With Great Britain as an enemy, Germany knew that she was hopelessly outclassed on the sea; but while "Britannia ruled the waves," she did not rule the depths of the sea, and so Germany decided to claim this realm for her own. Little attention did she pay to surface vessels. Except in the Dogger Bank engagement and the Battle of Jutland, the German first-class vessels did not venture out upon the open sea, and even the lighter craft merely made occasional raids under cover of fog or darkness, only to cut and run as soon as the British vessels appeared. The submarine boat, or _unterseeboot_ as the Germans called it, was virtually the only boat that dared go out into the high seas; consequently, the Germans specialized upon that type of craft and under their close attention it grew into a highly perfected war-vessel. But the Germans were not the only ones to develop the submarine, as we shall see. CONSTRUCTION OF THE U-BOATS When the great war broke out, the German U-boat was a comparatively small craft, less than 150 feet long, with its main hull only 12 feet in diameter. It could make a speed of 12 knots on the surface and only 9 when submerged. But as the war progressed, it grew larger and larger, until it attained a length of over 300 feet and its speed was increased to 12 knots when submerged and 18 knots on the surface. Figs. 16 to 18 show the construction of one of the early U-boats. The later boats were built after the same general plan, but on a bigger scale. [Illustration: Courtesy of the "Scientific American" FIG. 16. Sectional view of one of the earlier German U-Boats] [Illustration: Courtesy of the "Scientific American" FIG. 17. Sectional plan view of a German U-Boat of the type used at the beginning of the war] It is not always safe to judge a thing by its name; to do so is apt to lead to sad mistakes. One would naturally suppose, from its name, that a submarine is a boat that lives under water, like a fish. But it is not a fish; it is an air-breathing animal that prefers to stay on the surface, only occasionally diving under to hide from danger or to steal upon its prey. During the war, the German U-boats did not average three hours per day under the surface! Because they were intended to run on the surface they had to be built in the form of a surface vessel, so as to throw off the waves and keep from rolling and pitching too much in a seaway. But they also had to be built to withstand the crushing weight of deep water, and as a cylinder is much stronger than a structure of ordinary boat shape, the main hull was made circular in section and of heavy plating, strongly framed, while around this was an outer hull of boat shape, as shown in Fig. 18. PUTTING HOLES IN A TANK TO KEEP IT FULL