signalling system and this is used to detect the presence of a U-boat when it is submerged and cannot be seen, though to do this the enemy boat must be near-by. This conduction scheme is very simple and you’ll get me fine as I explain it.
Water, as you know, conducts sound waves to much greater distances than air does. You must have often made the experiment when in swimming of ducking your head under water and listening while another fellow would strike a couple of stones together under water at a distance of thirty or forty feet away from you;
