Mundy communicated this idea to Elisha Gray, and the two, working together, evolved a successful submarine signal system. It was on the last day of the nineteenth century that they were able to put their experiments into practical working form. Through a well in the center of the ship they suspended an eight-hundred-pound bell twenty feet beneath the surface of the sea. A receiving apparatus was located three miles distant, which consisted simply of an ear-trumpet connected to a gas-pipe lowered into the sea. The lower end of the pipe was sealed with a diaphragm of tin. When submerged six feet beneath the surface the strokes of the bell could be heard. Then a special electrical receiver of extreme sensitiveness, known as a microphone, was substituted and connected at the receiving station with an ordinary telephone receiver. With this receiving apparatus the strokes of the bell could be heard at a distance of over ten miles. This system has had a wide practical application for communication both between ship and ship and between ship and shore. Most transatlantic ships are now equipped with such a system. The transmitter consists of a large bell which is actuated either by compressed air or by an electro-magnetic system. This is so arranged that it may be suspended over the side of the ship and lowered well beneath the surface of the water. The receivers consist of microphones, one on each side of the ship. The telephone receivers connected to the two microphones are mounted close together on an instrument board on the bridge of the ship. The two instruments are used when it is desired to determine the direction from which the signals come. If the sound is stronger in the 'phone on the right-hand side of the ship the commander knows that the signals are coming from that direction. If the signals are from a ship in distress he may proceed toward it by turning his vessel until the sound of the signal-bell is equal in the two receivers. The ability to determine the direction from which the signal comes is especially valuable in navigating difficult channels in foggy weather. Signal-bells are located near lighthouses and dangerous reefs. Each calls its own number, and the vessel's commander may thus avoid obstructions and guide the ship safely into the harbor. The submarine signal is equally useful in enabling vessels to avoid collision in fogs. Because water conducts sound much better than air, submarine signals are far better than the fog-horn or whistles. The submarine signal system has also been applied to submarine war-ships. By this means alone may a submarine communicate with another, with a vessel on the surface, or with a shore station.