PART X. BRIDGES, AND PONTOONS.
Reading edition
BRIDGES.
To find the number of planks required to form a float, to support a given weight.
1st. Find the content of one plank (vide Practical Geometry, Part 12), and multiply it by the specific gravity of the wood; the product will be the weight of the timber.
2nd. Multiply the same solid content by the specific gravity of water: the product will be the weight of an equal bulk of water.
Then take the difference of these two products, or weight, and it will be the weight one piece of timber will support without sinking. Hence by Proportion, the number required to support the given weight may be found.
To find the number of casks required to form a raft to support a given weight.
1st. Find the solid content of one cask in cubic inches (vide Practical Geometry), and multiply it by the specific gravity of water; the product will be the weight of a quantity of water of equal bulk with the cask.
2nd. From this product, or weight, subtract the weight of the cask, and the remainder will be the weight it will support without sinking. Then by Proportion, the number required for the formation of the raft may be found.
To find the number of boats, or pontoons, required to support a given weight.
The burthen a boat, or pontoon, will support without sinking beyond a given depth (the form of the boat, or pontoon being known) must first be found, thus—
1st. Find the solid content of the part to be sunk, in cubic feet (vide Practical Geometry, Part 12), and multiply it by the specific gravity of water (vide Gravity, Part 12).
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.
At close order the bridge will pass any part of a heavy train.
Dimensions, and weight of cylindrical pontoons, manufactured in the arsenal at Woolwich.
Extreme length. Diameter. Weight.
ft. in. ft. in. cwt.
Large Pontoon,
with hemispherical ends 22 3 2 8 4
Small Pontoon,
with conical ends 15 0 1 8 1¼
ONE CARRIAGE LOAD.
Dimensions.
Number. Length.* Breadth. Depth. Weight.
ft. in. ft. in. ft. in. cwt. qrs. lb.
Pontoons 2 22 2 7 2 7 9 1 9
Baulks 12 14 2 5 3 5 1 1
Chesses 8 12 1 2 1½ 1 3 19
Half-chesses 4 11 5 1½ 1 3 0
Gunnels 2 12 1 2 4 1 3 19
Side-pieces 5 10 4 5 2½ 1 1 0
Anchor 1 3 1
Buoy 1 3
Cable (20 fathoms) 1 168 3 3 1 18
Oars 7 14 3 14
Boat-hook 1 14 7
{ Body 3 8 9 4 20
Lashings { Gunnel 4 15 1 1 } 8
{ Carriage 4 22 1 1 }
Rack-sticks, and lashings 8 14
Buoy-line 1 60 1½ 1½ 5
Breast-line 1 60 1½ 1½ 5
31 1 0
Weight of Carriage 1 13 3 20
Total 45 0 20
* The length includes the hemispherical ends.
2.—SMALL PONTOONS.
Five pontoons, with their appurtenances, form the load of one carriage. Length, 15 feet; diameter, 1 foot 7 inches; displacement, 27½ cubic feet; buoyancy, 1718 lb., or 15¼ cwt., from which deduct 2 cwt. for the weight of pontoon, and share of superstructure. A bridge of this nature is so light that it may be made on shore, and carried by hand entire. It will support as many men as can be placed on it, and, by removing the chesses over the gunnels, it may be bent so as to be passed without difficulty down a steep bank, or counterscarp.
ONE CARRIAGE LOAD.
Dimensions.
Number. Length.* Breadth. Depth. Weight.
ft. in. ft. in. ft. in. cwt. qrs. lb.
Pontoons 5 15 1 7 1 7 5 0 0
Gunnels 5 8 4 9 3 3 12
Baulks 30 6 6 3 1½ 1 1 9
Chesses 20 8 1 4 1½ 4 0 20
Paddles, used as side-pieces 5 1 27
Rack-sticks, and lashings 10 6
Carriage lashings 2 5
Anchor, and cable 1 2 0
12 1 23
Weight of the Carriage 1 9 0 0
Total 21 1 23
* The length includes the ends.
SCALING LADDERS.
Scaling ladders are made in portions, 12 feet, and 7½ feet long; which are joined together by placing the end of one portion into staples at the end of another, and securing them together with a lashing of rope. Four men are sufficient to carry an 18 feet scaling ladder.