latter being the angle which it made with the north and south line. Another appliance which now rendered great service was the wireless telephone. The commanding officer at once began talking with the other two boats, asking if they had picked up the noise. Unless all three vessels had heard the disturbance, nothing was done; but if all identified it nearly simultaneously, this unanimity was taken as evidence that something was really moving in the water. When all three vessels obtained the direction as well as the sound it was a comparatively simple matter to define pretty accurately its location. The middle chaser of the three was the flagship and her most interesting feature was the so-called plotting-room. Here one officer received constant telephone reports from all three boats, giving the nature of the sounds, and, more important still, their directions. He transferred these records to a chart as soon as they came in, rapidly made calculations, and in a few seconds he was able to give the location of the submarine. This process was known as obtaining a "fix." The reports of our chaser commanders are filled constantly with reference to these "fixes"--the "fix" being that point on the surface of the ocean where three lines, each giving the direction of the detected sound, cross one another. The method can be most satisfactorily illustrated by the following diagram: [Illustration: HOW THE LISTENING DEVICES LOCATED A SUBMARINE.] In this demonstration the letters A, B, and C, each represent a subchaser, the central one, B, being the flagship of the division. The listener on A has picked up a noise, the direction of which is indicated by the line _a a_. He telephones by wireless this information to the plotting-room aboard the flagship B. The listeners on this vessel have picked up the same sound, which comes from the direction indicated by the line _b b_. The point at which these two lines cross is the "fix"; it shows the spot in the ocean where the submarine was stationed when the sound was first detected. The reason for having a report from the third subchaser C is merely for the purpose of corroborating the work of the other two; if three observations, made independently, agree in locating the enemy at this point, the commanding officer may safely assume that he is not chasing a will o' the wisp.