Instead of cutting the stones square or oblong, as is usually done, he had them made so as to “dovetail” into each other, much after the fashion of a child’s puzzle toy, or the junctions at the edge of a box. Thus, each stone fitted into those around it, while the lowest tier was dovetailed in similar fashion into the rock.
The stone employed was that which is called Moorstone, a very hard variety of granite. Each course of stones was carefully fitted together on shore, and their accuracy tested, and they were then taken to the Eddystone rock, and fixed in their places. Beside using these precautions, Smeaton fixed the stones in their place with the strongest cement, and furthermore fastened the stones together and united the several courses by strong oak treenails and iron clamps. As none of the stones weighed less than a ton, and some of them were double that weight, the strength of such an edifice may be imagined.
The accompanying illustration shows the arrangement of these dovetailed stones in one of the courses. It will be seen that the central stone must be laid first, and then the others arranged round it. The whole edifice is rather more than eighty-five feet in height, so that the elements have every chance of demolishing it, as they did that of Winstanley. More than a hundred years have now passed since it was built, and, although the fury of the tempest has been such that the waves have washed completely over its summit, it stands as firmly as it did when it was finished in 1760.
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.