First stroke (down) draws in a mixture of air and gas.Second stroke (up) compresses the mixture. Just at the top of this stroke an electric spark firesthe mixture, causing an explosion which drives the piston downwards, thus making theThird stroke (down), during which the power is developed.Fourth stroke (up) expels the waste products of the explosion.
First stroke (down) draws in a mixture of air and gas.
Second stroke (up) compresses the mixture. Just at the top of this stroke an electric spark firesthe mixture, causing an explosion which drives the piston downwards, thus making the
Third stroke (down), during which the power is developed.
Fourth stroke (up) expels the waste products of the explosion.
Although all of them work on this same cycle, in which they resemble the engines of the motor-car, there are several much-used types of aero-engine in which the mechanical arrangement of the parts is quite different. Of these the best known is the famous Gnome engine which has a considerable number of cylinders arranged around a centre like the spokes of a wheel. The centre is in fact a case which covers the crank, and the cylinders are placed in relation to it just as the spokes are placed around the hub of a wheel.
There is only one crank and all the connecting-rods drive on to it. Owing to their position around it they thus act in succession, giving a nice regular turning effort.
Further, these engines differ from all others in that the crank is a fixture while the rest of the engine goes round, exactly the opposite of what we are accustomed to. The engine, in fact, constitutes its own flywheel. Rushing thus through the air, the cylinders tend to keep themselves cool, doing away with the need for cooling water and radiators. Consequently engines of this type are the very lightest known in proportion to their horse-power. A fifty horse-power engine can be easily carried by one man.
It would be possible to go on much longer with this most interesting subject of engines, but having treated the three types which are most used in warfare, it is now time to pass on to something else.
Except for the submarine the most prominent craft during the war has undoubtedly been the destroyer.
All warships are in one sense destroyers, since it is their prime duty to destroy other ships, so why should one particular kind of boat be given this name specially? Like many other of the terms which we use it is an abbreviation, a mere remnant of a fully descriptive title. "Torpedo Boat Destroyer" is what these ships are called in the Navy List.
Even that full title, however, only tells us what their original purpose was: it leaves us very much in the dark as to the many various functions which they perform.
The invention of the torpedo called for the construction of small boats whereby the new weapon could be used to best advantage, and so we got our torpedo boats. They in turn called forth another boat whose duty it was to run down and destroy them, and in that way we get our destroyers. From that bit of naval history we can almost see for ourselves what the characteristics of the destroyers must be. They have to be bigger than the torpedoboats, but as the latter were quite small the destroyers, though larger, are still comparatively small craft, latterly of about one thousand tons. Then they have to be very fast, in order to be able to chase the others and, finally, they need one or two guns, comparatively small so as not to overburden the ship and yet large enough to dispose of anything of their own size or smaller.
Unquestionably, their greatest feature is their speed. They are the fastest ships afloat, rivalling even a fairly fast train. Some of them can exceed forty miles an hour. They are very active and nimble, too, being able to turn in a comparatively small circle. For warships, too, they are cheap, so that a commander can afford to risk losing a destroyer when he would fear to risk another vessel. For all purposes except the actual hard-hitting they are the most useful weapon which the commander of the fleet possesses.
When the main fleet puts to sea a whole cloud of these smaller craft hover round looking for submarines or for the surface torpedo boats which might try to attack the large ships under cover of darkness, while keeping a sharp look-out, too, for mines or any other kind of floating danger, and thus they screen the more valuable ships.
Likewise do they convoy merchant ships sometimes, especially through waters believed to be infested with submarines. They also sally forth on little expeditions of their own, knowing that they can fight any craft equally speedy and show a clean pair of heels to any heavier ships, while by adroituse of their own torpedoes they may even "bag" a cruiser or two.
They are pre-eminently the enemy of the submarine, for the under-water boat is necessarily less active even when it is on the surface than they are, so that a submarine caught by a destroyer stands a very good chance of being rammed by it, which means that the destroyer deliberately rushes at it, using its own bow as a ram wherewith to knock a hole in it. Or if that be not practicable the destroyer, while dodging the torpedo of the submarine, may plant a single well-aimed shot into its opponent and the fight is over. A cleverly-handled destroyer appears to have little difficulty in avoiding the comparatively slow torpedo, but no ship ever built could avoid a properly aimed shell, two facts which are clearly indicated by the very few cases in which, during the war, a destroyer has succumbed to a submarine. The gun of the latter, if it has one, is no match for the guns of the destroyer.
Naval strategy and tactics, when one thinks about them carefully, reveal a very close resemblance to those of the football field. The destroyers are like the forwards, quick, light and nimble, valuable chiefly because of their ability to run swiftly and to dodge cleverly, while the heavy, stolid backs represent the battleships in their ability to withstand the heavy shocks of the game. Any imaginative boy will be able to carry this simile farther still and a comparison of the description of the battle of Jutland with his own knowledge of the game will reveal a surprising parallelism.
Thus the reader will to a very large extent be able to see for himself the manifold uses to which these wonderful little ships lend themselves, and he will see that above everything else it is their speed which counts, which fact gives us the key to their peculiar construction.
To commence with, they are made as light as possible. The material used is different from that of ordinary ships, being "high-tensile" steel, a steel into which a little more carbon than usual is introduced, resulting in about 50 per cent higher tensile strength but also involving, alas! rather more brittleness. When made of this material the whole framework of the vessel can be made of lighter beams and the covering can be of thinner plates than would be the case if the mild steel ordinarily employed for shipbuilding were used. The high-tensile steel is lighter for a given strength and therefore a ship built of it is lighter than it would otherwise have to be.
Besides the use of this particular material every resource in the way of ingenuity and skill on the part of the designers is bent towards saving weight. No unnecessary part is ever put in, but, on the other hand, necessaries are skinned down to the utmost limit consistent with safety in order to produce a light ship. How difficult this problem is is hardly realized until one thinks of the conditions which prevail when a ship floats in the water. The upward support of the water is exerted in a fairly regular way all along the ship while the weights inside which are pressing downward are concentrated in lumps. The engines, for example, represent a very heavy weightconcentrated in one fairly confined spot. Thus the vessel has to have sufficient stiffness to resist the action of these opposing forces which are thus tending to break her in two. That, moreover, occurs in the stillest water; when the sea is rough still worse stresses are brought to bear upon the comparatively fragile hull, for a wave may lift each end, leaving the middle more or less unsupported, or one may lift the middle while the ends to a certain extent are left overhanging. All this, too, is in addition to the knocks and buffets caused by huge volumes of water being flung against the ship by cross seas in the height of a tempest. In the case of ordinary ships where speed is not of such great importance, the problem is simplified by the use of what is termed a high "factor of safety," which means that the designers calculate these forces as nearly as they can and then make the structureamplystrong enough. In other words, care is taken to keep well on the safe side. In a destroyer, however, there is no room for such a margin of safety. Risks have to be taken, and it is only the high degree of skill and experience possessed by our ship designers which enable these light ships to be made with, as experience shows, a very considerable degree of safety. They have to be continually choosing between strength on the one hand and lightness on the other and the way in which they combine the two is marvellous.
The weight thus saved is used for carrying engines, boilers and fuel. Relatively to its size, the destroyer is about as strong as an egg-shell, but its engines are of extraordinary power.
The destroyers are generally organized and operate in little groups or flotillas of perhaps twenty or so with a small cruiser or a flotilla leader as a flagship, on which is the officer in command of them all. There is also usually a depot ship for each flotilla.
The flotilla leaders are what one might call super-destroyers, about double the size of the ordinary large destroyer, which is to say, about two thousand tons, and capable of very high speed.
The depot ships form a kind of floating headquarters for their respective flotillas. They are usually old cruisers which are specially fitted up for the purpose, and although they are of comparatively slow speed they can by wireless telegraphy keep in touch with the destroyers, which can return to them when occasion permits or demands. They carry workshops in which small repairs can be carried out, spare ammunition and stores of all kinds and spare men for the crews. In fact they can look after the smaller craft much as a mother looks after her children, and for that reason they are sometimes called "mother ships."
As has been said, the destroyer was originally intended to destroy torpedo boats, but small torpedo boats have almost gone out of existence or rather the class have so grown in size as to have become merged in the destroyers, which, it must be remembered, are well armed with torpedoes which they have at times used with great effect. It is not surprising, therefore, to find that a still newer class of ship has arisen which has been described by one authority as "destroyer-destroyers." Officiallyknown as "light armoured cruisers," not very much is known of their details. They are, however, about 3500 tons, with 10 guns, large enough that is to dispose of any destroyer which they might encounter.
Thus, to review the whole class of ships of which we have been speaking, we may say that there are the destroyers, all the more recent of which are about 1000 tons but diminishing as we go backward in time to about 350 or 400; the flotilla leaders about twice the size of the largest destroyers; and the destroyer-destroyers nearly twice as large as the flotilla leaders: all are characterised by high speed and by guns just large enough for the work for which they are intended. All are armed, too, with the deadly torpedo for attack upon larger ships than themselves.
They are essentially night-birds, much of their time being spent stealing about with all lights out, in pitch darkness, seeking for information or for a chance to put a torpedo into some chance victim. These night operations are very hazardous, but so skilful are the young officers who have charge of these boats that seldom do we hear of mishaps.
But although, as has been said, the torpedo boat has almost vanished, its under-water comrade has recently assumed a place in the first rank of importance, and perhaps to us the most valuable work of all done by the destroyer is that of hunting down and sinking these modern pirates.
Perhaps the greatest war invention of modern times was the British battleshipDreadnought.
Of course, there have been battleships for centuries. In history we read of fleets consisting of so many "ships of the line" or in other words "line-of-battle" ships, meaning ships which were considered capable of taking their place in "line of battle," as distinguished from "frigates" which correspond to the modern "cruiser."
The "line-of-battle" ships were stout and strong with plenty of guns. They went into the thick of the fight, since they were capable of giving and receiving hard blows, while the lighter frigates hovered around seeking an opening to use their higher speed to cut off stragglers or to prey upon merchant ships.
Although so different in form and material that a sailor of the old days, could he revisit the earth, would not recognize them, the battleships of to-day are the real descendants of the "line-of-battle" ships of those times. They are stout and strong, with the heaviest guns, capable of giving and taking the hardest knocks, and it is they who form the backbone of the fleet. As we saw in the accounts of the battleof Jutland, the German Fleet tackled our cruisers and lighter vessels but discreetly withdrew when the battleships came up.
Looked at in another way, we may say that a battleship is a floating fortress. Its speed is not great, when compared with other ships, but it is constructed to carry enormous guns. It is also armoured with steel plates of great thickness and of special hardness placed upon the outside of the hull so as to cover its vital parts and protect them from the shells of the enemy. Its chief function, we may say, is to carry its guns: to enable it to do this with safety, it is armoured: and to enable it to get to grips with its enemies it has engines and boilers. Those are the three features of greatest importance in a battleship, its guns, its armour and its engines. All else is of minor importance.
It is strange to think how short a time the iron or steel ship has been with us. In the American Civil War, for instance, only about sixty years ago, the battleships were made of wood. It was during that war that Ericcson thought of the idea of putting iron plates to protect the sides of a ship from the hostile shots, and from that improvised armouring of a wooden ship has arisen the iron-clad or, more correctly, steel-clad monsters of to-day.
It is just about fifty years ago since the last iron-clad wooden battleship was launched for the British Navy. Her name wasRepulse, and she took the water in 1868. With a tonnage of 6190 and a horse-power of 3350, she had a speed of 12 knots. Her armouring of iron was in parts 4½ inches and inother parts 6 inches thick, while she carried 20 guns of sizes which to-day would seem mere toys. If all her guns were discharged together she would throw a total weight of 2160 lbs. of projectiles.
Now, for comparison, let us take a modern battleship, theOrion, for example. The tonnage is 22,680, the horse-power 27,000.
She is more than twice the length of the older ship and is armoured with steel 12 inches thick. Her 10 large guns, each 13½ inches in diameter, if fired together (as I once heard them, like thunder, though 10 miles away) throw a weight of 12,500 lbs.
From this we see the wonderful growth in size, speed and in hitting power during the comparatively short period of fifty years. But there is a more striking comparison still.
TheRepulse'sguns threw 2160 lbs. and theOrion'sthrow 12,500. But that takes no account of the energy with which the weight is thrown. A tennis ball hit hard, might really contain more energy and do more damage to anything it hit than a cricket ball thrown gently, which illustrates the fact that in comparing the power of guns we need to consider something more than the mere weight of the projectiles. To arrive at a real comparison we take the weight of the projectiles in tons and multiply it by the speed at which they leave the guns infeet per second. And we call the answer so many "foot-tons."
Now the energy of theRepulsethus reckoned comes to just under 30,000; that of theOrionto just under 690,000. TheOrioncan hit twenty-threetimes as hard as could its forerunner of only fifty years ago.
Since theRepulseall our battleships have been built of wrought iron or mild steel. Speaking generally, there was a steady development in size and horse-power and in speed until 1906, in which year there was launched the world-famous H.M.S.Dreadnought. Previously no battleship had been faster than 19 knots: she was designed for 21 knots. Her tonnage was 17,900, exceeding by more than 1000 tons anything that had gone before. But the great change was in the guns. Pre-Dreadnoughts had, or one ought to say "have" for there are still many in existence, four of the biggest guns, a number of medium-sized guns and a still larger number of smallish guns intended for the purpose of keeping off torpedo craft and such small fry.
At one stroke Lord Fisher, who was then the First Sea Lord of the British Admiralty, changed all this. He swept all the medium-sized guns away and gave this new shipTENof the largest guns then in use.
The advent of this ship startled the whole naval world, for it was seen at once by all those able to judge that there was a vessel which might be expected to sink with ease any other ship afloat. The onslaught from those ten guns would be more than any other ship could stand. So other powers set to work to copy more or less exactly, while Great Britain quickly built more like her. So important was this new invention that very soon the strength of the naval powers began to be reckoned entirely on the number of Dreadnoughts they possessed, the older ships beingleft out of account as though they did not make any difference one way or the other.
But Great Britain was not content with theDreadnought, for each succeeding ship or set of ships was improved until, only four years later, there was launched theOrionalready referred to, nearly 5000 tons bigger, with 2500 more horse-power, and with 13½-inch guns instead of 12-inch. TheOrionand her sisters are often spoken of as super-Dreadnoughts.
The Dreadnoughts as a class are often referred to as "all-big-gun" ships, since that is the feature which most distinguishes them from those which went before.
These large guns are mounted in turrets as they are called. We might describe these as turn-tables with a cover over something like a small gas-holder. There are usually two guns in each turret, although there are a few ships whose turrets have three in each.
The turrets seem to be standing on the deck of the ship and it is by turning them round that the guns are trained or pointed at their target.
The originalDreadnoughthad one turret in front and two behind, all on the centre-line of the ship, and two more, one each side, amidships. In late vessels all five turrets are on the centre-line. Thus theDreadnoughtcan fire six guns ahead, eight astern and eight to either side, while the newer ships can fire four ahead, four astern and all ten on either side.
There are other battleships with even more guns than these, such as the U.S.A. shipWyoming, with twelve 12-inch guns, but the British Navy seems toprefer to stick to the original number of ten. The reason for this is that every such ship is a compromise between three alternatives.
The three great features have already been pointed out, namely, the guns, the armour and the propelling machinery. Either of these can be increased at the cost of one or both of the others, but all cannot be increased without sinking the ship, unless indeed, the ship be made larger and then other considerations crop up.
And that brings us to another class of ship often ranked among the battleships. These remarkable vessels are also termed cruisers and the fashion seems to have established itself of combining the two names and calling them battle-cruisers. They gave a fine account of themselves during the war.
The first three of these, of which theInvincibleis usually taken as the type, made its appearance the year after theDreadnought, and like the latter were the offspring of the fertile brain of Lord Fisher. TheInvinciblewas about the same size as theDreadnought, but had nearly twice the horse-power (41,000), which enabled it to attain an actual speed of nearly six knots more, namely, 28·6.
For guns it had eight of the same large weapons, and it was armoured with 7-inch steel armour-plates instead of 11-inch.
Thus we see illustrated what has just been said, less guns and thinner armour, to allow for more engine power and higher speed. Or, to put it the other way, we observe how higher speed was attained at the expense of the guns and the armour.
But just as theDreadnoughtwas followed by other still greater improvements in the same direction we get, in 1910, the famous shipLion, a vessel not unknown to the Germans, a "super-Invincible."
This ship has a tonnage of over 26,000 and 70,000 horse-power. It was designed to do 28 knots.
We saw the use of these ships in the Jutland battle, when, using their high speed, they attacked the German battleships and kept them engaged while the slower battleships came up. Though they suffered severe losses, which probably the more heavily armoured battleships would have escaped, they held the Germans so that it was only the failing light which saved them from utter destruction.
Another example was the way in which they hunted down Von Spee and his squadron off the Falklands, when they caught the Germans because of their higher speed and then sank them by means of their heavier guns with practically no loss to themselves.
We saw them again in the Heligoland battle, coming up to the assistance of the lighter vessels just in the nick of time and scattering the enemy like so much chaff.
A fact little known to most people and productive of much surprise is that these battleships and cruisers are not such very large vessels, when compared with those of the merchant service. TheLionis 660 feet long and 86 feet wide, theAquitaniais 930 feet long and 98 feet wide, and theOlympicis 882 feet long and 92 feet wide.
The mightyOrionmakes a poorer showing still inpoint of size, since she is only 545 feet long and 88 feet wide—little over half the length of theAquitania.
It is difficult to compare the tonnage of a warship with that of a merchant ship, since they are not measured in the same way. The former is the "displacement" or actual weight of water displaced: in other words the precise weight of the vessel in tons of 2240 lbs.
The tonnage of a merchant ship, however, has nothing to do with weight but is based upon capacity and is arrived at by a purely arbitrary rule, thus: all the enclosed space in the ship is measured in cubic feet and the total is divided by one hundred. That gives the gross tonnage. To arrive at the net tonnage the space occupied by the engines and all other space necessary for the working of the ship is excluded. Originally the tonnage of a merchant ship was the number of "tuns" of wine which it could carry.
Thus, you see, comparing the tonnage of a warship with that of a merchant ship is somewhat like comparing a pound with a bushel. Net registered tonnage is generally considerably less than the displacement tonnage of the same ship, so that a warship is usually less than a merchant ship of the same nominal number of tons.
And now let us turn to some of the internal arrangements of these wonderful ships, more particularly to the means for working the guns.
Each turret is placed over the top of what we might call a well, running right down deep into the inside of the ship. At the bottom of this well is the magazine, where the shells are stored and also thecartridges containing the explosive which drives the shell from the gun.
Underneath the turret, forming a kind of basement to it, is a chamber called the working chamber, and up to it the shells and cartridges pass by means of lifts. For safety's sake only a small quantity of explosives is kept here at any one time, but it is from here that the guns overhead are fed. Shells and cartridges alike pass up as required by means of hoists right to the guns. Indeed, the hoists are ingeniously contrived so that in whatever position a gun may be the hoist stops exactly opposite the breech, or opening at the back of the gun through which it is loaded. Then a mechanical rammer drives the shell or cartridge into its place in the gun.
The hoists are worked by hydraulic power or electricity, and in most cases by both, arrangements being made so that either can be used at will, thus serving as alternatives in case either should get out of order.
The turrets themselves are also turned by power. Indeed, so heavy are the weights involved that only by the use of carefully designed machinery is the operation of such great weapons made possible. A single shell of the 13·5-inch gun weighs 1250 lbs.
Around each turret there is placed a wall of thick armour plate as high as it is possible to make it without interfering with the movement of the guns. This is called the barbette armour and the space enclosed by it, in which the turret stands, is called a barbette, an old fortification term meaning a place behind a rampart.
The turret is covered over, as has already been remarked, by a steel hood, so that altogether the guns and their crews are about as well protected as it is possible to be.
That all this means a considerable burden upon the ship is shown by the fact that a pair of 12-inch guns with their turret and barbette armour will weigh something like 600 tons, and if there be five of them that means 3000 tons in all.
Down below in the magazine there are lifting appliances whereby the shells can be readily picked up and run to the hoist. Moreover, there is elaborate machinery for keeping them cool. Our allies the French had, years ago, several bad accidents through the explosives going off spontaneously in their ships, and this is quite likely to happen if the magazines become too hot. So refrigerating apparatus is installed similar to that employed in meat-carrying ships, which provides a constant flow of cool air into the magazines.
The ships also are subdivided to the greatest possible extent consistent with efficient working, so that in the event of a collision or a torpedo making a hole below water the ship may not sink. As far as possible the divisions or bulkheads are made to run right from top to bottom without any openings, but that obviously is a very inconvenient arrangement, so in many places there have to be doorways through them, leading from one part of the ship to another. In such cases these are closed by water-tight doors, which can be shut before the ship goes into action or into any dangerous region.
The engines of these vessels are now always turbines. This type of engine has many advantages over the older type, in which certain parts move to and fro, that motion being changed by cranks into a round and round action. For one thing, they are lighter for a given power, so that more power can be put into a ship without adding to the weight. That means higher speed. Then there is less to get out of order. Anyone who has been into a ship's engine room where to and fro or reciprocating engines are at work will realize this, for there is a maze of rods and cranks all moving together, and many parts which need to be oiled while in motion and which would get hot and tight if they were not carefully looked after. All this in an enclosed space with possibly an uncomfortable motion of the whole ship used to make the engineer's life at sea a very hazardous and unhappy one.
But the turbine is entirely enclosed. There is nothing to be seen moving at all. Indeed, there is only one moving part, and that is coupled directly to the propeller-shaft, so that nothing could possibly be simpler.
When it is decided to build a certain ship, the first thing to be done is to draw it on paper. The Admiralties of the world, and also the great shipbuilders, have each their own chief designer installed in a big, light, quiet office fitted with large strong, flat tables at which work a number of draughtsmen.
The naval authorities tell the "chief" in general terms what they want the ship to be capable of, and he determines its size and form. Then the draughtsmen work out his ideas on paper, themselves deciding upon the minor details, until they have produced exact representations of the ship which is to be. Some draughtsmen deal with the actual hull of the ship, while others design the various fittings and minor details, all working, of course, under the constant supervision of the chief.
In this connection one may perhaps allude to a matter which the general public often seems to misunderstand—the work and functions of a draughtsman. I have heard people say of a boy that he is good at drawing so they think of making a draughtsman of him. Now the point is that the actual drawing is perhaps the least important part of a draughtsman's work. He has to knowwhat to draw. He is given just a rough idea of something and from that he has to produce a perfect design, bearing in mind that the thing to be made must well fulfil its purpose, must be easy and cheap to construct, must be strong enough yet not too heavy, must be made of the most suitable material and so on. He has to possess a good deal of the knowledge of the skilled workman, he has to be something of a scientist and a good mathematician in addition to his ability to make neat and accurate drawings. So, you see, these men whose minds conceive the details of our great ships are men of long training and experience, with far greater knowledge and skill than we sometimes give them credit for.
Anyway, there they stand, each at his own table, bending over his own drawing-board, each doing his own particular share towards producing the perfect ship.
But when all is said and done, there are limitations to the cleverness of the cleverest among us, so the next step, after the draughtsmen have done their best, is to test what they have done by experiment.
Years ago a certain Mr. William Froude interested himself in the question of the best shapes for ships, and he found that by making an exact model of a ship and then drawing that model through water it was possible to foretell just how that ship would behave. He built himself a tank for the purpose of these experiments at Torquay, where he lived, and by its aid he added a very important chapter to the science of shipbuilding.
Nowadays the Admiralty have a large and well-fitted tank at Portsmouth, the United States Navy have one at Washington, private shipbuilders have the use of a national tank at Bushey, near London, while several of the large firms have tanks of their own.
The national tank at Bushey, by the way, was given to the nation by Mr. Yarrow, a famous shipbuilder, in memory of Mr. Froude, it being called the "William Froude Tank" in recognition of the great work done by him.
Now these tanks may be described as rather elongated swimming-baths. Such a structure is generally a little narrower than the average bath, but it is longer and much deeper.
At one end there are miniature docks in which the models float when not in use, while at the other there is a sloping beach upon which the waves caused by the models expend their energy harmlessly.
Along each side there runs a rail upon which are supported the ends of a travelling bridge. Driven by electric motors, this bridge can run to and fro from end to end of the tank, and its purpose is to drag the models through the water.
Carried upon the bridge is a platform which bears a number of instruments, chief among which is a self-recording dynamometer.
Now a dynamometer is an instrument for measuring the force of a "pull," and when we call it self-recording we mean that it automatically takes a record of a series of pulls or of a varying pull. In this case there projects below the bridge a lever, tothe end of which the model under test is attached. As the bridge rushes along it pulls the model through the water by means of this lever, and the force which is expended in doing so is recorded in the form of a wavy line upon a sheet of ruled paper.
If the model slips through the water very easily there is little pull upon the lever and the line drawn by the pen of the instrument remains low down upon the chart. If, however, much power is needed and the pull is a strong one the pen moves and the line rises towards the top of the paper. Any change, whether increase or decrease, is thus shown by the rise or fall of the ink line.
One model can be thus tried at various speeds and its behaviour noted under different conditions. Other matters can be investigated too, such as whether or not the bow rises in the water or falls when the boat is in motion, also how much such rise or fall may amount to.
The suitability of a certain shape of vessel, moreover, can to a certain extent be seen by observing the commotion which it makes in the water. Everyone has noticed the way in which a ship throws up a wave at its bows, and that bow-wave, as it is termed, represents so much energy being wasted. The power of the engines is absorbed to a certain extent in making that wave. It is impossible to make anything which when forced through the water will not make some wave, but certain forms cause less of it than others, and the designer of a ship seeks to find that form which will make the smallest bow-wave.
In like manner the eddies which a ship leaves inits wake are the result of wasted energy, and the ship must be so shaped that they too will be reduced to a minimum.
Shipbuilders find that there are three things which retard a ship's movement: skin friction, or friction between the water and the sides of the ship; wave making at the bow and eddy making at the stern. The first depends largely upon the smoothness of the ship's surface, the second and third depend upon its shape. If a model behaves badly in the tank the fault may be either too much wave making or too much eddy making, and which of these it is the dynamometer does not of course tell. In many cases the experienced eye of the tank officials furnishes the clue to the trouble, but in some cases a cinematograph is used to make a complete series of photographs of the model and the water around it as it rushes from end to end. These can then be studied in conjunction with the chart and the cause of the fault discovered.
The real aim, it is obvious, of all these tank experiments is to find out the lowest horse-power necessary to drive the ship, or the best form of ship to get the highest speed out of a given horse-power.
The cost of keeping up these large tanks and making the models and conducting the experiments is very great, for not only are the premises very large (I know one in which the water alone cost nearly a hundred pounds) but a highly skilled staff is necessary. The saving effected in the cost of ships and the superior efficiency of the ships makes it well worth while however.
There is still one other point about this matter which will possibly be puzzling the observant reader. What are the models made of and how are they made? They are made of paraffin wax, and a very important department of the experimental tank is that where the models are formed.
First of all a rough mould is fashioned by hand in modelling clay and into this is poured melted wax, the result being a very rough model of the ship. This is then placed in the model-making machine.
Those of my readers who are familiar with an engineer's shop will know what a planing machine is like, and from that they can form an idea of the general structure of this remarkable tool. There is, first of all, a travelling table which, as the machine works, travels to and fro. Spanning this table is a beam which carries on its under side two revolving cutters, so that as the table passes beneath them the cutters can operate upon anything placed upon the table.
Another part of the machine is a board upon which is placed the drawing showing the external shape of the proposed ship, and working over this board is a pointer connected by a system of rods and levers to the cutters just mentioned. The rough block of wax, then, having been placed upon the table and the to and fro motion set going, the attendant guides the pointer along the lines of the drawing, and as he does so the cutters so move as to carve away the soft wax into the precise shape of the model.
A little smoothing by hand is all that is necessary to complete the conversion of the rough piece of waxinto a perfect model. It is then placed in the water and ballasted with little bags of shot until it floats at just the correct depth, and finally a light wooden frame is fitted to it for the purpose of making the connection to the lever by which it is pulled along.
Thus, after much thought and experiment, the designs for a new ship are completed. Tracings are then made of them on semi-transparent paper or cloth, which tracings are then used as "negatives," from which a number of photographic prints are made, just as the amateur photographer makes prints from his negatives. At least that is how they used to be done, in a huge printing frame, but nowadays a machine is more often employed which passes the tracing or negative with a piece of photographic paper behind it slowly past an electric light, thus doing the work more quickly and more conveniently, for the drawings of ships are often very long and would either require an enormous frame or else would have to be made in pieces and joined together.
The prints are finally passed out to the works to be translated in terms of iron, steel and wood.
Perhaps the most important part of a shipyard is the mould loft, a large apartment on the floor of which the ship is drawn out full size. Then from these full-size drawings moulds or templets are made of wood or soft metal, showing the exact size and shape of the various parts. The moulds or templets go thence to the workshops, where the bars and plates of steel are cut to the right shape and perforated with holes, and some of the pieces are there joined together with rivets.
The Tripod Mast.Here we see one leg of the tripod mast of a warship. These masts have greater stability and freedom from vibration than others. They are used for observation and range-finding, and have a fighting-top on which guns of small calibre are mounted. Here is shown a sailor carrying a wounded comrade.
The Tripod Mast.
Here we see one leg of the tripod mast of a warship. These masts have greater stability and freedom from vibration than others. They are used for observation and range-finding, and have a fighting-top on which guns of small calibre are mounted. Here is shown a sailor carrying a wounded comrade.
From the workshops the various pieces or parts go to the yard where the slip is on which the vessel is being built. This slip is by the water's edge, conveniently placed with a view to the fact that later on the great structure, weighing possibly thousands of tons, has got to slide down into the water.
Where the keel of the ship is to go a row of timber blocks is placed a few feet apart, and upon these blocks the plates of steel which form the lowest part of the ship are laid. Upon them are laid other parts, and upon them others, the joints being made by riveting. Thus the great ship grows from the keel upwards. As she gets bigger and bigger there comes the danger of her tipping over, and that is provided against by the use of props or shores along both sides.
By the time the hull is ready for launching it is often of great weight, all of which is borne upon the wooden blocks underneath the keel. Consequently, if the ground be not good, piles have to be driven in or concrete foundations laid to enable the huge mass of the ship to be supported. For this reason a large vessel cannot be built anywhere but only on a properly prepared "slip," and it is the possession of a large number of such places which enables Great Britain to build so many ships at once.
Along each side of the slip there is usually a row of tall masts with a beam projecting out sideways near the top of each, forming cranes by which the heavier parts can be hoisted into position.
In other yards, again, there is a tall iron structure called a gantry along each side of the slip, while travelling cranes span across from one to the otherover where the growing ship lies. These travelling cranes, worked by electricity, permit heavy weights to be handled with ease and safety. Other subsidiary cranes, meanwhile, carry the heavy hydraulic riveting machines by which riveting is done.
Much riveting is done by hand, men working together in squads of four. Of these one, often quite a boy, heats the rivets in a small furnace, after which he throws them one by one to man number two, who inserts each as he receives it in its proper hole and holds it there with a big heavy hammer or else a tool called a "dolly." Number two is called the "holder-up," since he holds the rivet up in its place while the remaining two hammer it over with alternate blows of their hammers.
In many cases, however, the two last described men give place to one, who is armed with a tool in shape much like a pistol and operated by compressed air obtained through a flexible tube. When he presses a trigger a little hammer inside the "pistol" gives a rapid series of blows to the rivet, completing the job more quickly than the two men can do with hand hammers.
A third way of doing this operation so important in the building of a ship is by the hydraulic machine suspended from the cranes. To the casual onlooker this has the notable feature of being silent, whereas riveting by hand and still more by a pistol hammer is terribly noisy. The reason for this is that the hydraulic riveter does not hammer at all, but, like a huge mechanical hand, it takes the rivet between finger and thumb and just squeezes it down.
One strange result of all this hammering in of rivets is that every ship by the time it leaves the slip has become a huge magnet, with somewhat disconcerting effects upon its own compasses, but of that more later on.
Thus the great ship grows, being made piece by piece in the workshops to the shapes indicated from the mould loft and put together and riveted on the slip, until finally in due time it is ready to take its first journey.
The launching of a big ship always strikes me as about the boldest and most daring thing which is ever done in the course of industry. For the huge structure, naturally top-heavy, weighing hundreds or thousands of tons, is just allowed to slide at its own sweet will. From the moment it starts until it is well in the water it is in charge of itself, so to speak, and if anything were to go wrong no power on earth could stop it once it had got a start.
That nothing ever does go wrong, or scarcely ever at all events, is due to the care with which all preparations are made before that critical moment when the ship is let loose and to the skill and experience of those in charge.
As the hull reaches that degree of completion when it can safely be put in the water, strong wooden structures termed launching ways are constructed one on each side of her. These really act like huge rails upon which in due course there will slide a gigantic toboggan. Tremendously solid and strong they have to be, as they have each to carry half the total weight of the ship.
Under each side of the ship and upon the launching ways there is built a timber framework capable of raising the ship bodily off the blocks upon which until now it has reposed. These two frames, being connected together by chains passing beneath the keel, constitute what is called the cradle, the "toboggan" which is to slide down the ways, bearing the ship upon it.
It is easy to see that being top-heavy something must be done to give the ship support before the shores on either side can be taken away, and it is equally clear that these latter must be removed before she can slide down to the water. Neither would it do to let the vessel slide upon her own plates, so we see that the cradle fulfils a twofold purpose, first enabling the ship to reach the water without ripping holes in her own plates, and secondly giving it the necessary side support to prevent it from toppling over on the way.
When all is ready, but a short time before the hour appointed for the launch, a curious operation is performed. Between the main part of the cradle and the part which actually slides upon the ways wedges are inserted, hundreds of them, and they are all driven in simultaneously. Their purpose is to make the cradle slightly higher and so to lift the ship off the blocks upon which it was built. If they were driven in one at a time each would only dig its way into the timber and nothing else would happen, but being driven all together a most powerful lifting action is produced which actually raises the mighty ship. So hundreds of men stand, each with hishammer ready to strike a wedge, while the foreman stands by with a gong. At a stroke on the gong the hundreds of hammers strike as one, and so the ship is raised off the blocks, which can then be removed, to facilitate which they too are built of wedge-shaped pieces which can easily be knocked apart. The shores, too, have ceased to serve any useful purpose and can be taken away until at last all shores and all blocks are gone and the vessel rests upon the cradle only. Meanwhile tons of grease have been put on the ways, and the ship, urged by its own weight, is straining to get down the greasy slope into the element for which all along it has been intended. At this stage the only thing which restrains it is a kind of trigger arrangement on either side which locks the cradle in its place. In some yards elaborate mechanical catches controlled by electricity are used for this, but in many the old device of "dog shores" is still used. These are simply two stout wood props which fit between a projection on the ways and one on the cradle, there being one dog shore on either side. Just over each dog shore there hangs a weight.
The person who performs the ceremony cuts the cord which holds the weights, the weights fall, the dog shores are knocked away, and the ship is free. Slowly at first, but gathering speed every moment, she moves majestically downwards into the water, being ultimately brought to rest by means of chains.
Whether done by the simple dodge of cutting a cord or by the more refined method of pressing an electric push, the launching is generally preceded bythe breaking of a bottle of wine against the bows and the pronouncement of the vessel's name.
Once safely afloat, the vessel is towed away and berthed alongside a wharf whereon are cranes and other machines which lightly drop on board of her the massive turbines and boilers which in time will propel her, and the guns with which she will fight. All the multitudinous little finishing touches are here put into her until at last she sallies forth on her trial trips to show what she is capable of, after which follow trials of her guns, and then she takes her place in the fleet.
Thus, briefly sketched, we see the history of the warship from her inception in the minds of her designers till she is ready to meet the foe.
In parts of South America there lives a little fish, which, if you touch its nose, gives you a severe electric shock. The natives call it the "torpedo." When an artificial fish came to be invented, capable of giving a very nasty shock to anyone touching its snout, that name was bestowed upon it too.
Even more than the submarine, the torpedo resembles a fish with its graceful outlines and its fins and tail, the chief difference being that the tail of the torpedo carries a couple of little rotating propellers. Looked at another way we may say that the torpedo is an automatic submarine. As a matter of fact, we all know it best as the weapon of the submarine.
It was originally invented by an Austrian who took it to a Mr. Whitehead, an Englishman who then had an engineering works at Fiume. This gentleman took up the idea and developed it into the Whitehead torpedo, which is to-day used by half the navies in the world, the rest using something very similar. It is curious to note that the German variety is calledthe Schwartzkopf, the meaning of which is "blackhead."
The smooth, steel, fish-like body consists of two separate parts, which can be detached from each other. The front part called the "head" is made in two kinds, the war-head and the peace-head. The former contains a large quantity of explosive and the mechanism for firing it on coming into contact with any hard body. It is only used in actual warfare. The peace-head is precisely the same shape and weight as the other but is quite harmless, so that when it is fitted to the torpedo the latter can be handled with perfect safety, a valuable feature during the frequent exercises through which our sailors go in their efforts to attain perfection in the use and handling of these valuable weapons.
So much for the head. The body of the torpedo contains a beautiful little engine precisely similar to a steam-engine but on a small scale, which is driven by compressed air, a store of which is carried in a compartment provided for the purpose.
Then there is an automatic steering apparatus controlled by a gyroscope, the purpose of which is to keep the torpedo steered in precisely that direction in which it is started. If any outside force, such as current or tide, deflects it from its path the gyroscope, acting through a rudder at the tail, brings it back again.
Like the submarine, moreover, it has rudders which can steer it upwards or downwards and these again are controlled automatically so that having been set to travel at a certain depth the torpedo can belaunched into the water with the practical certainty that it will descend to that depth and then maintain it.
This remarkable result is attained by the use of two devices acting in combination, namely, a hydrostatic valve and a pendulum. Either of these alone would set the thing going by leaps and bounds, at one time above the required depth and at another equally below it, and so on alternately. The hydrostatic valve consists of a flexible diaphragm, one side of which is in contact with the water outside, so that since the pressure increases with increasing depth, it is bent inwards more or less as the depth varies. This deflection is made to control the horizontal rudders. Suppose that things are adjusted for the rudders to steer the torpedo horizontally when at a depth of ten feet: if it descends to twelve feet the increased deflection of the diaphragm will so change the rudders that they will tend to steer slightly upwards: if, on the other hand, it rises to eight feet the contrary will happen, with the result that it will descend. As has been said already, this alone would result in a continually undulating course, so the pendulum is introduced to check the too decided changes in direction and so produce a practically straight course.
There is an interesting feature, too, about the propeller. It is "twin" but not, as in ships, two screws side by side. Instead, they are both set upon one shaft or rather upon two concentric shafts, like the two hands of a clock. The hour-hand of a clock is on one shaft, a solid one, which itself turns insidethe shaft of the minute hand, which is hollow. The propellers of the torpedo are likewise, one on a tubular shaft and the other on a solid shaft inside it. These two shafts turn in opposite directions, but since the two propellers are made opposite "hands" they both equally push the torpedo along. The reason for this arrangement is that without it the action of a single propeller would tend to turn the torpedo over and over. Instead of the torpedo turning the propeller the propeller would to some extent turn the torpedo.
The range of the torpedo depends, clearly, upon the quantity of compressed air which it is able to carry and that is limited by certain practical considerations. One of these is the space required to store it, and a very ingenious method has been invented whereby the limited supply is eked out so that in effect its quantity is increased. As the air is used up the pressure in the air-chamber naturally falls and when that has gone on to a certain extent chemicals come into action which generate heat, whereby the remaining air is raised in temperature. This, of course, increases the volume of air and the result is just the same as if a greater quantity were carried to commence with.
The explosion is brought about by the pressing in of a pin which normally projects from the nose or point of the torpedo, and it would be very easy to knock this accidentally, causing a premature explosion, were not precautions taken to prevent it. These take the form of a little fan which is turned by the water as the torpedo proceeds through it. The firing-pin is locked by means of a screw so that it cannot be operated until it has been released by the withdrawal of the screw and that can only be done by the fan. Thus, while on the submarine or whatever ship carries it, the torpedo cannot be fired: it only becomes capable of explosion after it has passed through the water for a certain distance, far enough, that is, for the fan to have undone the screw. Thus the maximum of safety is combined with the maximum of sensitiveness when the object aimed at is struck.
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 shore, 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 shore 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 thespouting 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.
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 asserted 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—thatis, 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. Ithas 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 feetthe 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 shell 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, untilonly 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 constitutes 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 constitute 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 compass 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 compass 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 compass 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 outerwater that as the ship dives its depth is recorded upon its dial: that in effect is to him what the compass 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.