2nd. Subtract this product from the weight of the boat, or pontoon, and the remainder will be the burthen it will support without sinking beyond the required depth.
Then by Proportion, the number required to support the given weight may be computed.
Note.—In the construction of bridges, should a rope require to be extended across a rapid river, the coil should be placed in the boat (instead of on shore) and be paid out as the boat advances.
PONTOONS.
Those called Blanshard’s (from their inventor, Colonel Blanshard, Royal Engineers) are of two descriptions.
1.—LARGE PONTOONS.
Displacement of water, 97½ cubic feet, equals 6088 lb., or 54½ cwt.
The buoyant power of a raft of two pontoons, its own weight deducted, is 77 cwt., about one-half of which is a safe load. Each raft, or one carriage load, forms 2 bays, or 20 ft. 8 in. of bridge: its own weight will sink it about 7 or 8 inches. The crew of a raft consists of 6 rowers, and 1 steersman.
At open order the bridge will pass cavalry, field artillery, or infantry, with closed files.