When shells or naval mines are charged with explosives, we may describe them as live shells or live mines, just as we speak of a wire carrying a dangerous electric current as a live wire. Of course our soldiers must handle live shells, and we know how every care is taken and how a safety-pin is kept in a shell until it is being placed in the gun.

You have heard of sailors engaged in mine-laying and in mine-sweeping, and I have no doubt that you realise that these are very dangerous occupations. A mine-laying vessel may cover quite a large area with explosive mines during a single night. When this area has been supplied with a lot of mines, all anchored in position, we call it a mine-field. These mines do not float on the surface but are held some distance below the surface, and they will be of little use unless they are all anchored at the correct depth. They must not be too low or the enemy ships may pass over them without striking them, yet we do not wish them to float on the surface, or the enemy will see them. How can this be arranged?

One boy suggests that the mine-layer finds out how deep the water is by means of a sounding line, and then arranges the length of the anchor rope of the mine to suit the depth. That would be far too slow a process; the mine-layer must be able to drop the mines quickly as she steams along, and no matter how the depth varies from place to place, the mines must go automatically to the correct depth. But how can this be done?

One girl suggests that the mine is made just too heavy to float on the surface, and yet not heavy enough to let it sink more than a few feet below the surface. Our answer is that this is easier said than done, and we fear that this girl has not caught on to the meaning of the floating torpedo mines described in the previous chapter. This mine did float just at the required depth, but it required machinery to keep it at the proper depth, and even then was liable to drift away from the position in which it was laid.

The only satisfactory way will be to use a mine which would float on the surface, but which is held down by an anchor and rope, the rope being just long enough to let the mine rise within a few feet of the surface. Held in that position, the mine will not be seen by the enemy, and if the enemy ship should pass over it, the mine will be struck and explode, and the great ship will be seriously damaged if not sunk.

But we are no nearer a means of anchoring each mine at the required depth, which, we have seen already, will vary for almost every mine that the mine-layer drops overboard. I think you could guess for a long time before being able to suggest how this difficulty can be overcome. And yet when you are let into the secret it seems quite simple.

Suppose that instead of an ordinary anchor you have a metal box in which there is a large bobbin or reel. The anchor rope is wound upon this reel. If you were to take this mine and its curious anchor out in a small boat, and put them overboard, the mine would float on the surface and the anchor (the box and the reel) would sink down to the bottom of the sea, paying out the anchor rope as it descends. It is evident that we must have something more, or the mines would still float on the surface. It is this additional part of the invention which is interesting.

If any boy or girl does not know what a "pawl" and a ratchet-wheel are, they have only to look at any mechanical toy, and in the clockwork they will see a wheel with teeth all round its rim and a little catch which falls in between two of the teeth and prevents the winding wheel turning back again. Suppose we put a ratchet-wheel on the end of the reel on which the anchor rope is wound, and then have a small catch or pawl to prevent the wheel turning. What will happen?

One boy suggests that the mine will float and hold the anchor-rope up, while another boy thinks that the anchor will pull the mine down to the bottom of the sea. Both boys may be right; it will depend upon how buoyant the mine is, and how heavy the anchor is, but for our purpose neither of these things must happen. You say that nothing else can happen, as the wheel is locked, so that the anchor rope cannot be paid out. But suppose we have some means of lifting the catch out of the way of the ratchet-wheel, what will happen then? The anchor will fall to the bottom of the sea, paying out rope all the way, then the mine will still float on the surface, but if we could drop the catch into the teeth of the wheel, just when the anchor is a few feet from the bottom, then the reel in the anchor would stop paying out rope, and in falling these last few feet, the anchor would pull the mine just as many feet below the surface of the water. But how can we know when to drop the little catch into the ratchet-wheel?

We cannot ask the man on board the mine-layer to do any such thing, for he must simply drop the mines overboard one after another as she continues steering along. But suppose we have a heavy weight attached to the anchor by a few feet of rope, so that the weight will hang down from the anchor. We can very easily cause the pull of this weight to keep the little catch free from the ratchet-wheel. So long as the weight is not pulling at the connecting rope the catch holds the wheel, but when we drop the mine, anchor and weight overboard, the weight, being the heaviest, sinks first, pulling the lighter anchor after it, and at the same time keeping the catch off the wheel. This leaves the reel free to pay out the anchor rope to the mine which is floating on the surface, but as soon as the heavy weight reaches the ground, it can no longer hold the catch off the wheel. As soon as the catch stops the reel the anchor refuses to pay out any more rope to the floating mine, and so the mine is pulled below the surface until the anchor itself rests on the bottom beside the weight which got down a little earlier.

Now you see how the men in the mine-layer can drop the mines one after another without worrying about differences of depth of the ocean at each place; each mine will sink to the required depth.

If I ask you how the men on the mine-layer can make the mine sink 5 feet below the surface, I think you could tell me. They have only to make the rope between the heavy weight and the anchor 5 feet long, and then the anchor will stop paying out rope to the floating mine for the last 5 feet, so that the mine will be pulled 5 feet below the surface. It is all very simple when you know how to do it, but I think you will agree that it is a clever invention.

As the anchor containing the reel and rope is in a square box, four small wheels are added to convert the box into a little truck, so that it may be pushed along a pair of rails on board the mine-layer. This little rail track runs down to the stern of the ship, and projects over the water. As one truck is pushed along after another, it drops into the water and anchors the mine in the manner already explained.

One girl asks if there is no fear of the mines going off with a bang when they fall into the water. That is quite a thoughtful question. If the mine is to be set off by a steamer bumping against it, very much the same thing should happen when the mine strikes the sea. This accident would really happen unless precautions were taken to prevent it.

The mine-layers have no time to place the mines gently in the water; besides, they might easily get a sudden jar while on board ship. How then are they made safe to handle? One boy suggests that there is a safety-pin such as is used in a torpedo, and until this pin is withdrawn the mine cannot explode, but when this boy is asked how the pin is to be pulled out after the mine has dropped from the rails and struck the water, he sees that his suggestion will not work.

We must have something self-acting; something automatic. I believe some boys might suggest a means if I reminded them of the manner in which the diving rudders of torpedoes are controlled. You remember how a flexible iron plate or diaphragm was made to bulge inwards more and more as the torpedo sank into deeper water; this bulging in was due to the pressure of the water.

Suppose we have a similar metal diaphragm, which while in its normal or usual position prevents the mine being exploded, but when the diaphragm bulges inwards it leaves the mine free to go off if struck by a steamer. In this way the mine will be quite safe so long as it is on board the mine-layer. It is also safe while it strikes the water, and not until it has sunk several feet below the surface is the water pressure sufficient to bulge in the diaphragm and leave the mine free to be exploded.

Even with every possible precaution, mine-laying remains a dangerous occupation. If a mine-layer is sighted by the enemy, and an explosive shell is landed in the boat laden with mines, it does not require much imagination to realise the disaster which would occur from the explosion of the mines.

When our Navy finds that the enemy has set a mine-field to trap them, how can they ever hope to get past it? They send out the mine-sweepers. You will agree that sweeping up live mines is a very dangerous occupation. The steam trawler used as a mine-sweeper may run on a mine at any moment, although a most careful look out is kept. But why is it called sweeping? Because two trawlers each take one end of a long cable, and drag it along at or near the bottom of the sea. This cable will catch the anchor ropes of any mines that happen to lie hidden in the space between the two trawlers. One boy suggests that he would call this mine-fishing rather than mine-sweeping. It is certainly quite like a fishing operation; so much so that the men for the mine-sweepers are recruited from fishermen. Sweeping for floating or drifting mines is even more like fishing, for in this case a strong net has to be used, as there are no anchor ropes to catch.

But when the mine-sweepers with the cable do catch an anchor rope of a mine, how are they going to get the live mine on board without exploding it? They have no intention of bringing the live mine on board. They merely wish to explode the mines and thus render them harmless. It is most likely that the mine will explode whenever the sweeping cable catches its anchor rope, for the mine will receive a sudden jerk. If the mine does not explode, then the sweepers may pull their cable so that the mine will rise to the surface, and being still at a safe distance from them, they may fire a gun at it and thus explode it.

During the Great European War some Dutch sailors found a drifting mine near their coast, and they tried to take it ashore. When they got it on land it exploded, and unfortunately killed several of the sailors.

One girl asks how the Navy can get men to do such dangerous work as mine-laying and mine-sweeping. The answer is that no sailor is forced to do this work; it is all done voluntarily, and we are proud to say that there are always plenty of willing volunteers.