The ship is turned towards the sound until the same volume of sound is heard from both receivers, when the bell is known to be dead ahead. So accurate is this in practice that a trained operator can steer his ship in the densest fog directly to a lightship or any other point where a submarine bell is sending its warning beneath the sea. It must be repeated that the medium in which these signals are transmitted is a constant one, not subject to any of the limitations and variations imposed on the atmosphere and the ether as media for the transmission of light, blasts of a foghorn, and wireless vibrations. At present the chief use of submarine signalling is from the shore or a lightship to ships at sea, and not from ship to ship or from ship to the shore: in other words ships carry only receiving apparatus, and lighthouses and lightships use only signalling apparatus. Some of the lighthouses and lightships on our coasts already have these submarine bells in addition to their lights, and in bad weather the bells send out their messages to warn ships of their proximity to a danger point. This invention enables ships to pick up the sound of bell after bell on a coast and run along it in the densest fog almost as well as in daylight; passenger steamers coming into port do not have to wander about in the fog, groping their way blindly into harbour. By having a code of rings, and judging by the intensity of the sound, it is possible to tell almost exactly where a ship is in relation to the coast or to some lightship. The British Admiralty report in 1906 said: "If the lightships round the coast were fitted with submarine bells, it would be possible for ships fitted with receiving apparatus to navigate in fog with almost as great certainty as in clear weather." And the following remark of a captain engaged in coast service is instructive. He had been asked to cut down expenses by omitting the submarine signalling apparatus, but replied: "I would rather take out the wireless. That only enables me to tell other people where I am. The submarine signal enables me to find out where I am myself." The range of the apparatus is not so wide as that of wireless telegraphy, varying from 10 to 15 miles for a large ship (although instances of 20 to 30 are on record), and from 3 to 8 miles for a small ship.