But this "fix" is merely the location of the submarine at the time when it was first heard. In the great majority of cases, however, the submerged vessel is moving; so, rapidly as the men in the plotting-room may work, the German has advanced beyond this point by the time they have finished their calculations. The subchasers, which have been drifting while these observations were being made, now start their engines at full speed, and rush up to the neighbourhood of their first "fix." Arrived there, they stop again, put over their tubes, and begin listening once more. The chances are now that the noise of the submarine is louder; the chasers are getting "warmer." It is not unlikely, however, that the direction has changed, for the submarine, which has listening devices of its own--though the German hydrophones were decidedly inferior to the American--may have heard the subchasers and may be making frantic efforts to elude them. But changing the course will help it little, for the listeners easily get the new direction, and send the details to the plotting-room, where the new "fix" is obtained in a few moments. Thus the subchasers keep inching up to their prey; at each new "fix" the noise becomes louder, until the hunters are so near that they feel justified in attacking. Putting on full speed, all three rush up to the latest "fix," drop depth charges with a lavish hand, fire the "Y" howitzers, each one of which carries two depth charges, meanwhile manning their guns on the chance that the submarine may decide to rise to the surface and give battle. In many of these hunts a destroyer accompanies the subchasers, always keeping at a considerable distance, so that the noise of its propellers will not interfere with the game; once the chasers determine the accurate "fix," they wire the position to this larger ship, which puts on full steam and dashes with the speed of an express train to the indicated spot, and adds ten or a dozen depth charges to those deposited by the chasers.