In 1836 the comparative merits of General Pasley's and General Blanehard's pontoons were tried upon the Medway, when large bodies of infantry and cavalry, besides three pieces of artillery, were made to pass over the river on bridges supported by vessels of the two kinds. The order of march was purposely such as to subject the bridges to the severest strains ; and a superiority of steadiness on this occasion appears to have led to a preference in favour of the pontoons proposed by the last-mentioned officer.
The breadth and depth of one of General Pasley's pontoons are 2 ft. 8 in. The diameter of one of General Blanehard's cylinders is 2 ft. Bin. The length of the former kind of pontoons is 22 ft. ; of the latter, 24 ft. 6 in.
The manner of forming the bridge, with both kinds, is nearly the same. A rectangular frame, whose length is about equal to the breadth of the platform for the intended bridge (12 feet), is laid down longi tudinally on the deck of the canoe, or on the surface of the cylinder, and is kept in its place by rope-lashings ; on the upper surface of this frame, in the direction of its breadth, are nailed pieces of wood in pairs, at equal intervals ; the distance between every two in each pair joist (2Y little more than equal to the breadth of a baulk, or oist (24 inches), one extremity of which is to be received between them, and the number of pairs being equal to the number of baulks which are to support the chesses or planks forming the roadway. A raft is formed with two of these pontoons, by placing them parallel to each other, at a distance, from centre to centre, equal to about 124 feet ; the ends of two baulks, or transoms, as they are called, are made to rest upon the frames before mentioned, the distance between them being equal to the intended breadth of the bridge, and they are kept steady by having near each extremity a hole bored through them, iuto which enters an iron pin fixed vertically for the purpose in the frame ; they are also made fast to the pontoons by ropes passing through rings on the decks. Three or more baulks are then laid down parallel to the transoms, with their extremities confined between the crosspieces nailed to the frames as above said : the chesses are laid close together above them, and their ends are kept clown by ribands, which are attached to the transoms by lashings passing over them, and under the latter, at intervals. In those riband are fixed rowlock pins ; and when the bridge is not formed, the ribands being then placed parallel to the lengths of the pontoons, at the sides of the raft, the latter may be moved on the water by the oars.
When the bridge is to be formed, a certain number of such rafts are rowed to their stations in a ]due across the river, the lengths of the pontoons being parallel to the banks, and there anchored; the distances between the nearest pontoons in two rafts being equal to that between the two pontoons in each raft. Then each raft carrying the materials which are to make a platform over the water between itself and the next, such platform is laid down in a manner similar to that which is employed in laying down the platform of the raft ; and from each of the extreme pontoons a like platform is extended to the shore of the river.
When the banks are favourable, the complement of men attached to each raft of two pontoons—namely, one non-commissioned officer and six privates—can dismount two vessels and their stores from the carriages, launch them, and form the raft in a quarter of an hour. All.
the rafts being put together at the same time, the whole bridge may be formed in another quarter of an hour. After the passage has been effected, the bridge can be dismantled in eight minutes ; the rafts can then be taken to pieces, and the vessels and stores re-packed on the carriages in a quarter of an hour.
In order to convey the pontoons with an army, each is placed, with all its accompaniments, on a four-wheeled carriage. General Pasley proposes to use only a cart with two whdels for each pontoon, the latter being separated into two demi-pontoons, which are placed side by side above their stores. The shortness of these carriages enables them to be turned within a smaller space than would be required for a four-wheeled waggon ; but the weight on the shaft-horse in hilly countries would be so great that it would be hardly practicable.
The inconvenience attending the transport of pontoons, or the necessity of passing a river when they are not at hand, has induced commanders of armies to have recourse to bridges supported on rafts of timber or on empty casks, which in Europe can always be procured. Each raft may consist of four or more rough trunks of trees lashed together, and may be retained in its place by anchors, or by being mado fast to a rope stretched across the river : the baulks and chesses are applied as in a pontoon bridge. Perhaps the greatest raft-bridge ever formed is that which was executed by General Sokolniki over the Niemen at Grodno, in 1792. Trunks of trees from 50 to 60 feet long, and from 20 to 28 inches in diameter, were united by tens, and every ten were bound togdther at both ends by transverse braces, so as to form a raft, which, as all the large ends of the trees were placed con tiguously, resembled the voussoir of a bridge. All the voussoirs, 57 in number, were placed in the river, side by side, in an arc of a circle having its convexity towards the upper part of the river ; and a sort of key-voussoir was formed by a vessel 116 feet long and 22 feet wide, which was moored in the river where tho current was the most rapid. Baulka and planks were 'placed, as usual, above the rafts to form the road. The breadth of the river at that place is about 1030 feet. (Berard, • Equilibre des Votttcs.') A strong raft-bridge was formed by the Russians and Prussians over the Elbe, near Dresden, in 1813: one course of logs disposed parallel wise to each other formed the bottom of each raft ; cross-timbers were laid above these ; and over them was a third course of timbers, each placed above the interval between two of those below it. The timbers were well lashed together ; a strong cable was stretched across the river on each side of the bridge, which was further retained in its place by anchors, and a road was formed of planks as usual. While the British army was serving in Spain during the Peninsular war, the roofs of buildings were occasionally taken to form rafts : the rafters served as baulks ; and, for want of nails, the planks were kept down _ _ )o the timbers by ribends formed of young trees split in two ongittulinally and tied by willow twigs.