To drive the U-boat when no air was to be had for the engines, electric motors were used. There was one on each propeller-shaft and the shafts could be disconnected from the oil-engines when the motors were driving. The motors got their power from storage batteries in the stern of the submarine and under the floors forward. The motors when coupled to and driven by the engines generated current which was stored in the storage batteries. The submarine could not run on indefinitely underwater. When its batteries were exhausted it would have to come to the surface and run its engines to store up a fresh charge of electricity. The electric motors gave the boat a speed of about nine knots. In addition to the main engines and motors, there was a mass of auxiliary machinery. There were pumps for compressing air to blow the ballast-tanks and to discharge the torpedoes. There was a special mechanism for operating the rudder and hydroplanes, and all sorts of valves, indicators, speaking-tubes, signal lines, etc. The tiny hull was simply crammed with mechanism of all kinds and particularly in the early boats there was little room for the accommodation of the officers and crew. The officers' quarters were located amidships, and forward there were the folding berths of the crews. In the later boats more space was given the men. The large U-boats carried a crew of forty and as the hazards of submarine warfare increased, more attention had to be paid to the men. FAT MEN NOT WANTED Oddly enough, small, slender men were preferred for submarine duty, not because of lack of space, but because it was apt to be very cold in a submarine, particularly in the winter-time. The water cooled off the boat when the submarine was traveling submerged, and the motors gave off little heat; while when the vessel was running on the surface the rush of wind to supply the engines kept the thermometer low. This meant that the men had to pile on much clothing to keep warm, which made them very bulky. The hatchway was none too large and a fat man, were he bundled up with enough clothing to keep him warm, would have a hard time squeezing through. In the center of the vessel was the main hatchway, leading up to the conning-tower, which was large enough to hold from three to five men. This was the navigating-room when the vessel was running submerged, and above it was the navigating-bridge, used when the submarine was on the surface. In the conning-tower there was a gyroscopic compass; a magnetic compass would not work at all inside the steel hull of the U-boat. And here were the periscopes or eyes of the submarine, rising from fifteen to twenty feet above the roof of the conning-tower. There were usually two periscopes. They could be turned around to give the man at the wheel a view in any direction and they were used sometimes even when the vessel was running on the surface, to give a longer range of vision. THE BLINDNESS OF THE SUBMARINE