A cask bridge ie sometimes formed in the following manner :—A certain number of casks are placed side by side, so is to form a sort of floating pier, having their axes in horizontal positions; the casks are lashed to two gunwale timbers, as they are called, which extend along the upper side of the pier at the extremities of the casks ; two of these piers are placed at an interval of about 10 feet from each other, and they are connected together by the baulks which support the platform. As many of these rafts are prepared as may be necessary, they are rowed to their places In the line, and the bridge it formed in the same manner as the pontoon bridge above described.
When the formation of a bridge extending over a river from one bank to another becomes impossible, numerous expedients are had recourse to for conveying troops across by means of what are called dying or moving bridges. We hewn from Arrian that the army of Alexander passed the Indus by maws of boats and rafts of timber, the latter being supported on begs made of skin and inflated with air; and in a similar manner the infantry of Hannibal is said by Livy to have crossed the Rhine. In India, at the present time, the passage of rivers is often effected by means of basket,-boata—, which are formed of split bamboos and covered with half-dressed hides. Each vessel is said to be capable of containing about 30 men.
In 1811, Captain Squires, of the Engineers, formed a bridge over the Guadiana by fixing trestle-piers in the shallow parts of the river and mooring boats in the middle ; but just as the bridge was finished, the river swelled and carried away the trestles. The boats were then converted into flying bridges, to convey over the cavalry and artillery ; and a slight narrow bridge was made for the infantry with pontoons and casks, the latter being taken from the neighbouring villages.
Captain Fowke, R.E., has lately invented a pontoon which appears likely to supersede those now in use. It is a boat pontoon, the boats consisting of canvas on wexelen frames. The accompanying drawings will explain their form. The advantages claimed for than by their inventor are best explained In his own words (' Royal Engineer Corps Papers,' voL rii.) 1. The buoyancy is nearly double.
2. The weight is less than half.
3. The length on the line of march is lose than half.
4. The first coat is only two-fifths.
5. The bulk, when packed, is only one-seventh. 9. The height of the roadway above the water is increased.
7. The stability is greater, and the tendency of the bridge to oscil• late from aide to side is avoided ; this motion is caused by the ends of the pontoons being alternately immersed or plunged downwards, and will, eateris paribas, always increase as the vertical section of the pontoon approaches the fonn of the solid of least resistance; and as a flat surL-tco offers considerably more resistance than one with a circular or curved section, it follows that, independently of all considerations of buoyancy, the cauraa pontoon will excel in stability from this cause.
8. The liability to damage from shot is diminished, as a grape shot or musket ball, which would make a serious wound in metal, will merely perforate the canvas with a very small round hole.
9. In the case of a pontoon grounding while under a heavy weight, where the metal one would be put quite hors de combat, the worst that would be likely to happen to a canvas one would be the fracture of a frame or two, which would not occasion a leek, and could easily and quickly be repaired.
10. The pontoon can be bailed out easily, even while in bridge ; and this operation is no longer dependent on the small pumps, always so slow in action, and so likely to choke and get out of order.
11. The weight of every individual piece of the bridge is diminished, as well as the number of pieces, and the superstructure is simplified; in forming a bridge, one operation, that of lashing on the saddles (often a difficult one), is entirely avoided.
12. It would appear from the considerations in the last paragraph that each waggon load could be formed into bridge with at least equal celerity as one load of the service equipment, and, if so, there will be a saving of nearly half the time in forming a bridge, eight waggon loads of the canvas pontoon forming as much bridge as fifteen of the service one.
13. The durability is greater : one of the greatest advantages which the canvas pontoon seems to possess over that now in use (as far as can be judged at present) is that while its liability to injury is diminished, in point of durability it appears to be very superior to the latter pontoon, whether on the march, in or in store. Jolting on the march, or rolling over uneven ground, or down a stony beach, is almost certain to start one or more joints of the metal pontoon, slightly perhaps, but sufficiently to admit a small quantity of water, so small as to be disregarded; on being brought out of the water the Pontoon is then stored with its inside and all its delicate framework quite wet, and the process of corrosion thus commenced, tends still further to weaken the joints and render them even more liable to such accidents ; and this accounts for the fact that few, if any, of the present pontoons have ever gone through a summer's practice at Chatham without having to be put at least once into the hands of the timnan for repair. The canvas pontoon, on the contrary, from the elastic and yielding nature of its materials, is lose liable to injury on shore ; running it on piles or rocks has absolutely no permanent effect on it, for a like reason; and on the march it is enclosed in its own defensive armour of hoops, which effectually protects it from injury. As an example of the durability of such a construction, may be cited the case of the fishing boats on the west coast of Galway, which are made of canvas stretched over a wooden skeleton, without any external protection, and which stand all weather out at sea for many years. As to durability in store, I have had an opportunity of very carefully examining eight canvas pontoons in store at Chatham; of these one was constructed in the beginning of 1853, two more in the course of the gaIlle year, and three in the early part of 1855 ; and although they have all been out re peatedly, and the latter frequently, I think it will be found that they are not deteriorated in any respect : the oldest of all would be perhaps the better of a coat of boiled oil, but nothing more is necessary for any of them.
34. The canvas pontoon could without much difficulty be made in the field, such materials as hoops of casks, canvas, poles, and oil, being almost always procurable; and one or more extra waggons may be attached to each bridge-train, carrying nine pontoons each, to be avail able in case of emergency.
15. A single pontoon can be used and pulled as a boat, for laying out arichors, &c., and as it is stable in the water it may be pulled or poled into its pLace In the bridge.
Three pontoons, similar to the one described, have been constructed at Chatham, and subjected to some severe tests as to buoyancy; in one Instance a heavy 32-pounder was put on a raft of two pontoons and the load was increased by men and sandbags to 95 cwt., with which load it was floated out into the 31edway, and it bore it without any apparent strain, and with coneidemble spare buoyancy. The only point in which these pontoons appeared to be inferior to those in use was in the aingle one of speed in rowing, the expression in the report being that they seemed to be too light to row well ; this appears to me to be a matter of such small consideration that I should not have noticed it but for an Idea that seems to prevail that the quicker a pontoon rows the better it will ride at anchor in swift currents or rough water, and which every sailor knows to be fallacious, the fact being that the safe riding depends on the vertical angle of the bow, while the speed is mainly influenced by the horizontal angle or " lines ; " a vessel with a bow that is straight up and down will bury her head much more than one with a "flare bow," that is, one which projects beyond the water line, even though the former may have the superiority in point of speed.
(For many interesting particulars relating to the passage of rivers in military operations, see Sir Howard Douglas, On Military Bridges.)