CHAPTER V.
EXTEMPORE BRIDGES AND MAKESHIFTS FOR CROSSING
RIVERS OR RAVINES.
The solitary traveller in a wild country will be very rarely compelled to construct his own bridge, for, as a general rule, he will only have to pass once, or at most to return by the same route. The labour of making a bridge would be greater, and more time would be lost, than by seeking for a practicable passage at some distant point, or, in case a river was the impediment, forming a float of some kind.
Swamp roads, to make.
There are, however, occasions when there is no alternative but bridge-making, as when exploring expeditions, accompanied by pack animals, or a field force on the march, have rivers, swamps, ravines, or, perchance, rotten ice, to pass over. Where there is not water enough to float a canoe, but where there is sufficient to cause the formation of deep pools and dangerous mire, over which few animals used for the conveyance of baggage could pass without the aid of some artificial footway, narrow deep channels may be very often rendered comparatively easy to cross by filling them up with bundles of brushwood or marsh reeds. We were constantly in the habit, when engaged in making forced marches through Central India, of making use of the stalks of the recently cut juhari for this purpose. Unsafe ice can be rendered firm and secure by strewing a thick layer of reeds over it, and then throwing water enough to cause the whole to freeze into a compact mass.
Before, however, proceeding to describe the various modes usually had recourse to for rendering trees available for bridging purposes, it will be well to give a few plain and practical directions for ascertaining the width of rivers, ravines, and the arms of swamps, without the aid of scientific instruments, and also for finding, by makeshift modes, the altitude of trees.
To find the Width of a River without Instruments.
Fig. 1. If you have a pocket compass, and the river runs, say east and west, and you are on the south side, choose a well-defined tree, A, or other object on the opposite shore, and bring it to bear north of you; mark your position by putting in a stake or peg, B, turn to one side, say the left, and walk westward till A bears north-east, which will be the case at C; then C B will be exactly equal to B A, or the breadth of the river, because from the point C, A will bear north-east, and B will bear east, subtending an angle of 45°, and as the line C B is east, and B A is north, they subtend an angle of 90°, or a right angle, and must be of equal length; the triangle you have formed being the half of a square, divided by a diagonal line from corner to corner.
If you have room repeat this by walking east till A bears north-west from D; and if the first operation has been correctly performed, the second will confirm it; or if the first be in error, it is likely that the second will be exactly as much in error the opposite way, and the mean of the two observations will be approximately correct.
