The principle upon which this system is based is simply that of sound waves transmitted through the water and detected at a distance by a submerged electrical transmitter. The sound transmitted is usually that of a submerged bell. It is possible for a person whose head is submerged to hear the ringing of such a bell distinctly for a long distance; but of course for practical purposes such submergence is out of the question. The receiving apparatus of the Mundy-Gray signaling device offers a substitute in the form of a submerged "artificial ear"--an electrical transmitter, connected with a telephone receiver. In the early experiments a small hollow brass ball filled with water and containing a special form of electrical transmitter was lowered over the side of a ship and connected by insulated wires to the receiver of a telephone in the pilot house. The sound of a submerged bell could be heard in this manner at a distance of ten or twelve miles. The location of the bell could be determined by having two such brass balls, one on each side of the hull of the vessel but not submerged to a depth below that of the hull, so that the ship itself acts as a screen in obstructing the sound waves coming from the bell. By listening alternately to the sounds of the bell transmitted through these two submerged balls it was found that the ball on the side of the ship toward the bell gave a distinctly louder sound. By turning the ship so that the sounds were of equal intensity the direction of the bell could be determined as either directly ahead or astern; and by using the compass the exact location could be determined. But such brass-ball transmitters can be used only when the vessel is moving at a rate not exceeding three miles an hour. They are, therefore, of little value for ocean liners whose reduced speed far exceeds this. But the inventors discovered presently that by using the inside of the outer steel skin of the ship's hull below the water line as one side of the brass ball, the transmitter would work equally well. Indeed, with added improvements, this hollow metal device fastened to the inside of the hull on each side, with connecting wires leading to the pilot house, in its perfected form will pick up the sound of the submerged bell equally well at any speed, regardless of calm or storm. The chief defect of this arrangement was that the sound of the pulsations of the engines of the ship were also heard, and interfered seriously with the detection of the sound of the bell. But presently a receiving device was perfected which ignored all sounds but those of the bell, thus giving the mariner a means of protection against accidents that could be depended upon absolutely at all times regardless of speed or weather conditions.