PLANK ARCH.
Very neat and effective bridges may be made on the rafter principle, which our engraving at [p. 303] will sufficiently illustrate. The roadway, either horizontal, as in Fig. 1, or with more or less rise in the centre, as in Fig. 3, being supported by the king-posts, a tolerably broad river may be spanned by a bridge supported on latticed-worked sides, like a number of X’s set up without intervening spaces; but this would require skilled workmanship and secure fastening. Arches of great strength may be made of thin planks laid one upon another; a dozen inch planks would thus give an arched beam 1ft. in thickness, and when properly clamped, or bound together, exceedingly strong. This beam may be easily lengthened, no joining or scarping of the individual planks is required, their ends may simply butt against each other; but care must be taken that no two joints come even nearly together, or much of the strength will be thrown away.
CHINESE SLAB AND TRESTLE BRIDGE.
Many Chinese bridges are constructed with slabs of stone, set up much in the manner of those at Stonehenge. A number of bridges of this description, formed entirely of granite, are to be seen on the Cornish moors.
An Indian army was once sent to act against an enemy accustomed to a colder clime. A broad river checked the advance of the Indians, who had to remain day after day shivering from cold which their enemies bore with impunity; but their leader observed that ice was forming on the river, and, though the strong current kept the centre clear, he found a place some miles below the hostile camp where it had nearly united. By pushing forward poles and faggots into the water he made a nucleus on which fresh ice was formed, and before morning he had sufficiently bridged the river to allow of the passage of a few men who held their position till the rest could cross.
Hints on fording rivers.
In fording deep and rapid streams, the tendency of the body to float greatly diminishes the power to resist the force of the current. We have seen a short and compactly built man, struggling against this difficulty, when a couple of tall aborigines came to his assistance, and with mistaken zeal put their hands under his arms to hold him up, till he explained that he wanted them rather to press upon his shoulders and keep him down. A detachment of our troops found themselves before a ford where the stream was deep and strong, and while they hesitated, the tall guide picked up a heavy stone, placed it on his head, and walked safely through. It is to be remembered, however, that the ford which is safe for the first man is not always so for the last. An ancient general attempted the passage of a river, but the trampling of the men and horses loosened the sand, the stream swept it away, reinforcements were unable to come up, the advance was beaten, and in attempting to recross the river, numbers were drowned, owing to the increase in the depth of water.
In most countries native paths will lead to the practicable fords, and very frequently villages will be established near them, so that information and guides may generally be obtained. In the Cape colony, people who live near the drifts or fords of large rivers frequently keep spans or teams of powerful oxen for the sole purpose of drawing the waggons of travellers across, and these are generally led by some stout young fellow on a horse that is also well accustomed to the locality. It will often happen in the course of exploration that the traveller will have to find the shallow places for himself. Reefs, or edges of strata, running across the river are, of course, obvious enough, but to find moderate shallows in a river of more even depth is not quite so easy; the best way, therefore, is to follow the windings, bearing in mind generally it is better to seek up the river than down where fresh tributaries increase it. In all the hollows where the stream impinges strongly upon the banks they are generally very steep, with considerable depth of water, while the points will be found to slope downward with shoals extending from them, and there is generally an eddy or return current on the upper side; therefore, if a point can be found, with another somewhat below it on the other side, a ford may reasonably be looked for, especially if the width between the points seems greater than usual. A ford seldom leads straight across a river, and there is little use in looking for one in a hollow or under a steep bank.