The Suspension-bridge.
The mention of these Ants brings us to another point in architecture. We have already seen that they can not only build arched tunnels, but also can form their own bodies into arches, and we shall presently see how they can form themselves into Suspension-bridges. We will, however, first take the Suspension-bridge, and its vegetable origin, before passing to the animal.
I have little if any doubt that the modern Suspension-bridge, with all its complicated mathematical proportions, was originally suggested by the creepers of tropical climates. There are few points in a tropical forest, no matter in what part of the world, more striking than the wonderful development of the creeping plants. The trees are very much like those of our own forests, and are in no way remarkable, but the creeping plants form the chief feature of the woods.
They extend themselves to unknown lengths, crawling up to the very summit of a lofty tree, hanging down to the very ground, if not caught by a midway branch, running along the earth, making their way up another tree, and so on ad infinitum. They interlace with each other, forming almost impenetrable thickets, as has already been mentioned while treating of Nets, and there is scarcely a tree that is not connected with its neighbour by means of these wonderful creeping plants.
Of course the monkey tribes make great use of them in passing from one tree to another, thus being able to avoid the ground, which is never to a monkey’s liking. Man, therefore, copies the example of the monkey, and makes use, either of the creepers themselves, or of ropes stretched from tree to tree in imitation of them.
In some parts of the world, where palm wine, or “toddy,” is manufactured, the native has recourse to an ingenious device which saves a vast amount of exertion. As the calabash which receives the juice of the palm-tree is always fixed at a considerable height, and as each tree only yields a limited supply, the toddy-maker would be obliged to ascend and descend a great number of trees before he could collect his supply of palm-juice.
In order to save himself trouble, he has the ingenuity to connect the trees with each other by two ropes, the one about six feet above the other. He then has only to ascend once, and descend once, for he ascends one tree, and by means of the ropes passes from tree to tree without needing to descend.
The mode of traversing these ropes is simple enough, the lower rope serving as a bridge, along which the man walks, and the upper rope being held by the hands. Those who see these palm-wine makers for the first time are always greatly struck. At some little distance the ropes are quite invisible, and the man appears to be walking through the air without any support whatever.
In Borneo the Rattan is continually put in requisition as a bridge. It runs to almost any length, a hundred feet more or less being of little consequence; it is lithe and pliant, and so strong that it can hardly be broken. The “canes” formerly so much in vogue among schoolmasters, and now so generally repudiated, are all cut from the Rattan. Chiefly by means of this natural rope, the Dyak of Borneo flings his rude suspension-bridges across chasms or rivers, and really displays a wonderful amount of ingenuity in doing so.
The one fault of these bridges is their tendency to decay, or perhaps to be eaten by the multitudinous wood-eating insects which swarm in that country. However, the materials cost nothing at all, and time scarcely more, so that when a bridge breaks down, any man can fit up another at the expense of a few hours’ work. As, moreover, the Dyaks have a curious way of building their houses on one side of a ravine, they find that a bridge of this kind saves them the trouble of descending and ascending the ravine whenever they wish to visit their house.
In many parts of America the Suspension-bridge is almost a necessity. The country is broken up by vast clefts, technically called “cañons.” These cañons are ravines in the rocky ground, with sides almost perpendicular. For the greater part of the year they are dry, but sometimes, and without the least warning, they become the beds of roaring torrents, rising to some thirty or forty feet in height, and carrying away everything before them.
Over these ravines are thrown suspension-bridges made almost entirely of creepers, and loosely floored with rough planks. Although they are very strong, they appear to be very fragile, and even under the tread of a human being swing and sway about in a manner that always shakes the nerves of one who is unaccustomed to them. Yet, even the mules of the country can cross them, the animals picking their way with the wonderful sure-footedness of their kind, and not in the least affected by the swaying of the bridge.
Passing from the vegetable to the animal world, we revert to the Driver-ants, which have already been mentioned. It has been seen that their soldier-ants can, with their own bodies, form a tunnel, under the shade of which the workers can pass, and we have now to see how they can, with the same materials, form a suspension-bridge.
It often happens that on their march they come to water, and, as they always advance with total disregard of difficulties, they must needs invent some very ingenious way of overcoming the difficulty. One of them climbs a branch which overhangs the water, clasps the undermost twig very tightly, and allows itself to hang from it. Another at once follows, and suspends itself from its comrade in like manner, the powerful and sicklelike jaws doing their duty as well as the legs. A chain of Ants is thus speedily formed. When the lowermost Ant touches the water, it merely spreads all its legs, and awaits the development of events. Another runs over it, holds to the first Ant by its hind-legs, and stands in the water, spreading its limbs as much as possible over the surface. Ant after Ant descends, until quite a long chain of the insects is formed, and is swept downwards with the stream. By slow degrees the chain is lengthened, until the Ants at its head are able to seize the bank on-the opposite side of the water. When they have succeeded in doing so, the bridge is complete, and over that living bridge will pour a whole army of Driver-ants.
Even in those cases where this mode of travelling would be too perilous on account of the rapid torrent, the Ants contrive to suspend themselves in long strings until they effect a communication with the trees of the opposite bank.
It is, perhaps, needless to give more than a passing reference to the Suspension-bridges made by Spiders, by means of which they can traverse considerable distances. The similar bridge of the Little Ermine Caterpillar has already been mentioned, when treating of the subject of Double Walls.
ARCHITECTURE.
CHAPTER IV.
LIGHTHOUSES.—THE DOVETAIL.—THE DAM.—SUBTERRANEAN DWELLINGS.—THE PYRAMID.—MORTAR, PAINT, AND VARNISH.
The Eddystone Lighthouse: its Position, and the Difficulties of building it.—Destruction of successive Lighthouses.—Smeaton’s Idea of Form borrowed from the Tree-trunk.—Mode of building.—Rooting it into the Rock.—Principle of the Dovetail.—Bones of the Human Skull, and their Articulation.—The Dam, and its Uses to Man.—The Lock and the Water-mill.—Dam of the Beaver: its Objects and Mode of Construction.—Popular Errors with regard to the Dam.—Subterranean Dwellings.—The Indian Palace, and its Use in Summer.—Subterranean Dwellings in Kamschatka, and their Use in Winter.—The Wood or Horse Ant, and its double Dwelling.—The upper and lower Nests used according to the Amount of Warmth required.—Section of the Nest, and a Glimpse into its Interior.—The Pyramid.—Derivation of its Name.—Natural Objects from which the Form was derived.—Subaquatic Mortar or Cement, and its Use to Man.—Subaquatic Cement used by the Caddis, the Stickleback, the Terebella, the Sabella, the Serpula, and others.—Paint and Varnish, and their Utility to Man.—Propolis as used by the Hive Bee, and the Source whence it is obtained.
WE now come to some points in Architecture which cannot well be grouped together, and must therefore be treated as Miscellanea.
Our first example is one which was avowedly based upon an imitation of Nature, namely, the celebrated Eddystone Lighthouse, and we shall see that in two points—first its form, and next the mode in which the stones were fixed together—Nature had been closely followed by the architect.
Unlike ordinary lighthouses, this edifice had to be constructed so as to endure the full force of waves as well as wind. A few miles from the southern coasts of Devon and Cornwall there is a rock which in former times greatly endangered the ships which passed along the Channel. Several attempts were made to build a lighthouse on this dangerous spot. Winstanley’s lighthouse, which was finished in 1700, was wholly swept away three years later, together with the architect himself, and some workmen who were engaged in repairs. So terrific is the force of the elements on this spot, that the lighthouse was entirely destroyed, and the only vestiges of it that were ever discovered were some iron bars and a piece of chain.
Another lighthouse was built a few years afterwards, but was burned down, it being of wood instead of stone. At last the work was put into the hands of Smeaton, who saw that he must build on a totally new plan. He took for his model the trunk of a tree, and determined to build his lighthouse of the same form as the tree-trunk, and to fasten it into the rock just as a tree is fastened by its roots. Accordingly, he struck out a new principle in the construction of such edifices, and his model has been followed ever since. The reader will see, by a glance at the illustration, how close is the resemblance in external form. I may mention that the tree in question was sketched from one in a paddock opposite my house.
Having settled the form of the lighthouse, and made it like a tree-trunk, the next business was to fix it firmly in the rock, and, in fact, to give it roots of stone. For this purpose, he made the base of the edifice as wide as the rock would allow, so as to correspond with the wide base of a tree-trunk, and traced a circle of about ninety feet in circumference. Instead, however, of merely laying the stones as is usually done, or even letting them into holes cut in the rock, he hit upon a singularly ingenious device, whereby the building was practically a single stone.
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.