Most lighthouses are fitted with fog signals of some kind which have a distinctive character the same as the lights. Some are horns blown at intervals by compressed air often obtained from a special air-pump driven by an oil-engine. Another thing is to let off detonators at stated intervals. But perhaps the most interesting of all is the submarine telephone. The trouble with audible signals is that they are apt to vary as the conditions of the atmosphere change. For, strange though it may appear, the air which is the natural medium by which sounds are carried to our ears is really a very bad substance for the purpose. Water is much superior. A swimmer who cares to try the experiment of lying upon the water with his ears immersed while a friend beats a gong under the water some distance off will be astounded at the result. So many modern ships are fitted with under-water ears, waterproof telephone receivers, really. One is fixed each side of the vessel, the wires from them being led to telephone receivers near the bridge. Many lighthouses and lightships in like manner are fitted with under-water bells which can be rung at intervals. The sounds so conveyed through the water are always the same. Atmospheric or similar changes have no effect upon them. And, moreover, the officer can tell which side of his ship the bell is. If it be on his port-side it sounds louder in his port telephone, and vice versa. By turning his ship until he hears them equally he knows that he is pointing directly to or from the bell. Thus if the bell belong to a warning light he can steer confidently right away from the danger even in the thickest fog. But science has not only provided the mariner with lights of marvellous power and of strange distinctive characters, and reliable sound-signals for foggy weather, it has also found him a reliable compass, but that is worthy of a chapter to itself. CHAPTER VII THE GYRO-COMPASS The magnetic compass has been for ages the mariner's guide over the trackless waters. In cloudy weather it has been his only means of knowing the direction in which his craft was heading. Indeed, it is not too much to say that the maritime commerce of the world was based upon the behaviour of that little piece of magnetised steel.