It was some days before the busy little animals began their work again, for they knew that the Wolverene might still be on the watch for them, and have crossed the river himself. So they "lay very low," as Father Beaver put it, keeping to the thick undergrowth of the brushwood, or playing hide-and-seek with their young ones in the deeply tunnelled banks. Phil soon found that though each tunnel had a separate entrance, they all led to the same spot, within easy reach of the winter houses. He was never tired of admiring these, but Father Beaver brushed his praise aside, so far as they were concerned. "Come and look at our dam," he said. "It's a very fine one, though perhaps I ought not to say so." The dam stretched quite two-thirds across the river, and was curved, somewhat in the shape of a half crescent. "That is because the current here is very rapid," explained the Beaver, "and an arch is stronger than a straight line, as your own bridge builders know. If the current were gentle, our dam would be straight, and this would give us much less trouble. But a rapid current is very useful, for if we have to go any distance for our building materials, it brings them quickly down to us, without any special effort on our part." "So that was why we carried all the trees that you had felled quite close to the river bank?" "Exactly. When we are ready to build we shall push all those into the current, and some of us will be waiting by our dam to stop them as they float past. See how the branches of the willow are sprouting!" They had reached the dam by this time; it seemed to Phil like a thick hedgerow on a solid bank, for not only were the willow branches in full leaf, but the poplars and birches, used to repair it from time to time, had taken root also. "If the snow on the mountains melts too rapidly, and flows down to the river in torrents, the water behind our dam is still quite calm, and our houses, built in its shelter, are undisturbed. We must always have a deep body of water in which to build our lodges; so when we take a fancy to some small river or creek in which the water is likely to be drained off at any time, Nature teaches us to build our dam right across the river, in order that we may prevent this." "How do you start building the dam?" asked Phil.