human
The great dike of West Kappel is there,
gpt
I don't see anything,
human
There is nothing particular to see on this side of the dike, interposed Professor Stoute, laughing at the astonishment of the captain.
gpt
What did you expect to see?
human
I hardly know. I have heard so much about the dikes of Holland, that I expected to see a big thing when I came across one of them,
gpt
They are a big thing; but really there is very little to see.
human
But what is a dike, sir? asked Paul, curiously.
gpt
I never supposed it was anything more than a mud wall.
human
It is nothing more than that, only it is on a very large scale, and it must be constructed with the nicest care; for the lives and property of the people depend upon its security. When they are going to build a dike, the first consideration, as in putting up a heavy building, is the foundation. I suppose you have seen a railroad built through a marsh, or other soft place.
gpt
Yes, sir; the railroad at Brockway went over the head of the bay, where the bottom was very soft. As fast as they put in gravel for the road, the mud squashed up on each side, making a ridge almost as high as the road itself. They built a heavy stone wharf at Brockway, the year before we sailed, and the weight of it lifted up the bottom of the shallow bay a hundred feet from it, so that boats get aground there now at half tide.
human
That is the idea exactly: The foundation is not solid; and that is often the chief difficulty in building a dike. The immense weight of the material of which it is constructed crowds the earth out from under it, and it sinks down faster than they can build it. In such places as this they find it necessary to drive piles, to build the embankment on.
gpt
They must cost a heap of money, then.
human
The annual expense even for repairs of dikes in Holland is about three millions of dollars of our money. Speaking of that very dike of West Kappel, added the professor, pointing to its long, inclined escarpment,
gpt
I see there are trees growing on the shore, farther up the river,
human
Those trees are willows; and wherever it is possible for them to thrive, they encourage their growth for two reasons: first, because the roots of the trees strengthen the dike; and, secondly, because the willow twigs are wanted in repairing and securing the embankment. The foundations of sea-dikes vary from a hundred and twenty to one hundred and fifty feet in width. The rampart is made of clay, which, as being impervious to water, forms the entire structure when the material is available in sufficient quantities. The maximum height of the dikes is forty feet; but of course they vary in this respect with the elevation of the land to be protected by them.
gpt
But I should think the mud and clay would be washed away by the beating of the sea.
human
So they are sometimes; and to guard against such an event, which is a calamity in this country, the dike is covered with a kind of thatch-work of willow twigs, which has to be renewed every three or four years. Occasionally the outer surface of the embankment is faced with masonry, the stone for which has to be brought from Norway.
gpt
A ship there is coming in,