You remember that when we were talking about bullets flying through the air we found that the ocean of air offered a great resistance to the passage of the quickly flying bullet. And yet you can move your hand to and fro in the air quite freely. You have no difficulty in realising that water is much thicker or what we call denser than air. When you are bathing, you find that you cannot move your hand to and fro under the water, without some effort. If the water did not offer considerable resistance to the movements of your hands and feet, you could not push your way along as you do in the act of swimming.

When you are throwing a stone through the air the resistance of the air does not worry you very much, but if you try to throw a stone under water, you will find that the great resistance of the water prevents your throwing the stone to any distance. And so it is apparent that if a submarine had to shoot a torpedo at the enemy ship, the submarine would require to be close up to the ship. It would also mean that the enemy ship would require to be at rest, whereas a ship may be torpedoed while it is steaming along. It would also mean that the submarine being so close would be blown up along with the enemy ship.

The boys or girls who suggested that a torpedo is a small ship that goes under the water and attacks an enemy ship were quite correct. A torpedo is just a small submarine; it has propellers and engines to drive it along; it has a steering rudder and also diving rudders to keep it at the required depth. But there is no one on board this small ship to control it. Therefore it must be self-acting, or what we call automatic.

Of course the engines could be started before the torpedo left the submarine, and the torpedo could travel along as your mechanical toys do. Those boys who have had clockwork boats will agree that a clockwork engine would not be good enough for a torpedo which has to travel a long distance and which must go very quickly if it is to hit the moving enemy ship. A steam-engine cannot be used, as the torpedo is a submarine. However, we may use compressed air to drive an air-engine instead of a steam-engine. This is the most convenient kind of engine to put on board a torpedo, so we have one part of the torpedo filled with compressed air.

We all had one complaint against our mechanical toys: they set off very well, but they fell off in speed very quickly, getting slower and slower, until they came to a standstill. The reason was that as the clockwork spring unwound, the pressure became less and less. Now there is the same difficulty about compressed air; as the air is withdrawn to drive the engines the pressure of the remaining air becomes less and less. This would not matter if the torpedo could reach the enemy ship before the falling off of the air pressure began to tell. But the submarine may be required to strike the enemy ship from a distance of, say, a quarter of a mile, and before the torpedo had travelled that distance its speed would have begun to fall off considerably. As for discharging torpedoes from battleships, it is not likely that the ships would ever get within such close range, unless in fog or darkness, for their great guns would enable them to destroy one another from a greater distance. You can understand how necessary it is that the speed of a torpedo must not fall off. In the great naval battle off Jutland (June, 1916) some of our warships discharged torpedoes at enemy ships which were about three miles distant. The invention which enables the torpedo to continue at full speed is very ingenious, but unfortunately it is not permissible to describe it.

You must not picture the torpedo travelling along at a speed similar to that of a mechanical toy. If the torpedo cannot travel quickly it is of no use; it must make haste if it is to strike the moving steamer. I ask the boys to guess at what speed the torpedo travels. Knowing that the great shells from some of our giant guns set off with a speed of 2000 miles per hour, one boy guesses that a torpedo travels about 1000 miles per hour. But this boy forgets the great resistance offered by the water, and he also forgets, what is of more importance, that the torpedo is not shot off like a shell. Remembering that a submarine does not travel any more than ten miles an hour while under water, another boy suggests that a torpedo travels at a similar speed, but a torpedo can travel four times as fast as a submarine. The speed of a modern torpedo may reach 36 knots, which means 36 nautical miles per hour, and as a knot is equal to about 14 land miles, 36 knots is equal to about 41 miles per hour.

We see how this little automatic ship, called the torpedo, can fly along through the water from the submarine to the enemy ship. But suppose a current of water should turn the nose of the torpedo to one side, off it would go in a wrong direction. That would render it useless, as it would not strike an enemy ship. But one boy says that the torpedo has a steering rudder to keep it travelling straight; if he thinks for a moment, he will remember that when out in a boat we do not use the rudder only when we wish to alter our course. We have to keep using it to counteract the effect of the water currents which would turn us out of our way. But the torpedo would appear to be in a hopeless state, as it has no one on board to control the steering rudder. It looks as though we must just set the rudder as we think best, and chance that being able to keep the torpedo on a straight course, but that is not what we do.

There is a very clever invention which controls the steering rudder of a torpedo. This is called a "gyroscope." The gyroscope was not invented for this purpose. It is far older than torpedoes, but it has been applied to the torpedo. Most of you will know what a gyroscope is. Some of you have played with a small gyroscope sold as a toy. It is like the fly-wheel of an engine mounted within a ring. Here is a drawing of a simple gyroscope.

You know how it resists any attempt to turn it into another position; it wants to stand steady in one position. Suppose you mount a small gyroscope on a toy boat. You could turn the boat to the right or the left, and the gyroscope would keep on pointing in one direction. In a torpedo the gyroscope will keep acting in the same manner, but this action does not prevent the torpedo turning one way or the other. The gyroscope is made to control the steering rudder. If the torpedo tends to turn to the left, the gyroscope, refusing to turn, pulls the steering rudder so that the torpedo cannot go to the left. If the torpedo tends to turn to the right, then the gyroscope pulls the rudder the other way and keeps the nose of the torpedo straight.

One boy suggests that a very strong water current, by keeping pressing the nose of the torpedo round, might cause such a strain on the rudder that it would in the end succeed in turning the gyroscope. He is quite right; the gyroscope could be forced to alter its position in this way, but the difficulty is overcome just in the same way as the difficulty in steering a great steamer is overcome. If a sailor had to turn the rudder of a very large steamer, he might find a current of water pushing his ship round so hard that he could not pull the rudder against it, and so the rudder might force his steering wheel round in the opposite direction to that in which he tried to move it. You know how this difficulty is overcome by making small engines do the actual work of turning the rudder. The man at the wheel only controls those little engines which turn the rudder. When he turns his steering wheel in one direction, one of the two engines pulls the rudder to the right, and when he turns the wheel in the other direction, the other engine pulls the rudder to the left. No matter what water current opposes the movement of the rudder, these do not worry the man at the wheel, as the engines take up all the strain. The gyroscope is just in the same position as the man at the wheel. All the gyroscope does is to control two small air engines which turn the steering rudder. Any opposition to the movements of the rudder does not reach the gyroscope. And so we see how the torpedo can be kept steering straight at the enemy ship. But it is most important that the torpedo does not duck down any lower in the water or rise to the surface.

We saw how the Commander of the submarine had to operate his diving rudders constantly, to keep the submarine at the required depth; how the Commander had to watch a pendulum arrangement to see that his boat was travelling in a level course; and how the water-pressure gauge told him at what depth he was. How is the torpedo to do all this on its own account?

Instead of an ordinary water-pressure gauge, which merely indicates the amount of pressure, and leaves the observer to take action, the torpedo has a special water gauge. The pressure of the water pushes against a little metal plate; what we call a diaphragm. You have such a diaphragm in a telephone receiver; it is made of a thin sheet of iron, and is fairly flexible. In the water gauge the diaphragm is caused to bulge inwards when the pressure increases, and you know that the pressure will increase if the torpedo should sink into deeper water. Therefore if the torpedo should tend to duck downwards this diaphragm will bulge inwards, and in so doing it will move the diving rudders, so that they cause the torpedo to steer upwards. On the other hand, if the torpedo should dive too far upwards, the diaphragm will not bulge in, and the diving rudders still steer the torpedo downwards. Of course the diaphragm and the diving rudders have to be arranged to balance each other at the depth at which the torpedo is to keep.

While this ingenious arrangement would ensure that the torpedo did not rise too high or fall too low, the torpedo's course might be something like that of a switchback; in any case, it is not likely that its course would be level, and so there is added a pendulum arrangement, which also controls the diving rudders and helps to keep the torpedo level.

By the very clever invention just described, we can depend upon the torpedo travelling straight to the enemy, provided it is well aimed to start with.

Although the torpedo is not to be shot at the enemy ship, it is necessary to give it a good send-off. We saw in the previous chapter that it is dispatched from a torpedo tube. As the submarine is not at rest, and as the enemy ship is almost sure to be in motion also, it is necessary to lose no time in setting off the torpedo. The necessary push-off may be given by compressed air.

When it reaches the enemy ship, the torpedo must explode, and how this takes place will be explained in the following chapter. But we have been talking about the torpedo as though it belonged entirely to the submarine, and if you knew nothing of the subject you might imagine that the idea of the torpedo had been got from the invention of the submarine. The torpedo is practically an automatic submarine, having propellers, steering rudders, and diving rudders, which might pass as copies of those of the submarine. But that is not how the torpedo came to be invented, for this ingenious torpedo was invented before we had any submarines.

Our great battleships carry torpedoes, but ships which have guns capable of hitting the enemy ten miles away cannot often have opportunities of using torpedoes. A torpedo may be fired from a tube on the deck as well as from a tube under water, as, once in the water, the torpedo will keep the required depth. Seeing that the great battleships were not likely to get close enough to the enemy to use their torpedoes, it was suggested that we should have special torpedo boats. Although it is possible to send a torpedo nearly three miles, it would not be easy to hit a moving ship at such a distance.

You have heard people speak of torpedo boats and torpedo boat destroyers, and I have been asked more than once what is the difference between these two kinds of boats. The torpedo boat was built specially to get close enough to the enemy ship to fire torpedoes at her. In the dark one of these torpedo boats might succeed in getting close enough to send a torpedo, but in daylight the only hope would be for a group of these boats to approach the enemy, and trust to one of them striking the enemy ship before the torpedo boats were all sunk.

The invention of these torpedo boats gave rise to the torpedo boat destroyers. These destroyers were made to travel so fast that they could overtake the enemy torpedo boats, and with quick-firing guns destroy the torpedo boat, thus ridding the great battleship from the danger of torpedo attacks. These destroyers were fitted with torpedo tubes, so that they became torpedo boats and destroyers combined, and therefore we have no need of the original boat nowadays. It requires very little imagination to realise that the submarine has a far better chance of using torpedoes, as the under-water boat can get within half-a-mile or even a quarter of a mile of the enemy ship without being seen.

In the illustration facing page 160 we see that a modern battleship still carries torpedoes in case she should happen to get close enough to an enemy ship. But for the invention of a submarine the torpedo would never have come to be the deadly weapon which it now is.