It is doubtful whether much use is made nowadays of permanent mines of the types just described, for they have, no doubt, been largely displaced by the temporary mine which can be laid in a moment by simply being dropping overboard from a ship, but it is quite possible that some of the defences of, say, the Dardanelles, were of the permanent nature.

So let us pa.s.s on to the temporary mines. These were used by the Germans from the first few hours of the war. One of the first naval incidents was when our ships discovered a small German excursion steamer which had been converted into a mine-layer strewing these deadly things surrept.i.tiously in the North Sea in the hope that some of our vessels would run upon them. Needless to say, that ship went on no more excursions.

Laid thus, it is evident that there can be no wires running ash.o.r.e, so that all mines of this cla.s.s must be contact mines. What makes them of extreme interest is the way they are laid. Just think for a moment what is involved. From the very nature of things their laying must often be done in secret. It is not the British practice to place them in the open seas, except avowedly, after due notice, in certain specified areas, where they are laid quite openly under the protection of adequate forces to ensure against interruption. There is little doubt, however, that they have laid many a mine field secretly in purely German waters, while everyone knows that the Germans have not hesitated to sow the shipping routes broadcast with these things, such work of course being done secretly and largely at night.

The mine can therefore only be laid by dropping it into the water and leaving it. Yet it must not float on the surface or it will be easily seen and picked up; it must float below, so that the unsuspecting ship may run upon it. And it is quite impossible to make a thing float in water anywhere except upon the surface. If it does not float upon the surface it sinks to the bottom: there is no "half-way house" between.

Many people are surprised to hear this, judging, no doubt, by the fact that a balloon floats _in_, and not on, the air and expecting an object floating in water to be able to do the same thing. The difference is due to the fact that air is easily compressible, so that the air close to the earth is denser, more compressed, and therefore heavier, than the air higher up owing to its having the whole weight of the upper air pressing downwards upon it. The density of the air diminishes, for this reason, as one ascends, and a balloon which displaces more than its own weight of air at the surface of the earth rises until it has reached just that height when the air displaced exactly equals in weight the balloon itself: then it goes no higher.

Precisely the same conditions exist in the sea except that water being incompressible is no denser at the bottom of the sea than on the surface. Therefore, if a thing sinks at all it sinks right to the bottom.

There is one very ingenious device for overcoming this difficulty by means of a motor and propeller. The mine has enclosed in its case a motor driven by a store of compressed air which operates a propeller. In this it is somewhat like a torpedo, but in this case the propeller is set vertically so that its action lifts the mine up in the water. Now the mine is so weighted that it just and only just sinks when dropped in, but on reaching a certain depth the motor starts and by means of the propeller raises it nearly to the surface again. On nearing the surface the motor stops and the mine sinks once more, only to be raised again in due course, so that the thing keeps on rising and falling; it never rises above a certain depth nor falls below a certain depth, but oscillates continually between its two limits.

The question then arises, what starts and stops the motor at precisely the right moments to produce this result? It is done by means of a hydrostatic valve. As just pointed out, the water at the bottom of the sea is supporting the weight of all that water which is above it. The water is not compressed by this, but the pressure is there all the same.

Obviously the degree of pressure at any point depends upon the weight of the layer of water above, and since the weight of that layer will obviously increase and diminish with its thickness it follows that, starting from the surface, where the pressure is nil, we get a perfectly steady and regular increase as we descend, until we reach the maximum at the bottom. Now within the mine is a small watertight diaphragm, the outer surface of which is in contact with the water and upon which, therefore, the water presses. As the mine descends, therefore, this diaphragm is bent inwards more and more by the pressure of water and that is made to start the motor. Adjustments can easily be made so that a certain degree of bending shall result in starting the motor, which is the same as saying that the motor shall start automatically at a certain depth. Likewise as the mine rises under the influence of the propeller the pressure decreases, the diaphragm straightens out and at a certain predetermined depth the motor is stopped.

When, finally, the store of motive power is exhausted the mine sinks to the bottom and is lost, a very valuable feature from a humanitarian point of view, since it means that the active life of the mine is short and it cannot go straying about the oceans for weeks or even months, finally blowing up some quite innocent pa.s.senger ship.

More often, however, this difficulty of depth is overcome by anchoring the mine at the depth most suitable for striking the bottom of a pa.s.sing ship. But here again there seem to be insuperable difficulties, for the depth of the sea varies and so the length of the anchor rope must be varied with almost every mine that is laid. It has been found possible, however, to make the mines automatically adjust the length of their own anchor ropes so that the desired result is attained without difficulty no matter how deep the sea may be. Let me describe how it is done in the Elia mines used by Great Britain. The inventor, Captain Elia, was an officer in the Italian Navy.

The mine consists of three parts: (1) the mine proper, a case containing the explosive, gun-cotton and the firing mechanism; (2) the anchor; and (3) the weight, all of which are connected together by suitable wire ropes.

The mine is lighter than water and so floats: the anchor, which bears no resemblance to the ordinary anchor but which is an iron case containing mechanism, only able to act as an anchor by virtue of its weight, is heavier than water and so sinks, while the weight of solid cast iron sinks more readily still.

The anchor is often fitted with wheels so that it forms a truck upon which the mine and the weight are placed, the whole running upon rails laid on the deck of the mine-layer. As this ship steams ahead the men push the mines along the rails, dropping them over the stern at regular intervals.

When the thing reaches the water, the weight sinks the most rapidly, thereby tugging at the chain whereby it is connected to the anchor. The latter, being less compact, sinks more slowly so that the pull upon the rope is maintained until at last the weight rests upon the bottom. _Then and only then is the pull relaxed._ Now inside the anchor is a winch, upon which is wound a length of flexible wire rope, the other end of which is attached to the mine. The latter, it will be remembered, is light enough to float and so, since it lies upon the surface while the anchor sinks, the rope is drawn off the winch. But there is a spring catch which is able to hold the winch and to prevent it from paying out rope, and that catch is only held off by the pull of the weight.

Consequently, as soon as the weight touches the bottom and its pull upon the anchor ceases, the winch is gripped by the catch, no more rope is paid out, and from that moment, as the anchor descends, it drags the mine down with it.

The result, then, is that the mine becomes anch.o.r.ed at a depth below the surface roughly equal to the length of the rope connecting weight to anchor.

Mines of this kind can, of course, be fired electrically by the tilting of a cup of mercury or similar device as already described. Another arrangement is to fit projecting horns upon the surface of the mine made of soft metal so that they will be bent or crushed by a strong blow such as a pa.s.sing ship would give. This breaks a gla.s.s vessel inside, liberating chemicals which cause detonation.

The method adopted in the Elia mines is to have a projecting arm pivoted upon the top of the mine. The mine is spherical (they are nearly all either spherical or cylindrical), with the rope attached to the South Pole, so to speak, and the arm pivoted to the North Pole. As the mine floats in the water the arm projects out horizontally. The effect of this arrangement is that when a ship strikes the mine the latter rolls along its side, but the arm being too long, simply trails along. Thus the spherical case of the mine turns while the arm remains still and that is made to unscrew and eventually release a hammer which, striking the detonator, fires the mine.

In other words, this type of mine is exploded not by the ship giving it a blow, but by its rubbing itself along in contact with the mine. The great advantage of this is that it is only a ship that can do this. No chance commotion in the water can do it: no chance blow from floating wreckage can do it: only the rubbing action of a ship can accomplish it.

Such a mine, too, is less likely to be affected by counter-mining, of which more presently.

Apparently the laying of these mines must be very dangerous work, for since a blow will explode most of them, what is to prevent their receiving that blow while on the deck of the mine-layer, or at all events as they are dropped into the water.

In all cases, precautions are taken against such an event. Sometimes a hydrostatic valve is employed, the arrangement being that the firing mechanism is locked until released by the valve, until, that is, the mine is immersed to a predetermined depth in the water.

Another device for the same purpose is a lump of sugar. The mine is so made that it cannot be fired until this lump has been melted by the action of the water: sal ammoniac is another substance employed for the same purpose. The technical term for this is a "soluble seal." The firing arrangement, whatever it may be, is sealed up so that it cannot come into operation until the seal has been dissolved away by the water, or until the mine has been in the water long enough for the mine-layer to get out of harm's way.

Another interesting feature of the Elia mine is connected with the source of the power which drives the hammer which causes the explosion.

The anchor, it will be remembered, pulls the mine down under water, the latter being of itself buoyant. There is a continual pull, therefore, upon the rope by which the mine is held under. It is that pull which works the hammer.

And now observe the beautiful result of that simple arrangement. Suppose the mine breaks its rope and gets loose, so that it can drift about and carry danger far and wide. It can break loose and it can drift about, but at the very moment of getting loose the danger vanishes, for the rope ceases to pull and the firing mechanism loses its motive power.

In other mines the same result has been sought by means of clockwork, which throws the firing arrangements out of action after the lapse of a given time. This scheme of Captain Elia's, however, whereby the very act of breaking adrift produces its own safeguard, is one of the most delightful instances of a happy invention.

In conclusion, just a word about the measures taken against mines.

Counter-mining is one. It consists in letting off other mines in the midst of a mine-field with the purpose of giving them such a shaking up that some of them will be exploded by the shock.

The simplest and indeed the only effective way, however, seems to be the simple primitive method of dragging a rope along between two light draught vessels and thus tearing the mines up by their roots, so to speak. The very act of thus dragging it along by its anchor rope often causes a mine to explode, well astern of the mine-sweeping vessels, but sometimes they are pulled up and fired or sunk by a shot from a gun which the sweeper carries for the purpose.

The sweeping up of the mine-fields is a duty often allotted to the steam fishing boats or trawlers, whose crews seem particularly well fitted for the work. It is a hazardous duty, and many lives have been lost through it. Let us hope that in time to come all submarine mines and the dangers connected with them will be a thing of the past, for they are mean, cowardly and contemptible weapons.

CHAPTER VI

MILITARY BRIDGES

Bridging has always been an important part of actual warfare. In my school days I studied "Caesar" from a textbook which is not much in use nowadays and which had very copious notes, prominent among which was a description, with drawings, of a bridge made by the Roman Legions in Gaul. And a fine bridge it was, too. How its details came to be known was partly through the description given by Caesar himself and partly by a study of certain old timbers found in the bed of the Rhone, which timbers were believed to be relics of the very bridge which the great Julius himself had had built.

This bridge of nearly two thousand years ago appeared to be built of baulks of timber fastened together in very much the same manner as that adopted by the engineering units of the great armies of to-day.

Every observant person has noticed how tall poles and short sticks tied together with ropes can be fashioned into the firm, strong scaffolding from which workmen can in safety raise great tall buildings. That mode of construction can always be used to form a bridge.

Equally well known, no doubt, are the gantries built over the footway while a large building is in course of construction. Generally of huge square baulks of timber, they are intended to carry very heavy loads of materials and to save the public pa.s.sing beneath from any possibility of damage through heavy objects falling from above. Those gantries furnish us with an example of another sort of construction in wood which can be and is often used in bridging.

When the Germans retired in Northern France they blew up all bridges behind them, and before the Allies could use those bridges they had to repair them. If only for foot-traffic, a contrivance of poles, lashed together after the manner of the builder's scaffold, is ample in most of such cases and by its means a strong and safe bridge can be made upon what is left of the old bridge in the course of a few hours. For light vehicles a similar structure but made stronger by more lashings and of poles closer together will suffice, but for heavy traffic, with guns and possibly railway trains, recourse has to be had to the heavy timberwork exemplified by the builder's gantry. This takes longer to make, since the timbers are big, heavy and not easy to move about: they are, moreover, not simply laid beside or across each other and tied, but are cut the right lengths, and one is notched where the end of another fits into or against it. The baulks are connected by bolts and nuts for which holes have to be drilled or by rods of iron with a sharply pointed p.r.o.ng on each end stretching across from one baulk to another, one p.r.o.ng being driven into each.

With the long-thought-out military operations of modern warfare it is just possible that steelwork for repairing certain particular bridges might be prepared in advance and simply launched across when the time arrives, but that is manifestly impossible except in certain cases and under particularly favourable conditions, such as railway facilities for bringing up the new bridge close to the site where it is to go.

Nearly every military bridge therefore has to be more or less improvised on the spot. In a highly developed country scaffold poles or baulks may be found or brought up by road or rail, in less civilised lands their equivalents may be cut and prepared from neighbouring forests, but all armies have, as a recognised part of their organisation, certain engineering "field companies," and "bridging trains," which carry with them large quant.i.ties of material carefully schemed out long in advance, so shaped and so prepared that it can be fashioned into almost anything, much as the strips of a boy's "meccano" can be adapted to form a great variety of objects.

First, there are pontoons, large though light boats or punts, about 20 feet long, constructed of thin wood with canvas cemented all over to give additional strength and water-tightness. Each pontoon rides upon its own carriage upon which there are also stowed away quant.i.ties of timbers of various sorts, anchors for holding the pontoons in place, oars for rowing them, ropes of different kinds, and so on.

Each pontoon, moreover, is divided about the middle into two pieces called respectively the bow piece and the stern piece. The two are normally coupled together by cunningly devised fastenings but they can be quickly separated, in which state they form two shorter boats.

Other carriages carry more timber and material intended for the purpose of forming "trestle bridges" but which is also usable in connection with the pontoons.

Of this material the chief sorts are "legs," long straight pieces which form the uprights; transomes, heavier beams which can be fitted across horizontally between two legs so that the three form a huge letter H or a very robust Rugby goal; "baulks" which are light timbers tapered off towards each end for the sake of lightness and of such size that they fit snugly into notches which are cut in the upper surface of the transomes; and planks called "chesses" for forming the floors of a bridge.

Probably the most dramatic incident of the war was when the British, having been apparently beaten by the Turks in Mesopotamia, driven far back and their General and many troops captured, suddenly turned the tables upon their enemies, driving them from Kut and sending them fleeing helter-skelter to Bagdad and then beyond. Now the capture of Kut and then of Bagdad were both made possible by the rapid bridging of the Tigris, and without doubt this is the sort of material which was used.

Let us see how it is done.

An army arrives at a river across which it is decided to throw a pontoon bridge. The pontoons are unloaded off their wagons and launched into the water. One is rowed out and anch.o.r.ed a little way from the sh.o.r.e, while upon the bank parallel with the river is laid a "transome." On the centre of the pontoon is a centre beam with notches in it like those in the transomes and from the one to the other "baulks" are pa.s.sed.

Meanwhile a second pontoon has been rowed into place and more baulks are pa.s.sed from the first pontoon to the second, while chesses are laid upon the baulks to form a platform or floor.

Thus, pontoon by pontoon, the bridge grows until it has reached the further bank.

If pontoons are scarce and the loads to be carried by the bridge are light they are divided in two, and instead of a row of pontoons joined by "baulks" there is a row of "pieces" joined by baulks. Pieces arranged thus form a light bridge, pontoons a medium bridge, while pontoons placed closer together form a heavy bridge. Which shall be built depends upon the number of pontoons available in relation to the width of the river and the nature of the traffic which will have to pa.s.s over.

An alternative arrangement is to make the pontoons up first into groups or rafts and then bridge from raft to raft instead of bridging between pontoons.