There were other craft that _Herr Kommandant_ had to look out for. His were not the only submarines in the sea. His foes also were possessed of submarines. They could not see under water any better than he could, but they could fight on the surface as well as he, and they could creep up on him even as he crept up on his prey. As a French submarine commander puts it: "The U-boats used to enjoy the advantage of remaining themselves invisible while all the surface and aërial craft which were sent in pursuit of them were boldly outlined against the sky and visible to them. This is one of the reasons we used submarines to ambush U-boats." Submarines were also used to accompany the convoys, so that the U-boat commander had to watch not only for the eyes of the ship's lookouts and the eyes in the kite balloons, but also for the periscope eyes that swam in the sea. TRAILING U-BOATS BY SOUND The troubles of the submarine-commander were multiplying. All over the world inventors were plotting his destruction. As long as we depended upon our eyes to ferret him out, the sea was a safe refuge, provided he dived deep enough, but when we began to use our ears as well, he found himself in a very serious predicament. Although light is badly broken up in its passage through water, sound-waves will travel through water much better than in air. The first listening-devices used were crude affairs and did not amount to much, particularly when the U-boats muffled their motors and engines so that they were virtually noiseless. But the French invented a very sensitive sound-detector. It consisted of a lot of tiny diaphragms set in a big hemisphere. There were two of these hemispheres, one at each side of the boat. When sound-waves struck these hemispheres, the diaphragms would respond. At the focus of each hemisphere there was a megaphone receiver; one of these carried the sound to the operator's right ear and the other to his left. He would turn a megaphone around until he found the diaphragm that produced the loudest sound. This gave him the direction of the sound-wave. Then the boat would be steered in that direction. He knew that it was aimed properly when the sound coming to his right ear was just as loud as that which came into his left ear.