44
PASSAGE OF RIVERS.
2 ndly, with a Sextant , Compass , or TheodoUte.
The lines may be laid down as in fig. 1, taking care to make the angle b a x =b d o, and the calculations may be made as before; or measuring a b as a base,and ascertaining the angles x a b and x b a, the distance a x may be found bytrigonometry.
10. To find the weight which any floating body will support, first calculate thenumber of cubic feet of water displaced by it, which for solid timber and air-tightcases may be considered equal to the entire bulk; but for open boats, which cannotbe safely immersed lower than within 9 inches of their upper surfaces, the bulk belowthat level can only be calculated on. Multiply the number of cubic feet of water dis-placed by 1000 for fresh water, and by 1026 for salt water, which will give the weightin ounces: from this must be deducted the weight of the body itself with the super-structure, and the difference will give the weight which can be supported. Theweight in ounces of the materials is found by calculating their cubic feet, and multi-plying that by their specific gravity, which for oak is 950, for elm 670, and for malefir 580; but it must be remembered, that if timber be not tarred, its weight will beincreased Jth by imbibing water after being immersed two or three days. The weightpressing upon a bridge during the passage of infantry four deep would be that of 20men, or about 3600 tbs., on a length of 12 J feet. Two cavalry horses abreast, withtheir riders leading them, would occupy 12 feet, and the total weight would be about2640 lbs. A medium 12-pr. brass gun and its limber occupies 14 feet without thehorses, and its weight is about 5000 lbs. including the limber and ammunition. Abrass 9-pr. or 24-pr. brass howitzer weighs about 4200ibs., and a light 6-pr. or 12-pr.howitzer about 3000 lbs. (For the weights of Ordnance and Carriages, see the firstvolume.)
In crossing temporary bridges, cavalry should always dismount, and infantry shouldnever be allowed to keep step : cattle should be driven over in very small numbers ata time.
TEMPORARY BRIDGES.
11. The following resources for facilitating the passage of troops across rivers areadded to the article ‘ Bridge, Field.’
Large trees may be felled to enable infantry to cross a narrow stream, placingthem so that their butts may rest upon the banks with the tops directed obliquelyup the stream: if one is not long enough, others may be floated down so as toextend across, being guided and secured by ropes: a footway may be formed bylaying planks or fascines or hurdles over them, and their branches should bechopped off nearly to the level of the water, and interlaced below; poles also maybe driven into the bed of the river, to aid in supporting the trees by attaching theboughs to them.
12. Wheel carriages may be used to form a foot-bridge, if the river is not toodeep, being pushed or hauled into the stream, and connected by beams; or a singlepair of wheels, with an axle-tree to admit two strong posts, may be attached andplaced in the centre of the stream, if not too wide, and poles reaching from eachbank may be secured to the posts, and the wheels would act as a trestle: with aflooring over the poles, a slight bridge could rapidly be constructed.
13. A Wicker Bridge.* —“ The bridge across the Zab at Ligan is of basket-work;stakes are firmly fastened together with twigs, forming a long hurdle, reaching from