The space between the inner and outer hulls was used for water ballast and for reservoirs of oil to drive the engines; and, strange as it may seem, the oil-tanks were always kept full by means of holes in the bottom of them. As the oil was consumed by the engines, water would flow into the reservoir to take its place, and the oil, being lighter than water, would float on top. The false hull was of light metal, because as it was open to the sea, the pressure on the inside was always the same as that on the outside. The reservoirs of oil and the water-ballast tanks protected the inner hull of the vessel from accidental damage and from hostile shell and bombs. There were water-ballast tanks inside the inner hull as well, as shown in the cross-sectional view, Fig. 18. The water in the ballast-tanks was blown out by compressed air to lighten the U-boat and the boat was kept on an even keel by the blowing out or the letting in of water in the forward and after tanks. [Illustration: Courtesy of the "Scientific American" FIG. 18. Transverse section through conning-tower, showing the interior (circular) pressure-resisting hull and the lighter exterior hull, which is open to the sea] A heavy lead keel was attached to the bottom of the boat, to keep it from rolling too much. In case of accident, if there were no other way of bringing the boat to the surface, this keel could be cast loose. At the forward end, where the torpedo-tubes were located, there was a torpedo-trimming tank. Torpedoes are heavy missiles and every time one was discharged the boat was lightened, and the balance of the submarine was upset. To make up for the loss of weight, water had to be let into the torpedo-trimming tank. A submarine cannot float under-water without swimming; in other words, it must keep its propellers going to avoid either sinking to the bottom of the sea or bobbing up to the surface. To be sure, it can make itself heavier or lighter by letting water into or blowing water out of its ballast-tanks, but it is impossible to regulate the water ballast so delicately that the submarine will float submerged; and should the boat sink to a depth of two hundred feet or so, the weight of water above it would be sufficient to crush the hull, so it is a case of sink or swim. Usually enough ballast is taken on to make the submarine only a little lighter than the water it displaces; and then to remain under, the vessel must keep moving, with its horizontal rudders tilted to hold it down. The horizontal rudders or hydroplanes of the U-boat are shown in Fig. 17, both at the bow and at the stern.