At this point it may strike you suddenly that you are now under water--you begin to wonder how deep you are, and why you have not perceived any change in the boat’s position. The answer is that the depth marked on the gauges is only twenty feet, and the angle of descent was therefore very slight--much too slight to be perceptible in the short length of a single compartment. The depth of twenty feet is now being maintained with surprising steadiness; the explanation is that two entirely separate forces are at work. First, there are the horizontal rudders or hydroplanes, fitted outside the vessel both fore and aft, by which she can be forced down, provided she has sufficient way on, in much the same fashion as an ordinary vertical rudder forces a ship to one side or the other. But this is only the diving apparatus; to keep her down, there is her water ballast--the water which was taken into her main ballast tanks, when the order to submerge was given. These tanks contain a sufficient weight of water to counteract the normal buoyancy of the boat, by which she would naturally float upon the surface. When they are emptied, she will neither sink nor rise of her own motion--she will lie or run at whatever depth she is placed, by her hydroplanes or otherwise. These, you will have noticed, were called the ‘main’ ballast tanks--there would seem then to be others. There are, and several kinds of them. First, there is an auxiliary ballast tank, which has a peculiar use of its own. A submarine must be able to float or submerge in fresh water as well as at sea; for her base or harbour will often be in the mouth of a river, or she may have to navigate a river, a canal, or a lake. It is a point that would not probably have occurred to you, but the difference between the density of fresh and salt water is sufficiently great to make a real difficulty here. Everyone knows that it is less easy to float in fresh water, and less easy to sink in salt. For practical purposes, a submerged boat is less buoyant in fresh water by 26 tons in 1000, and _vice versa_; so that when a submarine of 1000 tons leaves a river for the sea, she must take an extra 26 tons of ballast to keep her down, and when she comes home again she must get rid of 26 tons, or she will sink so much deeper in the fresh water. For this purpose she has a special tank of the right size, proportioned to her tonnage; and it is placed in the middle of the ship, in order that it may not interfere with her trim when it is filled or emptied.