WHETHER the original inventor of the “dovetail” took his idea from Nature I cannot say, but he certainly might have done so. On the left of the illustration is part of a human skull. The skull is not, as many persons seem to think, made of a single bone, but it is composed of many bones, united by “sutures,” which are, in fact, natural dovetails. Although in early life these sutures are comparatively loose, they hold the various parts together so firmly, that if the head be violently struck, the bones may break, but the sutures do not give way. Perhaps some of my readers may ask how it is possible to take a skull to pieces without cutting it or fracturing the sutures. It is done in a way equally simple and ingenious. The skull is filled through the opening with dried peas, and then sunk under water. The peas expand with the moisture, and, as they exert an equable force in all directions, they slowly and quietly pull the sutures asunder, without injuring the bones. THE DAM. In many human operations, where a certain depth of water is required in a running stream, the reasoning powers of man have enabled him to attain his object by building a dam, or obstacle across the stream, which forces the water to rise to its level before it can find a passage. Such, for example, are the Locks which render rivers navigable, and allow even the heavily laden barges to traverse miles of water which would otherwise have been closed to them. Those mills, again, which are worked by water need that a sufficient amount of water should be ready in order that it may by its weight force the wheel round. Such a Dam is shown on the right hand of the illustration, the height to which it raises the water being shown by the level of the stream below the Dam, and that of the water as it tumbles over in a miniature cascade. Putting aside the natural dams made by accumulations of the various debris that are washed down by a swollen stream, and which sometimes raise the water to a very great height, we have an example of a natural dam in the curious structure made by the Beaver, for the same purpose as that of the lock in the mill-stream, namely, to insure a depth of water sufficient for the needs of the beings that make them. Every one has heard of the Beaver’s dam, but there is so much misconception on the subject, that a few words will not be out of place.