Nor is the foghorn much better: the presence of different layers of fog and air, and their varying densities, which cause both reflection and refraction of sound, prevent the air from being a reliable medium for carrying it. Now, submarine signalling has none of these defects, for the medium is water, subject to no such variable conditions as the air. Its density is practically non variable, and sound travels through it at the rate of 4400 feet per second, without deviation or reflection. The apparatus consists of a bell designed to ring either pneumatically from a lightship, electrically from the shore (the bell itself being a tripod at the bottom of the sea), automatically from a floating bell-buoy, or by hand from a ship or boat. The sound travels from the bell in every direction, like waves in a pond, and falls, it may be, on the side of a ship. The receiving apparatus is fixed inside the skin of the ship and consists of a small iron tank, 16 inches square and 18 inches deep. The front of the tank facing the ship's iron skin is missing and the tank, being filled with water, is bolted to the framework and sealed firmly to the ship's side by rubber facing. In this way a portion of the ship's iron hull is washed by the sea on one side and water in the tank on the other. Vibrations from a bell ringing at a distance fall on the iron side, travel through, and strike on two microphones hanging in the tank. These microphones transmit the sound along wires to the chart room, where telephones convey the message to the officer on duty. There are two of these tanks or "receivers" fitted against the ship's side, one on the port and one on the starboard side, near the bows, and as far down below the water level as is possible. The direction of sounds coming to the microphones hanging in these tanks can be estimated by switching alternately to the port and starboard tanks. If the sound is of greater intensity on the port side, then the bell signalling is off the port bows; and similarly on the starboard side.