Part 5 (1/2)
There is still another way of making the bridge, and that is to put it together alongside the bank, afterwards swinging it across the river like the opening or shutting of a door. Anyone can see that there must be many advantages in this latter method when it is practicable, since more men can work at once and with greater safety, for all will be near the bank.
It is evident that such a structure depends for its security entirely upon the anchors. Those which are carried for the purpose are like those of a s.h.i.+p but there may not be enough or they may not suit every kind of river-bed. They are often improvised therefore. Two wagon wheels lashed together, with heavy stones clipped between them, are said to be a very effective anchor. Under certain conditions a net filled with stones is surprisingly effective. Two pickaxes tied together form a good imitation of the conventional anchor, as also does a harrow sunk and held down by stones thrown upon it.
Trestle bridges are made in quite a different way. The trestles are formed of two legs or uprights with a transome between, a shape which resembles, as has been already remarked, a very robust Rugby goal. The transome is connected to the legs by a special form of band which permits it to be fixed at any height without having to drill any special holes for the connections. The legs are so shaped at their ends that they can be shod with steel shoes provided for the purpose, enabling them to get a good foothold even on s.h.i.+fty soil. The trestles are put together ash.o.r.e, and each is taken out in a boat or on a pontoon to the place where it is to stand. Then it is launched feet foremost into the water, the boat being on the side away from the sh.o.r.e, so that a rope from the trestle to the sh.o.r.e will enable men on land to pull the trestle into an upright position.
[Ill.u.s.tration: AN INCIDENT AT LOOS.
This picture gives us some little idea of the devastation caused by modern weapons. It also shows the inventiveness of the soldier who makes his rifle into a battering-ram. Incidentally we see a kind-hearted soldier rescuing a little girl from danger. This incident really happened.]
Thus trestle after trestle is added until the bridge has grown right across the water to the further bank. The trestles cannot fall over sideways because of their own width, they cannot fall forwards or backwards because of the ”baulks” which pa.s.s between them and carry the floor, but as a precaution diagonal ties of rope are always added here and there along the bridge, that is to say, two trestles are tied together with two ropes, each rope pa.s.sing from the bottom of one trestle to the top of the other, a form of tying which is very effective and very easy and simple to carry out.
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 pa.s.s 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 a.s.sumed 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 sh.o.r.e.
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.
But very often warfare has to be carried on in the most outlandish places where armies can only travel light, and where, hampered by bridging material of the conventional sort, they would have no chance in catching up with a fleet and agile native enemy. Yet bridges are needed even more under those conditions perhaps than under any other.
There are many examples of this in the wars just beyond the frontier in Northern India. Then ingenuity has to make good the luck of prepared material and the bridges are made of those materials which happen to be procurable.
An army in India once wanted to cross a river, where no materials of the ordinary kind were available. The river, however, was lined with tall reeds. A reed has for centuries been a favourite example of weakness and untrustworthiness, so how can reeds be made to form a safe bridge? This is how it was done.
Great quant.i.ties of reeds were cut and were made up into neat round bundles about a foot in diameter. Ropes were scarce too, but these likewise were improvised by twisting long gra.s.ses into ropes. It is surprising what good ones can be made in this way, and they served their purpose well. Many bundles having thus been made numbers of them were tied together so as to form rafts. Each bundle in fact was a small pontoon, and the rafts which were thus const.i.tuted differed only in size from the regulation rafts made of pontoons.
While this work was being done two ropes were got across the river and secured on both banks: then rafts were floated down in succession, each one on arrival being tied up under the two ropes. Finally a track of boards was laid over the centre and the bridge was strong enough for men in fours to walk over it.
Had it been necessary, the floor could have been made of brushwood, interlaced so as to form a kind of continuous matting or of a layer of branches covered with canvas. Floors for bridges can be made in many ways.
A dodge which soldiers in the British Army are taught is how to make boats for bridging purposes out of a tarpaulin or piece of canvas, supported on a framework of light wood poles or twigs. The outline of the boat is first drawn roughly on the ground. Then three posts are driven in on the centre line of the boat and to the top of these three a horizontal pole is tied, thin, flexible branches stripped of their bark, being fixed by having their ends stuck in the ground on either side. The ends are driven in on the outline already marked out so that when done the branches form a framework like the ribs of a boat upside down. Other branches are intertwined among these so as to bind them together and finally a tarpaulin or canvas sheet is laid over all. A number of boats formed after this fas.h.i.+on can be used as pontoons to support a bridge, or several can be made into a raft and towed to and fro--a sort of floating bridge.
Another scheme is to make a number of crates like those in which crockery and other things are often packed. These are of very simple and easy construction, consisting of sticks slightly pointed at the ends driven into other pieces which are perforated with suitable holes to receive the ends. The only tools necessary are an axe (or even a pocket-knife will do) to sharpen the ends and an auger to make the holes. Almost any sort of wood can be made to serve. The cover for this, and indeed for most of these improvised rafts, is tarpaulin or canvas, the latter of which, being the material used for so many purposes, is almost sure to be available in some form or other.
For instance, every one of those familiar ”General Service Wagons” has its large canvas cover. In fact, a general service wagon, taken off its wheels and wrapped up in its own canvas cover, makes quite a serviceable boat, pontoon, punt, barge or whatever you like to call it.
Then there is an ingenious type of little bridge which can be quickly and easily made where bamboos or similar light canes or sticks are available. The only tool required in making this is a couple of poles ten feet or so in length. To commence with, these poles are laid side by side upon the bank with one end of each pointed out over the water, overhanging it by about four feet. Two men then climb along these, while others sit upon the insh.o.r.e ends to keep them from tipping into the water.
Seated, then, on the outer ends of the poles the men drive some bamboos or whatever they are using into the water, after which they tie a crosspiece to the uprights, so forming a light trestle. Then the poles are pushed forward until they overhang another four feet beyond the trestle just made, the other men, of course, continuing to sit upon the rear ends. And so the bridge grows until it entirely crosses the stream.
Between the trestles other light poles are laid and tied, forming the floor upon which men can cross in single file.