There are other forms of torpedo which although little used are by no means lacking in interest. There is the Brennan, for example, at one time much favoured in the British Navy. Its propellers were operated from the sh.o.r.e, by the pulling of two very flexible steel wires. The effect was much as if the thing were driven by reins, as a horse is driven. On sh.o.r.e was a powerful engine with two large drums on which the wires could be wound and by which they could be drawn in at a very high speed. By pulling one more than the other the torpedo could be steered and it is said that such a torpedo could be made to follow a ship through complicated evolutions and fairly hunt it down, finally overtaking and striking it.

The purpose of such weapons was clearly to defend a port or roadstead against enemy craft which might try to rush in. It needed to be controlled by someone perched upon an eminence of some sort from which he could watch its course and guide it as might be necessary.

Compare this with the ease with which the Whitehead torpedo is just slipped into the water and then left to itself. A submarine has in its bows either one or two tubes just large enough to hold the torpedo easily. At the front is a flap door which is kept closed while the torpedo is slipped into its place. Then the similar door at the rear of the tube is closed after which the front one can be opened. Water of course flows in and surrounds the torpedo when this takes place and a little push from some compressed air sends it floating out. As it emerges from the tube the engines are set going automatically and likewise the gyroscope which steers it, after which it continues to proceed in a straight line, soon seeking and maintaining the desired depth.

Other vessels besides submarines have submerged torpedo-tubes like these, but others again have tubes of a different kind. These are fixed on the deck and have the advantage that they can be pointed in any direction almost like a gun, whereas the others are either fixed rigidly in the vessel or are only slightly movable. In the case of these other tubes the torpedo is shot over the side of the ship, off which it leaps into the water somewhat like a man diving.

One other kind of steerable torpedo may be mentioned because of its ingenuity, although so far as is known it is not in actual use. It is called the Armorl, a compound of the names of its inventors, Messrs.

Armstrong and Orling. It is controlled by wireless telegraphy in a very simple but effective manner. The rudder which steers it is connected to a small crank in such a way that as the crank revolves it turns the "helm" first to one side and then to the other. Suppose that, to commence with, the rudder is straight: a quarter of a revolution of the crank sets it to one side, say, the right: another quarter sets it straight again: a third quarter sets it to the left: and so on. The crank is turned by a wound-up spring, the effect of which is, however, normally held in check by a catch. When a wireless impulse comes along the catch is lifted for a moment, the crank slips round a quarter of a turn and the rudder is moved accordingly. Every impulse changes the position of the rudder and by sending suitable series of impulses it can be set as desired and changed at any moment.

A difficulty with all these guided torpedoes is that they must carry some indication whereby their place at any moment will be made visible to the man in control. A little mast and flag would do, for example, but it would be a fair mark for the enemy's guns and being shot away would leave the torpedo uncontrollable. The same objection seems to apply to the wireless antenna which this last type must carry with which to receive their guiding impulses, but that can be made light and almost invisible. It is when the thing is clearly visible that the danger arises, and, of course, to serve its purpose it must be visible. The way in which this difficulty was overcome by Messrs. Armstrong and Orling is a beautiful example of ingenuity. They cause a jet of water to be blown upwards by compressed air, something like the spouting of a whale, so familiar in books of natural history. That forms a mast which is clearly visible, yet the enemy may blaze away at it to their heart's content without damaging it in the least.

CHAPTER XIX

WHAT A SUBMARINE IS LIKE

The precise details of the submarines of our own navy or of any other for that matter are wrapped in mystery. Those who might tell do not know and those who know must not tell. True, there have been fully descriptive articles in many books and magazines, but it may be safely a.s.serted that those descriptions are nothing more than what this chapter avowedly is, reflections by the authors on what such a craft must be like, more or less. It is just as well that this should be clearly understood, and the following description does not claim to be any more than that.

Just as an aeroplane follows the general design of a bird of the swallow type, which soars without flapping its wings, so the submarine necessarily follows much the lines of a fish. It has fins which help to guide it, it has rudders which compare with the fish's tail, and while it cannot use either fins or tail to push itself along as the fishes do, it has one or more propellers which serve that purpose admirably. It is rather remarkable that, while we often imitate nature very closely, there is one very important mechanical feature which almost invariably distinguishes man-made schemes from natural ones--that is, that man uses rotary motion for many purposes whereas nature practically never does. To be perfectly honest, the natural mechanisms are far too difficult for us to copy or I expect we should do so. For example, watch a goldfish and see how cleverly it uses its tail. Man could never hope to make anything so perfect as that tail. Absolutely under its owner's control, it serves a double purpose of propelling and steering in a manner which is equally beautiful and impossible to imitate.

For certain definite purposes, however, a rotary propeller is quite as good as anything which the fishes can show us. As a straightforward, simple, forward-pushing device it is equal to anything that a fish possesses. It has to be given that one duty, however, and no other, the steering being the task of a separate device, the rudder. There again, too, we see how nature does two things with one kind of mechanism while we have to use two, for the fish steers itself to right and to left with its tail in a vertical plane, but if it wants to steer upwards or downwards it twists its tail over somewhat towards a horizontal plane.

The submarine, however, needs two distinct and separate rudders, one for right and left steering and one for up and down, the latter being generally a pair, one each side the vertical rudder for the sake of symmetry and balance.

So we find that a submarine has a body like that of a fish except that it is rather more rotund, perhaps, than the most portly fish usually seen. It has certain fixed fins projecting from its sides, which together with the rudders enable it to be guided. It has also certain long fins called bilge keels for the purpose of keeping it from rolling too much. Also, it has one or more propellers and the two kinds of rudder already referred to.

A fish, never wishing to get outside itself and walk about upon its own upper surface, needs no deck, in which the submarine differs from it, for the crew require somewhere where they can enjoy a breath of fresh air when opportunity offers. It is not a very commodious place, one could not exactly take a long walk upon it, nor even play deck-quoits, but on the back of the submarine there is an undoubted deck where the men can get out and upon which they can stand when she is on the surface.

A fish, moreover, takes little heed of things upon the surface: its interests lie almost entirely below. Hence it has no conning-tower or periscope, but without these the submarine would be useless. The former is a little oblong tower something like a chimney, which projects upward from the deck, while projecting to a higher level still is the tall hollow mast with prism and lenses at the top called the Periscope, through which the commander of the submarine, himself comparatively inconspicuous, can sweep the horizon for enemies or victims.

The problem of constructing a ship to travel under water is quite different from making one to travel on the surface in the ordinary way.

When deep down the pressure of the water tending to crush the vessel is something enormous. Roughly speaking, it is a pound per square inch for every two feet in depth, so that if a submarine dives to a depth of fifty feet the water presses upon it with a force of about twenty-five pounds upon every square inch of its surface. On a square foot, that means over a ton. And there are many square feet of the surface in even a small submarine. Consequently, the whole sh.e.l.l of the ship has to be of very substantial construction. Moreover, there are curious strains which come upon the vessel when it dives to which surface ships are not subject. All these have to be reckoned as far as possible and allowed for.

The size of the modern submarine is not known with any certainty, but we may put it down roughly as two hundred feet long and at least a thousand tons displacement, which means that that is its actual weight, including everything and everybody on board, when it is just about to submerge.

Of course, a submarine, alone among boats, has two "tonnages." When it is on the surface it is comparatively light. Indeed, "running light" is the technical term describing it when it is riding upon the surface of the water like an ordinary ship. Then, by increasing its weight, it can cause itself to sink until the little promenade or deck called the superstructure is just submerged and little can be seen above water except the conning-tower. That is termed the "awash" position, and it is clear that it is then displacing more water than when running light, and hence its displacement tonnage must be more.

When it is desired to sink, the vessel is set in motion in the awash position, from which it is gradually steered downwards by the diving rudders, until only the periscope, or it may be not even that, is left showing above. Then the maximum of water is being displaced. It is then actually displacing more than its own weight of water, for if left to itself it will rise rapidly and it is only the speed and the action of the rudders which keep it under. We see, then, that the action of a submarine in submerging itself is a real genuine dive. It sinks upon an even keel until it is awash, after which it goes under "head-first,"

just as a swimmer does. It also rises bow first.

This tendency to rise when the combined action of movement and rudder ceases const.i.tutes a very considerable safeguard, for should anything happen to the propelling machinery the vessel simply rises. At one time weights were attached to the under side of the hull which could be detached from the inside so that in the event of the vessel descending against the wish of her commander, she could be simply forced to the surface by the great excess of buoyancy resulting from shedding these "safety weights." Of course, in the event of a serious perforation of the hull neither of these forms of surplus buoyancy would bring the boat up.

Let us now trace the operations of diving right through, supposing that our submarine is first running light. In that condition she is being driven by the oil engines which const.i.tute her primary propelling power.

The hatch or door at the top of the conning-tower is open, as also, it may be, is the one lower down, just at the foot of the tower. Men are standing upon the little platform formed by the tower, and one of them is steering by means of a wheel, keeping his eye, moreover, upon a compa.s.s also provided there, that being in fact, to the submarine when light, what the bridge is to the ordinary steamer. Other members of the crew may be upon the superstructure or deck just below, while others again are down inside, attending to their duties there.

Under these conditions the inside is by no means an unpleasant place.

Plenty of fresh air comes down through the open hatches and through the ventilators, it being drawn down through the latter by means of a fan.

Preparations are then made for submerging. The hand-rail along the little deck is removed. The upper steering wheel and compa.s.s are covered up or shut away into the coverings provided for them, the wireless apparatus, if provided, is removed and the mast shut down. Hatches are securely closed and valves in the ventilating pipes are closed. In fact every opening is shut and made water-tight so that no risk shall be run of diving prematurely and taking in water accidentally.

The quarter-master transfers himself to the steering wheel inside, where he has another compa.s.s to guide him, not of the magnetic variety this time but a cunning application of the gyroscope. The commander, too, having descended before the last hatch was closed down, takes his stand at the eyepiece of the periscope, since that is now his only means of seeing what is going on above.

Another man takes his place at the wheel which controls the diving rudder, conveniently near to which is a pressure gauge so connected to the outer water that as the ship dives its depth is recorded upon its dial: that in effect is to him what the compa.s.s is to his comrade at the other wheel.

With every movement of men there needs to be adjustment made to keep the ship on an even keel. Otherwise she would go down by the bow or down by the stern according as the men's weight shifted towards either end. This is arranged for by two small tanks formed in the structure of the vessel, one at either end. Connected together by pipes and controlled by compressed air, water can be transferred from one to the other at will and so the balance be always kept. Quite simple manipulations of a valve serve to accomplish this delicate balancing performance. It is perhaps not of such importance at this stage, but in a moment, when the whole vessel will be under water, a very little movement indeed will suffice to upset the equilibrium.

Next water ballast is admitted into certain other s.p.a.ces in the ship's structure, these s.p.a.ces being called, because of the use to which they are put, ballast tanks. Gradually, as the incoming water increases the weight of the vessel, she sinks until she is awash. Then the diving rudders are set at the right angle (a pendulum serves to show the angle at which the boat points) and down she goes. As the pressure-gauge indicates the approach to the required depth the rudder is flattened out a little until just that position is found which keeps the boat under at the desired depth.

Of course, when all hatches and openings were closed the supply of fresh air was cut off and after that the crew had to depend upon the air contained in the submarine. Also, they had to stop the engine, for without air it cannot work: nor can it work without giving off fumes, which, if admitted to the ship, would soon suffocate the crew. Just before closing up, therefore, the engine is stopped and electric motors take up the task of driving the ship.

Now suppose that, while running submerged, the commander espies, through his periscope, an unsuspecting enemy. He tries forthwith to get as close as he can. Having noted the direction of the vessel and which way she is going and as far as possible her speed, he submerges more deeply, in all probability, lest the white streak which represents the wake caused by his periscope should reveal his presence. For possibly she is one of those terrible destroyers in fair fight with which he has but a poor chance. His only safety lying in complete invisibility, he therefore submerges entirely, trusting to his calculations to lead him in the desired direction. Thus he attempts and, if he have good luck, he succeeds in getting reasonably near to his foe.

Then he must try so to man[oe]uvre that his bow shall at the right moment be pointing towards the quarry, for his torpedo tubes are in the bow and they are fixed, or nearly so at all events, so that he can only fire them in a direction nearly, if not precisely, in the direction of the centre line of his ship.

Nay, he must do even more than that. It will not do to fire the torpedo directly at the ship, for a torpedo is comparatively slow. Suppose it is capable of forty miles an hour, and the other ship is a mile away: the torpedo will take ninety seconds to reach it. And in that time it may have travelled a mile or so itself. So the submarine man has to allow for that.

Occasionally, therefore, he comes up a little for a moment in the hope of getting a sight of the enemy while not revealing his own presence.

Or perhaps he may decide to risk being seen and caught, trusting to the chance of getting his own blow in first. He needs to be a most resourceful man, with clear and keen judgment and supreme self-confidence, or he can never grapple with such a task.

Supposing, then, that he succeeds in getting undetected into a favourable position, as he thinks; at the critical moment the other ship may change its course, and the whole scheme goes awry. Perhaps he then tries to follow, but that is bad, for the end of a ship is not nearly so good a target as the side and the part hit is not so vulnerable. The first torpedo may, however, so disable the vessel as to give him chance to get into position for a second and better shot.

Anyway, when he thinks he has got his best chance he lets off a torpedo, immediately diving to be safe out of harm's way for a while. Then he rises to see the result of his work. If successful he would be sure to hear the sound, for water is an excellent sound-conductor and a submarine is like a gigantic telephone ear-piece.

It must be a nerve-racking job at the best of times, for the submarine is a very vulnerable craft. A member of the crew of a German submarine captured during the war is reported to have said that out of ten submarines attacked, nine were sunk. That may or may not be true, but it is certain that a very little damage, which would hardly affect an ordinary craft, is enough to sink a submarine. That is because, in order to be able to sink at will, the reserve of buoyancy has to be very low.

An ordinary surface ship has at least as much of its bulk above water as below: hence it can take on board a weight of water almost equal to, if not exceeding its own weight before it sinks. At the best a submarine has not more than 30 per cent of excess and so it sinks if water amounting to only 30 per cent of its weight gets into it. In other words, the reserve in one case is at least 100 per cent: in the other at most 30 per cent.

During the war a submarine saw and tried to track down, somewhat after the manner described, a slow, steady-going collier which plies between London and the north carrying coal for a London gas-works. Having, as it thought, got into position for discharging its torpedo it rose for a final look when (it must have been to the amazement of the crew) the collier was seen making straight for them. What they really thought no one will ever know, for the collier had the best of the encounter, the submarine was crushed beneath her blunt bows and sank, no doubt, for ever. The mere fact that a slow, clumsy, heavily-laden collier could ever thus vanquish an up-to-date submarine is eloquent testimony to their vulnerability.

Many a submarine, too, has fallen to the sh.e.l.ls of an armed fishing trawler simply because the sh.e.l.ls of the latter were so much quicker in action than a torpedo, coupled with the fact that one well-placed shot, by preventing a submarine from diving, renders it almost helpless.

Some submarines, however, have a gun on the deck, so that when light they can fight like a destroyer or other lightly-armed vessel. The gun shuts down into a cavity when the vessel goes below.

The periscope, which forms such an important part of the submarine's equipment, is really very little more than a telescope. On the top there is a little mirror, or more probably a prism or three-cornered piece of gla.s.s which serves precisely the same purpose in that it reflects exactly as a mirror does. This is so placed that it throws the light from distant objects down the tube into the interior of the ship. In the tube are lenses very like those of an ordinary telescope and the light may be made to throw a picture upon a little table or screen or else can be viewed through another prism directly by the eye. In either case the periscope is just like an ordinary telescope set up vertically with a prism at the top so that it can "see" at right angles, and possibly another at the bottom so that the picture can be viewed at right angles to the direction of the tube. The latter is necessary only for the convenience of the observer, since otherwise he would have to be upon his back to look up the tube. The whole apparatus can be rotated mechanically and a scale forms a means of measuring the precise direction in which the prism or mirror is at any moment pointed. This is useful for measuring roughly the position of the "prey," and it may even be used as a rough means of getting the range.

Another feature is the gyroscope compa.s.s, to which a pa.s.sing reference has already been made. It is fairly well known that an object when spinning exhibits properties quite different from those which it possesses when still. A boy's top is a familiar ill.u.s.tration, for while spinning it will stand perfectly steady, supported only upon a tall peg with a sharp point, a pose which it will absolutely refuse to maintain when not spinning. Now fortunately for the present purpose it so happens that one of the peculiarities of the gyroscope or spinning-wheel is this: that if mounted in a certain way it persists in placing its axis in the same plane as that in which the axis of the earth lies. If you imagine for a moment a plane or flat surface of which the earth's axis forms a part you will see that wherever that plane cuts the surface of the earth will be a line in a north and south direction. Consequently, if any horizontal object has its axis in that same plane it, too, will always point north and south. A wheel, small but heavy, is therefore mounted with its axis supported horizontally upon a little metal raft floating in a trough of mercury and driven round at a very fast speed by a small electric motor fixed in it.

Whatever its position may be to start with, this revolving wheel will in a short time slew itself round upon the supporting mercury until its own axis is in the same plane as the axis of the earth: until, in fact, its axis points due north and south. Arrived in that position, it will remain there no matter how the ship upon which it stands may turn. Since it floats freely upon mercury the motion of the ship has little effect upon it, so little indeed, that it has no difficulty in following its own peculiar bent, even if the ship be describing circles.

The advantages of this are various: two of them may be stated. First, the apparatus points to the actual geographical north and not to the magnetic north, which is a slightly different direction and one, moreover, subject to frequent variation. Second, it is absolutely unaffected by the presence of iron or other magnets, a very fruitful source of error in the magnetic compa.s.s when used upon an iron ship close to steel guns and electrical machinery. Surrounded with iron as is the compa.s.s in the interior of a submarine, the magnetic needle practically refuses to work at all, so that, although employed on other ships, it is on the submarine that the gyro-compa.s.s finds its most important field of usefulness.

The pressure-gauge or manometer, which indicates the depth, is probably not different in any respect, except in its dial, which is marked in feet-depth instead of in pounds-pressure, from the pressure-gauge used on steam boilers. It has either a little cylinder with a piston in it which the water presses upwards more or less against the force of a spring, a diaphragm which is bent more or less, or a bent tube which tries to straighten itself out as the pressure inside it increases.

The older submarines derived their power from petrol engines similar to those which drive high-power motor-cars, but nowadays these have given place to engines of the type invented by the unfortunate Diesel who, after making one of the most brilliant and successful inventions of modern times, committed suicide, apparently in the height of his success.

These engines burn cheap heavy oil in place of the costly refined petrol: they are exceedingly reliable and well-behaved, and are free from many of the troubles which affect the petrol motor. They are referred to in more detail in another chapter.