Gin Lashing ([Fig. 129]).—The three legs of the gin are placed so that the centre one is pointing in the opposite direction to that of the others, all being parallel, the upper ends resting on a skid and two inches apart. The lashing is commenced with a clove-hitch on an outside spar and carried over and under loosely and without riding six times. Two frapping-turns are taken in each interval and the whole finished off with two half-hitches round one of the spars. Iron chain is much better than rope for these fastenings.
Fishing Spars.—This consists in strengthening spars by lashing others parallel to them. The fishing spars should be placed against the spar to be fished so that they may take off as much strain and be in as close contact as possible. Lashing-ropes should then be applied round all of the spars until they are all firmly bound together.
Frapping.—The drawing together the several returns of a lashing so as to tauten it. The rope used for frapping is generally the running-end of the lashing itself; after having frapped, the end of the rope is made fast by two half-hitches, one around all the returns, the other on half only.
BRIDGES.
Weight.—Infantry in column of fours when crowded is allowed per lineal foot 560 pounds.
Cavalry under same conditions is allowed 700 pounds.
Field-artillery in column of sections weighs 400 pounds per lineal foot, so that a bridge built for infantry will carry it.
In calculating the strength of parts of bridges the live load must be doubled to bring it to dead load if weight is to be applied suddenly.
Formula for weight that may be borne safely by a beam supported at both ends and loaded in the centre:
| W = | bd2S | , |
| l |