Now you are in the forward torpedo compartment, and there are the tubes. I need not say anything about their size or number--you will realise at a glance that when a couple are loosed off at once, a good deal of weight goes out of the ship. The ordinary 18-inch fish is 17 feet long, and takes some handling. The explosive alone in her war-head weighs as much as a big man, say 12½ stone, and a 21-inch fish carries twice as much as that, packed in some four feet of her length. Behind that comes the air chamber--another ten feet--with the compressed air to drive the engine, which is in her stern. The air is stored at a pressure of over 2000 lbs. to the square inch; so the steel walls of the chamber must be thick, and this makes another heavy item. Lastly, there is the engine-box with its four-cylinder engine, two propellers, gyroscope and steering gear. Altogether, an 18-inch fish will weigh nearly three-quarters of a ton, and a 21-inch over 2000 lbs., so that the amount of compensation needed when you fire, is considerable. To see how it is done, we will imagine ourselves firing this starboard tube. The torpedo is got ready, and special care is taken to make sure that the firing-pin in her nose is not forgotten. Cases have been known in which a ship has been hit full by a torpedo which did not explode--just as a good many Zeppelin bombs were found in London, after the early raids, with the detonating pin not drawn. The fish is now ready to come alive, and is slid into the tube. The door is shut behind it, and the water-tight outer door, at the other end of the tube, is now ready to be opened by powerful levers. But the immediate result of this opening would be an inrush of sea-water which would weigh the boat’s head down; for though the fish’s belly fits the tube pretty closely, there is a good deal of empty space where it tapers towards the nose and tail. Here comes in the tank system. When the tube is loaded, this empty space is filled by water from within the ship, so that no change of weight occurs when you open the outer door. But when the firing-button has been pushed, and the torpedo has been shot out by an air-charge behind it there is no possibility of preventing the whole tube from filling with water, and this water must be got rid of before the tube can be reloaded. To do this, you first close the outer door again; then you have to deal with the tubeful of water. A good part of it is what the ship herself supplied to fill the space round the torpedo; and this must be pumped back into the special tank it came from. The remainder is the sea-water which rushed in, to take the place left empty by the departing torpedo: and this must be pumped into another special tank to prevent the ship feeling the loss of the torpedo’s weight. When you get a fresh supply of torpedoes, these special compensating tanks (which are really a kind of dummy torpedoes) will be emptied out, one for each new torpedo. Meantime, you have now