The most modern of the German submarines at the time had a length of 213 feet and a beam of twenty feet, these dimensions giving them sufficient deck space to mount thereon two rapid-fire guns, one of 3.5 inches and another of 1.4 inches. Their displacement was 900 tons, and they could make a speed of 18 knots when traveling "light" (above water), and 12 knots when traveling submerged. These speeds made it possible for them to overtake all but the fastest merchantmen, though not fast enough to run away from destroyers, gunboats, and fast cruisers. Their range of operation was 2,000 miles, and in the early months of 1915, it was possible for Germany to send two or three of them from their base in the North Sea to the Mediterranean. Germany was at the same time experimenting with a larger type, with a displacement of 1,200 tons and an operating distance of 5,000 miles. The ordinary submarine in service at the beginning of the war could remain below the surface for twenty-four hours at least. Reserve amounts of air for breathing were carried in tanks under pressure, and in the German type there were also chemical improvements for regenerating air. Contrary to the opinion of laymen, submerging was accomplished both by letting water into ballast tanks, and also by properly deflecting a set of rudders; every submarine had two sets of rudders, one of which worked in vertical planes and pointed the prow of the ship either to the left or the right; the other pair worked in horizontal planes and turned the prow either upward or downward. A pair of fins on the sides of the hull assisted action in both rising and diving. The action of water against the fins and rudders when the ship was in motion was exactly the same as that of the air against the planes of a kite; to submerge one of the craft it was necessary to have it in motion and to have its horizontal rudders so placed that the resistance of the water would drive the ship downward; the reverse operation drove it upward. And here lay a danger, for if the engines of a diving submarine stopped she was bound to come to the surface. Her presence, while moving entirely submerged could be detected by a peculiar swell which traveled on the water above; if submerged only so much as to leave the tip of her periscope still showing, the latter left an easily discernible wake.