That last remark will put you in mind that, in any kind of navigation, the trim of the boat is a delicate and important matter. Even in very large and heavy ships you may be able, by shifting guns or cargo, to slip off a shoal, or right a leaking vessel after a collision. In a tickle boat like a submarine, it is necessary to have some means of trimming the vessel, fore or aft, at any moment, and especially when about to dive, or when caught by some under-water obstruction. Tanks are therefore fitted for this purpose at each end of the boat. They are comparatively small, because the effect required is in ordinary circumstances very limited, and in a desperate emergency they may need to be supplemented by rushing the crew fore or aft, as living ballast. An example of this will be found in a later chapter. You may now feel that you have heard enough of tanks; but your instructor will insist on showing you a whole additional series. He will make a point of your recognising that a submarine, when submerged, is in reality hanging in the water as a balloon hangs in the air, and for every loss of weight she must be instantaneously compensated, or she will begin to rise. What loss of weight can she suffer while actually under water? It is not perhaps very hard to guess. There is, first of all, the consumption of oil by the engines; secondly, the consumption of food and fresh water by the crew; and thirdly, the departure from time to time of torpedoes. Also, when on the surface, there may be gun ammunition fired away, or other things heaved overboard, and allowance must be made for this when the boat goes down again. The modern submarine is prepared to keep her balance under all such circumstances. She has compensating tanks, and they are placed as near as possible to the oil-tank, fresh-water tank, or torpedo-tube, for whose diminished weight they are to compensate. You are probably more interested in the torpedo-tubes than in the oil-tanks. It is time then to go forward. You pass through the battery compartment, where the officers’ quarters are, and are shown (under the floor) the accumulators, ranged like the honey sections in the frames of a beehive, and very carefully covered over with flexible waterproof covering as well as with close-jointed planking. What would happen if water did find its way down to the batteries? An instant discharge of chlorine gas, blinding and suffocating. What would you do then? Come to the surface at all costs--and lucky if you are in time! The Germans know all about that--and not long ago one of our own boats was only saved by the good goal-keeping of a lieutenant, who caught up a lid of some sort, and stood by the leak, neatly fending off the water spurt from the door of the battery compartment.