They permit the passage of troops of all arms, and of the heaviest carriages.
The entrance to and exit from them is easy.
They do not interrupt navigation; and they are not liable to be injured by floating bodies which, either by accident or design, are carried down-stream by the current.
The current should not be less than one yard per second.
To Construct the Raft.—The raft is formed of six pontons. Two pontons are lashed stern to stern, and to these a third, breaking joints. A second set similar to the above are placed at a distance from the first of 26 feet from set to set. The two sets are connected by six balks over which four courses are lashed. Then fifteen balks in a manner suitable for receiving chess. The extreme chess are nailed down, and the outer courses secured by side-rails. The length of the cable should be at least one and a half times the width of the river. One, two, or three anchors are used, depending on the strength of the current. The cable is supported by pontons. The boat nearest the anchor is the largest; the distance between the boats should be such that the cable shall not touch the water between the first boat and the raft; each boat is fitted with a staging, composed of two short balks, and a supporting block, on which the cable rests and to which it is lashed. The cable is also connected with the bow of the boat by a line of such length that the boat is allowed to turn just enough to keep parallel with the raft. After the raft is attached to the cable it is passed from shore to shore once or twice, using a stern veering-line if necessary, until the anchors are firmly imbedded and the cable is stretched; the two abutments are then constructed; these do not differ from the first bay of the ordinary bridge.
The proper angle for the axis of the boat to make with the current is about 55°. This angle is gradually increased on nearing the shore, until the way of the raft is diminished sufficiently to prevent it from striking the abutment with a shock.
Trail Bridges.—When the river is not more than 150 yards wide, a sheer-line may be used in place of the anchor and cable; the sheer-line must be taut enough to keep above water.
If the banks are not high enough, the sheer-line should be elevated at each shore by passing it over a frame formed by three poles, arranged like an artillery gin. Upon this line a pulley is fixed, so that it can run freely from shore to shore; through the eye of the pulley-block a line is passed, one end of which is attached to the bow of the first, and the other to the bow of the second, boat forming the raft. The raft is manœuvred in the same manner as the flying bridge; or one end of a line may be made fast to the running-block on the sheer-line, while the other passes through a snatch-block near the stern of the raft on the up-stream side; by hauling in or letting out this line the proper direction is given to the raft.
Rope-ferries.—The rope-ferry is used when the velocity of the current is not sufficient to propel the raft. It consists of a raft or flat, provided with a standard near each end on the up-stream side. These standards are forked on top to receive the sheer-line, which is stretched across the stream in the same manner as for the trail bridge. The raft is propelled across the stream by men on its deck hauling on the sheer-line.
Prairie Raft.—It frequently occurs in the Western country that expeditions, unaccompanied by regular ponton-trains, are compelled to cross streams so situated that it is impossible to obtain timber or other material suitable for the construction of rafts or bridges. Under these circumstances, a raft may be constructed of two canvas pontons, by means of which loaded wagons may readily be ferried over the stream. All the material required for such a raft is easily carried in one ponton-wagon. The construction is as follows: The wagon to be floated is backed into the stream until the rear wheels stand in about one foot of water. A canvas ponton is placed on each side of the wagon, parallel to and one foot from it. A balk is placed against the tail-board of the wagon, and resting upon the gunwales of the pontons. A second balk is similarly placed against the front-board of the wagon. On each side of the wagon a strong rope is made fast to the front balk, passed under the axle-trees round the rear balk, and thence back to the starting-point, where it is made fast. The raft and wagon are pushed into the stream, and, as soon as the latter is clear of the bottom, the balks are lashed to the gunwales of the pontons. A line is attached to the wagon-pole, and coiled in the bow of one of the pontons. This raft may be conveyed across the stream either by rowing, or in the manner of a trail bridge. On approaching the opposite shore, it should be turned with the wagon-pole toward the bank. As soon as the wagon grounds, the balks are removed and the wagon is drawn on shore by means of the rope attached to its pole. A single hinged canvas ponton, which is readily packed in an ordinary quartermaster wagon, will suffice for the crossing, if the wagons are unloaded and taken apart.