this way, as the stream can no longer flow in its channel, and can only get by the obstacle placed in its way, very slowly, by spreading out and flooding the surrounding country, the result is that a great pond or basin of water is formed on the up-stream side of the dam, and this the beavers have all to themselves. Of course, when the water is checked in its flow, it begins to rise against the dam that confines it, and as only a small quantity percolates through, it sinks and runs away in a much smaller volume, on the other side of the obstruction. When a flour-mill, which is to be worked by water-power, is erected by the side of one of our small streams, exactly the same principle is employed, a dam being built across it, from bank to bank, and the water running off by a side-channel. Beavers, however, existed long before there were any millers, and moreover, they make better dams than our millers do, or, at least, they construct them upon more scientific principles. The mill-dam runs, as a rule, straight across the stream, but the beavers curve theirs a little up into it, so that the water does not rush against it so violently as it would if it were straight, but flows smoothly off upon either side. This is how we make our sea-dams—at least when it is possible—and where any structure has to resist a great force of water, as, for instance, the buttresses of a bridge across some large river, it is always shaped like this, only more so; that is to say, we turn the curve into an acute angle and present a sharp edge, instead of a rounded surface, to the impetuous rush of the stream. In this way the water is cut in two, as if by a knife-blade, whereas, if the masonry presented a broad surface for it to rush against, the first flood might wash the strongest bridge away. Practical experience seems to have led to the beaver’s employment of the principle, though probably he has no very clear ideas as to what the principle is. He could not “formulate it”—as we say—and to say the truth, neither could I myself at this moment.