. It is a
constant struggle with nature, in which the victory rests with man. As the
dyke is the bulwark of Dutch prosperity in peace, it might be converted
into the ally of despairing patriotism in war.

There are marked differences among the canals. The two largest and best