from total destruction by the scuttling and sinking of some of its piers. Such a misfortune could not be repaired in a moment; and it was only by the hasty equipment of a flying bridge, and the sending over upon it of supports to the troops on the far side, that the latter managed, with hard fighting, to keep their ground.

The principle on which we construct a bridge on rafts, on casks, or inflated skins, is the same which guides us in the laying down of pontoon bridges and bridges of boats. We find or construct piers sufficiently buoyant to sustain the weight which it is proposed to lay upon them, and surmounting these with appropriate saddles, we arrange on them our balks and chesses, just as we should do were boats or pontoons there to support them. Rafts, casks, and inflated skins are, however, to be pressed into the service only when other and more obviously suitable means of passing a river are wanting. The materials for piers of rafts are found chiefly where wood is abundant; and there are some trees which prove more available for the purpose than others. Taking the specific gravity of water at 1,000 oz., it will be seen that while American pine varies from 402 to 632, oak weighs 750, heart of oak 1,170, larch 530, and English poplar only 383. This last, therefore, which no shipbuilder would touch, so long as other wood could be procured, is the best adapted of any for raft purposes; whereas heart of oak-the pride of our navy-must be classed in the same category with the box of Holland and of the Brazils, the pomegranate, the American ebony, and the vine. It cannot be used at all in the construction of piers for a floating bridge.

The materials of which we now speak are to be procured either by cutting down trees or taking possession of felled timber, or removing from barns or houses such beams as may appear to suit our purposes. There are, however, limits both to the size and to the cubic contents of such beams. Anything below 25 ft. in length, a mean girth of 30 in., and a content of 12.5 at the least, is too small to be serviceable. Anything beyond 35 ft. in length, with a mean girth of 78 in. and a content of 118.3, is too large to be removed with the appliances generally at our command, from the spot where it has been felled. We must refer the professional reader to Sir Howard Douglas's work for the rules by which he is to he guided in calculating the specific gravity of his materials, as well as for instructions how to convey them in every emergency to the point where they are wanted. But we should do injustice both to our author and to the important subject which he discusses, if we failed to transcribe the following sentences:

'Bridges of rafts being in general only temporary expedients, resorted to upon sudden emergencies, the construction should be such as to effect the purpose with the least means consistent with safety; and this we are now to consider.

'A fir-tree of 4.33 feet mean girth, and 30 in length, contains 44.9 cubic feet, and will float 1128 lbs. Six of these will float 6768 lbs. Deduct the weight of 5 balks of oak, 4 inches wide, 6 inches deep, and 26 feet long; also the weight of 2-inch planks, sufficient for a floor 10 feet wide, and 12 long; in all about 1981 lbs.; and there will remain 4787 lbs. for the weight which each raft is capable of bearing. If the rafts are 12 feet distant from centre to centre, and the whole bridge covered with infantry filing across, each will have to sustain 12 men nearly; which, taken at 180 lbs. per man, is 2160 lbs., leaving 2627 lbs. for the remaining buoyancy of each raft.

'If the timber be well seasoned, and has not been long in the water, infantry may pass under a front of 4 men, ranks 30 inches from each other, by which each raft will have to bear 16 men (2880 lbs.); retaining a buoyancy, by calculation, of 1907 lbs. But, in general, the excess by calculation should be greater than this, to allow for loss and deficiency† of buoyancy.

*

'The mean diameter of a tree whose mean girth is 4.33 feet, is 1.38 feet. A raft of 6 trees of this size will be 8.28 feet wide; and the interval between two rafts, placed 12 feet between centres, will be 3.72 feet; a space sufficient to allow a great part of the superficial current to pass.

'Cavalry may pass by twos, but intervals of 3 feet should be left; when each float will have to sustain about 2632 lbs., the weight of two men and their horses.

'The weight of a light six-pounder and limber, complete, with 4 horses and 2 drivers, is about 8000 lbs. ; but as these cover about 34 feet in the march, they will always be sustained by 3 floats directly, and further supported by the adjoining two. Three floats could

bear this weight, viz., 2667 lbs. each, as we have seen; but, to render the operation quite safe, intervals of 15 feet should be allowed between guns in crossing, in order that the weight may be borne by all the floats which are reached by the balks actually pressed upon.

'Heavy guns should be unlimbered, and drawn across by hand, with long ropes to keep the men at a distance from them. The weight of a 9-pounder gun and carriage is about 2856 lbs., which, being more concentrated than any of the other weights here considered, will be most severe upon the bridge.'

The following description of a bridge of this sort, constructed at a critical moment by a portion of that corps of which we

* By imbibing water. If the ends are not tarred, dry timber will increase its weight one-sixth by two or three days' immersion.'

By being full of sap when cut.'

hope ere long to see the revival, will interest the general reader:

In July, 1809, when Sir Arthur Wellesley's head-quarters were established at Placencia, it became necessary to secure the means by which a junction might be formed with Cuesta. Two companies of the Staff Corps were accordingly sent, with a strong working party, to Baragona, to make a bridge across the Tietar. This river, though deep only on one side at that part, is so wide as to have required 15 pontoons to form the bridge which the French constructed there some time before, and which, upon retiring to Talavera, they of course entirely destroyed.

"The officer sent to re-establish a communication across that river could find no other materials with which to effect this than the timber of a large inn and its outhouses, about a mile and a half distant, and some pine-trees that grew in a neighbouring wood. The building was therefore immediately unroofed, and timber of the following descriptions and dimensions procured from its demolition. Six beams of dry fir, each about 20 ft. long and 2 ft. square; three or four hundred rafters, about 10 ft. long, and 6 in. by 4, in section; six large doors; and 20 running feet of mangers from the stables.. The six large beams were formed into a raft about 20 ft. long and 12 wide; the buoyancy of which was therefore about 13,500 lbs. The rafters formed the beams, and the planks of the mangers the floor. This raft had to support a floor about 30 ft. long, to which it was fully adequate; its extreme buoyancy being sufficient to float 60 or 70 men, exclusive of the weight of floor; and half of that buoyancy being not much more than was required for the weights that had to cross it. In the shallower parts of the section piles were driven into the bottom of the river, and caps of light material laid across: the beams were formed of young pine-trees, 30 ft. long, and about 7 inches in diameter. The doors and mangers of the inn, being too thick for the only nails that were forthcoming, were secured to the beams by ribands formed of young pines split thus, which were laid over the ends of the planks, and tied, with willow twigs, to the outside beams and to the caps of the piles. The raft was made fast to a sheer-line, attached to a tree on one side of the river, and to a stake driven into the sand on the other. A trestle, and two large wooden mallets, made on the spot, were used to set and drive the piles. On the 18th of July the army crossed the Tietar by this bridge, and by Miajados marched to Jalayuela.'

The fitness of casks for bridge purposes, as well as of all other light vessels, which, whether made of deal or skins, can be rendered air-tight, needs, after what has been already stated, no demonstration. With casks, indeed, provided they be large enough, bridges may be constructed on an emergency which shall sustain, not infantry alone, but cavalry and field artillery also. Six wine-pipes, lashed together, would make a very good raft.

Its weight would not much exceed that of one formed with two large cylindrical pontoons, and it would sustain a burthen less than a pontoon raft by four or five cwt. only. And small armies or corps are continually liable to be thrown into situations where a knowledge of this fact may be of the utmost importance to them. Thus, in mountainous and difficult countries, where carriages are unable to travel, it will always be possible to transport on mules, or even by hand, empty casks enough, with the necessary planks and beams, to bridge over the sort of rivers which may be expected, in such districts, to cross our line of march. And if our field of operation compel us to traverse a desert, the casks and skins in which water for the use of the troops is conveyed may prove of immense service after we shall have emptied them. Thus Alexander's army passed the Oxus on rafts made of skins stuffed with dry grass. (Árrian, lib.iii. cap. 29.). We are informed by Livy that Hannibal's Spanish infantry crossed the Rhone by swimming with the assistance of inflated leathern bags; and Cæsar states that it was a common practice with the Spanish and Portuguese light infantry to pass rivers in this manner. Nor is modern authority wanting in support of these statements. Colonel Chesney, in his Expedition for the Survey of the Euphrates,'-a valuable work, to which full justice has not been done, describes in detail the sort of rafts (consisting of logs of timber supported on inflated skins) which are still used, for trading and other purposes, in the rivers of Western Asia. These are facts which every educated and thoughtful officer will carefully register against the hour of need; for where cattle follow an army (and the army is in a bad plight which is not so attended), materials for the construction of rafts never can be wanting. We have only to take the skins of the slaughtered beasts, and deal with them properly, and a very effective floating vessel is at our disposal.

The following experiment,' says Sir Howard Douglas, 'was made with an ox-hide trimmed into a circular shape, of about 5 ft. 6 in. in diameter. The skin was drawn together at the edge, and firmly bound round a tube made of alder-tree, having the pith removed; and a piece of leather was nailed upon the inner end, as a valve to prevent the air from escaping. The vessel was inflated by a common handbellows, and floated 300 lbs.; and, without any application to close the pores of the skin, remained nearly fully inflated for five hours; at the end of twenty-four hours, it was still found capable of floating 150 lbs. The weight of the skin was 45 lbs.; so that by this expedient great power of floatation may be packed in small space, and easily transported. Skins may be preserved for a con

siderable time by common salt; and if covered with a solution of gum, or any glutinous substance, more particularly at the part which corresponds to the back of the animal, where skins are always most porous, they will retain the air for a very considerable time. By means of the wooden tubes, the skins may be re-inflated in succession at any time, without withdrawing them, by merely turning up the tube, taking off the lashing, which, as a precaution, should be made to close the vessel effectually below the valve, and then using the bellows as before.'

It is obvious that, in order to accomplish our purpose with rafts thus hastily constructed, we must have means at hand for making them fast in the river; and as we are supposed to be in all respects reduced to our shifts, -as regular anchors and cables cannot be within reach, how shall we supply their places? In many ways. For cables, a quantity of stout cord, such as is used in packing, well twisted together, will suffice; failing these, willow-withs or coarse grass may be turned to account. In lieu of anchors, heavy stones with ring-bolts let into them, wooden grapnels, and loaded fascines, may be resorted to.

Perhaps, of all the expedients to which armies in the field are forced to have recourse in order to facilitate their movements, and to keep up their communications, the construction of bridges upon trestles is the least satisfactory. A bridge on trestles can hardly be put together at all if the bed of the river be rocky or muddy, or the river itself be very deep or fluctuating in its course. Such a bridge will never hold its ground against any sudden rise, for if one vessel be swept away, the undue pressure upon the rest becomes fatal. Still, forasmuch as a trestle bridge is perhaps the easiest of all bridges to construct, and that it serves every purpose, where the stream happens to be quiet and of moderate depth, the student will do well not to overlook the section in Sir Howard Douglas's work which treats of such bridges generally. He will there be shown of what materials trestles can be formed; how they are driven, as well in deep water as in shallow; what sort of balks are most suitable for connecting one with another; what weight of flooring is required, and how it is to be procured and laid down. For our own parts, we must content ourselves with giving, in the language of our author, two instances: one of failure in a trestle bridge through an accident which could not be guarded against; another, of its complete accomplishment of a proposed object in the face of difficulties of which the extent seems to have been enormous.

The little reliance that can be placed on trestles was seriously experienced in the operations on the Guadiana in 1811,