Chapter V: Mines, Submarine and Subterranean

The word mine in its military sense originally meant just the same as it does in the ordinary way, but like many other words it has got twisted into new uses the connection of which with the original meaning is very obscure. One of the most striking of these verbal puzzles is the submarine _mine_. There seems at first sight not the remotest connection between the floating barrel of explosives concealed beneath the water and what we ordinarily call a mine. The explanation of this is that the term has acquired this meaning after passing through a series of stages.

When soldiers "mine" for the purpose of blowing up their enemies they dig a hole in the ground, and conceal therein a quantity of explosives so arranged that they blow up when the enemy pass over or near. The operation of digging the hole in the earth is clearly akin to the work of the miner and so such is quite appropriately called a "mine."

The hole may be dug from the surface downwards, the marks of excavation being afterwards covered up and obliterated as much as possible. In other cases the hole may be a tunnel starting from a trench and driving towards the enemy's position. The idea, of course, is to burrow until the end of the tunnel is just under some important part of the enemy's works or fortifications. When the end of the tunnel has reached the right spot explosives can be placed there, the tunnel partly stopped to prevent the explosion from driving back upon those who make it and the whole fired at the desired moment.

This tunnelling is also called "sapping" and the tunnel itself a sap. Military engineers are often spoken of as "sappers and miners" as if the two things were clearly different, but as a matter of fact both are often used to describe the same thing. Roughly, we may say that a mine which stays still in the hope that the enemy will walk upon it is a mine proper, while a mine which itself progresses towards the enemy until it ultimately goes off beneath him, is a "sap" and the making of such a thing is "sapping." Or we might say that sapping is under-mining, in which sense we use it in general conversation when we speak of something sapping a man's strength. Soldiers speak of their engineering comrades as "sappers" just as they term artillerymen "gunners," but the only reason why they call them by that name instead of miners is because the latter is a well-known term applied to those who work in coal mines.

A subterranean mine, then, is nothing more or less than a hole in the ground, made in any way that may be convenient, filled with explosives and fired at a suitable time to do damage to the enemy.

In other words, it is simply some explosive _concealed in the ground_ with means for firing it, and when the sailor _conceals explosives in the sea_ so that they may blow up the enemy's ships, he borrows his military comrades' term and calls it a "mine" too.

Counter-mining is the enemy's reply to mining. Suppose I was foolish enough to wish to blow up my neighbour who lives in the house opposite to mine. I might start from my cellar and dig a tunnel under the road until I knew that I had arrived under his dwelling. But suppose that he got to know of my little scheme: he could then try counter-mining. In this case it would mean starting a tunnel of his own from his cellar towards my tunnel: then, as soon as the two tunnels had come sufficiently near to each other, he could let off his explosives thereby wrecking my tunnel and putting an end to my operations while yet I was only half-way across the road. Thus he would stop me before I had had time to harm him, and since he need only tunnel just far enough to render the necessary explosion harmless to his house, while I to succeed would have to tunnel right across the road, the man who is counter-mining always has a slight natural advantage over the man who is doing the mining. If only he gets to know what is going on in time he can always retaliate.

All forms of land mine are improvised on the spot according to circumstances. Not so, however, with submarine mines on which much ingenuity has been expended, the mines being made in workshops ashore ready for laying and then laid by ships and sometimes by divers.

Of these there are two main kinds, those which are put in place in times of peace for the protection of particular harbours and channels, and those which are simply dropped overboard from a mine-laying ship during the actual war.

They all consist essentially of a case of iron or steel plates riveted together just as a steam boiler is made, in fact the cases are made in a boiler shop. The charge is gun-cotton fired by a detonator, the latter being excited by a stroke from a hammer, as in a rifle, or else by electricity. In the latter case, a tiny filament of platinum wire is in contact with the detonator, and the wire being heated by the current, just as the filament of a lamp is, the detonator is fired by the heat.

Of the permanent mines whereby the entrances to important channels are protected arrangements are often made for firing by observation, that is to say, by the action of an observer ashore. Being laid by divers and securely anchored to heavy weights laying on the bottom, wires are carried from the mines to the observation station. The observer watches and fires the mines at the right moment by simply pressing a key thereby making the electrical circuit.

More often, however, mines are fired by contact. Observation mines have the advantage that while they may be exploded under an enemy they will allow a friendly ship to pass in perfect safety. Contact mines, on the other hand, will afford protection against attacks by night when enemy craft may attempt to creep in under cover of darkness.

This photograph was taken looking down upon the deck of the ship. The mines run upon rails, and are pushed by the men towards the stern, whence they are dropped one at a time into the water. The splash indicates that one has just fallen.]

Contact mines are often fired electrically, sometimes by batteries of their own inside their own cases, or else by current from the shore through wires, the circuit being completed by an automatic device of some sort actuated unwittingly by the unfortunate victim.

One of these contact devices will illustrate the general character of them all. Imagine a little vessel with mercury in it: it is, generally speaking, of some insulating material, but right at the bottom is a metal stud with which the mercury makes contact. The rim may likewise be of metal or a metal rod may project downwards into it: it matters not which, for we can see at once that it is quite easy so to arrange things that whereas, while upright, the mercury shall be well clear of the upper contact, it shall when the vessel is tilted flow on to it, thereby bridging from lower contact to upper contact and completing the circuit.

Of course, a mine must only go off when actually struck by a ship and not when it is gently swung to and fro by the action of tide or current in the water. That is easily arranged, for the vessel and contacts can be so shaped that contact is not made until an angle of tilt is reached which no tide or ordinary commotion of the water could bring about.

It is clearly possible, too, to combine the contact and observation arrangements in such a way that contact mines can be made safe for friendly ships during the daytime. It is only necessary to adopt the shore battery arrangement already mentioned and disconnect the batteries during the day or when no enemy is in sight, restoring the connection during the darkness or in the event of hostile ships trying to rush the passage.

Another interesting scheme for keeping mines safe until required is to anchor them in what is termed a "dormant" condition. This means that a loop is taken in the wire rope by which they are anchored, the loop being fastened by means of a link. This link, however, contains a small quantity of explosive which can be fired from the shore. This has the effect of breaking the link, releasing the loop and allowing the mine to float upwards to the full length of the rope. Thus the mine is down deep, well below the bottom of the biggest ship until released for action.

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 pass 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 surreptitiously 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 ashore, so that all mines of this class 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 passenger 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 passing 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 anchored 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 passing ship would give. This breaks a glass 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.