One boy asks what would happen if anything went wrong with the pumps while the submarine was deep down in the water. How could it ever rise again? The answer to that question is that the submarine is not dependent upon the pumps to get the water out of her tanks. The pumps are used to force the sea-water into her tanks, but there is plenty of compressed air stored in the submarine, and this can easily force the water out. Here is an old-fashioned toy which explains the matter very simply. In case you may not have played with one of these toys, I shall tell you about it.

Inside a glass jar filled with water there floats a little glass man. The mouth of the bottle is covered with a piece of strong rubber, so that the little man is a prisoner and cannot rise out of the water. But how does the little man manage to stand erect? Because he is hollow, and a little water has been put inside him to weigh his feet down.

If you now press your finger firmly upon the rubber cover the man immediately goes down and stands at the bottom of the glass. The moment you release the pressure upon the cover, up he comes again. You may get the little man to sink to any desired depth; he may stand half-way down or at any other place you wish. But what has all this to do with submarines? The principle of sinking the little glass man in the toy is the very same as that employed in sinking and floating a submarine.

What really happens to the little doll is that when you press upon the rubber cover you force some water into the doll through a very small hole in the glass. In doing so the doll becomes heavier and sinks, and as the doll was filled with air the water presses the air into a smaller space to make room for itself. When you release your pressure upon the rubber the compressed air forces the water out of the little doll, and thus regaining its former lightness, up it comes to the surface.

In the submarine we force the water into the tanks by means of pumps, but we need not worry about compressing the air in the tanks, as there is plenty of compressed air stored within cylinders in the submarine. Therefore when it is desired that the submarine should rise, all the crew have to do is to open certain valves leading to the tanks and let the compressed air force the water out again, and up rises the submarine.

Another boy asks how the commander of the submarine can tell to what depth he has sunk his boat. This is done very easily and by means of a pressure gauge. The open end of this pressure gauge passes out to the sea, and the deeper down the submarine goes, the greater will be the pressure of the water upon it, and so more water will be forced into the pressure gauge. The pressure will always be the same for the same depth, so the gauge is marked off to indicate feet, and by looking at the position of the water in this pressure gauge the Commander knows exactly to what depth he has taken his boat. The actual instrument will be made with a dial and an indicating finger to point to the number of feet, but this indicator will be moved directly by the rise and fall of the water in the gauge.

Another boy asks how the Commander can keep his boat level while under the water. That is quite a sensible question, for it is apparent that although the boat will remain level while on the surface of the sea, it might travel at almost any angle while wholly immersed in the sea. One boy suggests that they could have a sort of pendulum arrangement which would show when the boat was level. If the bow of the boat were tilted upwards then the bob of the pendulum would swing towards the stern, and so on. When the pendulum, hanging straight down, is perpendicular with the floor of the submarine, then the boat will be level.

But suppose the Commander finds his boat has set her nose to dive upwards or downwards, how can he right her? He has two diving rudders, one on either side of the ship at the stern. These rudders go out sideways from the boat, what we describe as horizontally, and are not upright or vertical, like a steering rudder. By moving these diving rudders the Commander can make the boat dive upwards or downwards at will. And so by operating these and watching the pendulum indicator he can run the vessel along on the level although he is far below the surface of the ocean.

Another question refers to the firing of torpedoes. How can these be shot out of a ship under water without the water rushing into the ship? One boy says that the torpedo does not require to be shot out of the submarine, as the torpedo has propellers to drive it along just like a miniature submarine. That is quite true of the torpedo, as we shall see in the following chapter. But the torpedo must be launched out of the submarine, and it must get a send-off in the proper direction, and so the submarine is equipped with torpedo tubes.

The torpedo tubes are in the bow of the submarine. The tube has a water-tight door at each end, so that by opening the inner door while the outer door remains closed, the torpedo may be placed in position in the launching tube. Then the inner door is securely closed before the outer door is opened, and now the torpedo is free to pass out of the submarine. We shall see how the torpedo goes when we come to look at this invention in the following chapter.

Another question is: How do the men get air to breathe down below the water? You know that a man in a diving dress gets a regular supply of air from the surface and that air is forced down from a boat by means of an air-pump through long rubber tubes, connecting the diver to the air-pump. This cannot be done in the case of a submarine, for the whole idea of a submarine is that it may be quite independent of any other vessel, and approach an enemy without being observed. However, the crew of a submarine have no fear of a shortage of fresh air, as they have such a quantity of compressed air stored away, and if necessary they can draw upon this store. In reality they may never have to open any of the air valves, for there are so many air tubes and valves from which there must always be some air escaping into the submarine, and this alone may be sufficient to give them plenty of air for breathing.

One boy asks if it would not be a good plan to carry some cylinders of compressed oxygen, as it is that gas which we use from the air in breathing. Probably this boy has known of someone, who was very ill, being given oxygen by the doctor's orders. The object of this is to revive the patient. If the crew of the submarine were to breathe pure oxygen it would have too much of a reviving effect; it would excite them, and their duties are such that they must keep very cool. It is true that it is the oxygen of the air which we use, but when we take a breath we inhale a mixture of nitrogen and oxygen. There is about four times as much nitrogen as oxygen, and our breathing apparatus is adapted for dealing with this diluted mixture.

One boy is anxious to know if the crew could escape from a submarine if the boat should happen to be run down and sunk. The answer to this question is that under certain circumstances they might escape, but it is not likely that they could, and if the boat sank into very deep water they certainly could have no hope. A plan of escape has been invented, and this has been tried in a large experimental tank in the Naval Dock at Portsmouth. At the bottom of the tank is what we might call a dummy submarine, and here the men may practise putting on a special diver's helmet and waist-coat. Equipped with this, the sailor may open the hatch of the conning-tower and float to the surface. But one boy suggests that if the submarine should fill with water the men would be drowned before they could find and don these divers' helmets, and one boy says that in any case the diver would only have water to breathe if he did succeed in putting on the helmet. That is what would happen unless the sailor could find air to breathe in the damaged submarine, but this can be arranged in the following simple manner. Inside of the submarine two partitions hang down from the roof as shown in the drawing on page 148.

You will see that even if a hole should be made right in the very top of the submarine the inrush of water would lock a certain amount of air between the partitions and the sides of the boat. It is in this space that the divers' helmets are kept, and with a little practice in the Naval Tanks the men can find the best means of donning the safety waistcoats and helmets.

But one boy asks where the sailor is to get air when he has once closed the helmet. The man could not live very long inside a closed helmet; he would soon be suffocated. Why? Because he would gradually use up all the oxygen, and breathing out carbonic acid gas, he would soon be choked. But there are some chemicals placed in the helmet, and the moisture of the man's breath causes the chemicals to take up the carbonic acid, and to give off oxygen. These chemicals will keep the air all right for about an hour, and if the man can get out of the submarine through the hatch, he will float to the surface. Here he can open the little window in the diver's helmet, and he is once more in the open air. To add to his safety he can blow air into a part of the waistcoat, which acts exactly like "wings" used by children while learning to swim. Then all the man requires is a friendly steamer to pick him up.

This safety helmet is of interest from the invention point of view, but I doubt if it is reckoned of much service in war time. It was invented before modern submarines had ever fought in war, and what suggested the invention was that on more than one occasion the submarine, while practising in peace time, was sunk by accident, and the crew were trapped in the sunken boat and drowned.

One boy asks how long a submarine can remain submerged in one position, with its periscope above the surface, on the look out for an enemy. I ask him to guess how long, but he says that his reason for asking the question is that he wondered if the submarine could really lie still at one depth. He is quite right in doubting this, and I am curious to know what made him think of this difficulty.

He tells me that on one occasion he filled a bath with water, and taking an empty glass bottle and a well-fitting cork, he tried to make the bottle represent a submarine. He had no difficulty in making the empty bottle float; representing a submarine on the surface. By filling the bottle with water instead of air, he could make his imitation submarine go to sleep at the bottom of his miniature ocean. He then tried filling the bottle part with water and part with air, and he could get it to sink very gradually. When he tried the bottle with a little less water, and then placed it under the water, the would-be submarine would rise upwards very gradually, but try all he could, he found it impossible to get the bottle to remain submerged at any given depth.

If this boy had a real submarine to experiment with, he would find that it acted in the very same manner as his disobedient bottle. The submarine has to keep on the move, or she would rise to the surface. She is always left with some buoyancy, so the Commander can steer her up and down at will. The only time that she can stay under water without moving is when she goes to sleep at the bottom of the sea. Of course she might be anchored at any depth, but that is not very convenient.

One boy asks if it is true that submarines carry a safety keel which they can unfasten in case of accident, and thus enable the boat to float to the surface, in the event of its being impossible to force out the water ballast. All I can say is that some submarines do still use this invention, which was first used in one of the very primitive submarines to which I referred in the last chapter. The Austro-Hungarian Navy allowed an American to describe one of the Austrian submarines which was launched in 1915, and which had a detachable keel weighing five tons.

At what speed can a submarine travel? It is apparent that it will be able to travel much faster on the surface, when it can use its powerful oil engines, than it can do under water, where it must depend upon the storage batteries to drive its electric motors. The speed on the surface may be as much as twenty miles an hour or more, but under water it cannot be much more than half that speed.

How far can a submarine travel on its own account without having to get a further supply of oil? The possible distance will depend upon the speed at which the boat travels. To go at a high speed, a ship requires to use a much greater proportion of fuel than when it is travelling at a low speed. This is due to the greater resistance of the water to any object moving quickly through it. If a submarine were to go on the surface at a high speed it might cover a distance of 3000 miles, but it could almost double that distance if it went more economically at a low speed.

That the submarine is a very useful war invention was proved at the very outset of the Great European War, when three British warships were sunk by one enemy submarine. Unfortunately the enemy began to use their submarines for destroying non-fighting ships, and drowning innocent passengers, which, of course, was entirely against the rules of warfare.