One interesting thing to notice is the form of the "baulks," in which connection I would like to remark that when I use the word without inverted commas I mean it in the ordinary sense as implying a big heavy timber, but when I use the commas I mean it in its technical sense as it is used in military engineering. In this latter sense it describes the timbers specially provided for the purposes just described. Large supplies of the ordinary heavy baulks could not be carried with an army: but strength is required nevertheless. Hence the military engineers have invented a form which combines strength with lightness.
If you stand a plank upon its edge, supported at each end so as to form a beam, its strength will vary as its width and as the square of its height. If then you double its width you only double its strength, but if you double its height you multiply its strength four times. If you halve the width of a given beam you halve its strength, but if you then double its height you quadruple that half, in other words, without
making the beam any heavier by these two operations you double its strength. Moreover, if you support a beam at each end and pass a load over it or spread a load permanently upon it, its greatest strength is required in the middle. You can shave away the ends without making the beam as a whole any less strong. So these "baulks" are made like planks, very oblong if looked at endwise, also thinner at the ends than in the middle. But if by chance they tipped over on to their sides they would for that very reason be very weak, and that is why the notches are provided in the transomes and the centre beams of the pontoons, in order that the "baulks," having been laid edgewise in them, cannot tip over. Thus a considerable saving is made in the weight of the bridging material to be carried.
It sometimes happens that when a trestle is dropped into the water one leg will fall into a depression in the river-bed or will sink more deeply if the bed be soft, leaving the whole structure lop-sided and useless. That, however, is easily overcome, since it is provided against. A little iron bracket, which is carried for the purpose, is clipped on to the leg which has sunk near its top and on to it is hung a pair of pulley blocks—one of those little contrivances which everyone has seen at some time or another by which one man pulling a chain quickly can raise, although slowly, a heavy load. By this means the end of the transome is raised until it is horizontal and the legs have assumed an upright posture, when the transome is refastened to the leg in its new position. Thus we see the advantage of clamping the transome
to the leg rather than fixing it with any arrangement of holes. The iron band, which is fastened on to the transome and which grasps the leg, is so arranged that the greater the load the more tightly does it hold, so that it is perfectly safe under all conditions.
The trestle bridge has a great advantage over the floating bridge if the height of the water varies at all, as for instance, with the tide. The former remains still, while the latter goes up and down, requiring a special arrangement to be contrived for connecting it to the shore.
Under some conditions a suspension bridge is the most convenient form of all, particularly if the banks are high and strong, or if the current be very rapid or the river-bed very soft. In such cases steel wire ropes are stretched across the water between two trestles. The latter may be made in the way just described, but more often they have to be stronger and are built specially out of big strong timbers securely fastened together. Their form does not matter much so long as they are strong and stiff, high enough to carry the ends of the suspension ropes and of such a shape as not to block the entrance to the bridge itself. The higher they are the better, because, according to the natural laws which govern such things, the more sag or dip there is in the ropes across the river the less severely will they be strained. They need to be very strong, as the whole weight of the bridge and its load falls upon their shoulders. The pull of the suspension ropes, moreover, tends to pull them forward into the water, so they must be held back by other strong ropes called guys, and the action of
these two sets of ropes entails the unfortunate trestles bearing really more weight than the actual weight of the bridge and load. The guys, too, require very strong anchorage or at the critical moment they may give way, when the whole contrivance, with possibly valuable guns or ammunition on board, will be precipitated into the water. The men may be able to swim but the guns will sink.
Having, then, constructed a trestle upon each bank, securely guyed it back and connected the suspension ropes to it, the next operation is to attach smaller vertical ropes to the suspension ropes at intervals, to support the ends of the transomes. Then upon the latter are laid "baulks" and upon them the flooring as usual. Or if ropes be not sufficiently plentiful, timbers may be lashed on to the suspension ropes instead, the transomes being fastened to them.
That is all that is absolutely essential to a suspension bridge, but one so formed would be rather flimsy and unstable. It needs to be stiffened by diagonal timbers at suitable places and often it has props placed upon the bank reaching out as far as their length will permit over the water to steady and consolidate what to commence with is rather too much like a spider's web. Those little strengthening dodges can be laid down in no books. They need to be left to the judgment of the men in charge to do what is necessary in the best way they can with the materials which happen to be at hand.