In perfectly still weather the sound of the fog horn carries far enough, and indicates location well enough so that two ships approaching each other, or a ship approaching a bell buoy, can detect its location and avoid danger. But this is under favorable conditions; and unfortunately such conditions do not always prevail. And if there is a wind stirring or the sea running high atmospheric sounds cannot be depended upon. A fog whistle whose sound ought to carry several miles under ordinary conditions, may not be heard more than a ship's length away. And scores of accidents, such as collisions between ships, have happened in fogs, when both vessels were sounding their fog whistles at regular intervals. When wireless telegraphy was perfected sufficiently to be of practical use, great hopes were entertained that this discovery could be used to give warning and prevent accidents to fog-bound vessels. But experience has shown that its usefulness is confined largely to that of calling for help after the accident, rather than in preventing it. Thus in 1908 when the wireless operator on board the steamer _Republic_ flashed his message broadcast telling ships and shore-stations for hundreds of miles around that his vessel had been run down in a fog and was sinking, he could only give the vessels that hurried to the _Republic's_ aid an approximate idea of where they could find her. The use of another electric appliance, of even more recent invention than the wireless telegraph, was necessary for locating the exact position of the stricken ship. This was the submarine signaling device, which utilizes water instead of air as a medium for transmitting sound. Benjamin Franklin pointed out more than a century ago that water carries sound farther and faster than air, and carries it with greater constancy. Density, temperature, and motion of the atmosphere act upon aerial sound waves to reflect and refract them in varying degrees; but these waves are not affected when water is the medium through which they are passing. The knowledge of these facts was turned to little practical account until the closing years of the last century when Arthur J. Mundy of Boston, and a little later Prof. Elisha Gray of Chicago, began experiments together that resulted finally in a practical submarine signaling apparatus which is now installed as a system on boats and buoys in dangerous places along the coasts, particularly near the great highways of ocean travel.