CORDWAY AND ONE-TREE BRIDGE.
One-tree bridge, to make.
In passing extensive marsh tracks, few expedients surpass the so-called American cordway, the subject of the illustration on [p. 325]. It is constructed as follows from the description of material usually abounding in marsh tracks: Trees and poles of almost any description will be found to answer. Cut as many as is thought requisite. Divide them into three classes—ground poles, cross poles, and stringers. The ground poles should be the largest and heaviest. The cross poles are comparatively short lengths, and lay across the ground poles with their ends projecting some distance beyond. They are laid closely together, and then secured and bound down by the stringers which lay on them. A treenail driven in here and there serves to keep all in place by nipping the cross poles tightly. The ends of the ground poles and stringers may be either scarfed and treenailed, or laid side by side and tied with withies or strips of suitable bark. It will be seen, on examining the illustration, that where the roadway ends a bridge begins. This is of the description known as a one-tree bridge, and is made as follows: Select a tree of sufficient length to reach, when felled, across the stream, and of fair average girth, say 9ft.; fell it with the axe in the manner before directed, and then walk out on the trunk and cut away all the branches from the upper surface; then log it up into lengths of about 10ft. a sufficient number of transverse pieces to reach, when placed side by side, from one end of the bridge log to the other. Then in the centre of each of these make a shallow notch by delivering right and left cuts with the axe. Next with your auger bore two holes as wide apart as the diameter of the bridge log will admit of. Lay your transverse piece on the main log, so that it shall rest in the form of a true cross, with the notch in the centre resting on the main log; then, whilst keeping the cross piece steady with one foot, bore down the auger holes about a foot into the solid timber. Treenails (see “[Treenails, to make]”) are then to be placed in the holes and driven home with a mallet or the head of the axe.
TREE-NAILED SWAMP BRIDGE.
Swamp bridge, to make.
Treenails, to make.
Another form of tree-nailed bridge, calculated for very wet or dangerously swampy ground, is formed by laying down two lines of stouter ground logs than those used to form the “cordway” just described, scarfing and tree-nailing the ends together as they are laid down. The transverse bars for the footway have a right and left chip taken out each end from the surface which is to lay next the ground log. They are then bored with the auger, one hole at each end being sufficient. They are then placed closely side by side. One man completes the hole which passes down into the log, whilst another drives home the treenails, as shown in the annexed illustration. Earth or sand thrown with a shovel between the cross pieces increases the stability of the arrangement. Treenails, to make. Treenails have been, and will be, frequently mentioned in the course of this work; it will, therefore, be well here to give directions for making and using them. To the shipwright they are invaluable, as by their aid he unites the various planks and timbers made use of in the carrying out of his art. In England they are usually made from straight-grained oak, which, after being sawn into proper lengths, is split up into the rough form of the required treenail. This, after being faced and hewn with the adze, is passed through a double-handed cutting instrument known as a treenail tool. A skilfully-handled axe, a spokeshave, or a drawing knife, will, although less expeditiously, produce with ease well rounded and serviceable treenails. In wild countries any tough straight-grained wood may be selected for treenail making. For bridge and roadway work pine wood will answer the purpose very fairly. A dead log is best when it can be obtained for making these wooden holdfasts. Cut it up into pieces the length of the proposed treenail, chop off the bark, and split them into either three or four (as directed at [p. 272]), according to the size of the log; then, with the froe or axe, split them into rough squares the length of the required treenail, round off the curves and corners, chop off the edges at the end, so that it may enter the auger hole freely, and the treenail is ready for use.