“Skipper’s up in the conning-tower, taking aim through the periscope,” explained the man who had told me about trimming-tanks. “The tubes being fixed in the bow, he has to train the whole boat like a gun. Likewise he’s got to figure out how far it is to the target and how fast the tug is towing it, how many seconds it’s going to take the torpedo to get there, and how much he’s got to allow for its being carried off its course by tide and currents. When he gets good and ready, the lieutenant’ll press a little electric button and you’ll hear--” “THUD!” went the compressed air in the tube, and the submarine shuddered slightly with the shock of the recoil. But that was all. “There she goes!” said my friend the tank-expert. “As soon as the Whitehead was expelled, a compensation-tank just above the tube was flooded with enough water to make good the loss in weight.” “What keeps the sea-water from rushing into the tube after the torpedo leaves it?” I asked. “A conical-shaped cap on the bow of the boat keeps both tubes closed except when you want to fire one of them. Then the cap, which is pivoted on its upper edge, swings to port or starboard just long enough for the torpedo to get clear and swings back before the water can get in.” Four of the ten torpedoes carried in the magazine were sped on their way to the unseen target. I returned to the turret as the wireless operator entered and handed a typewritten slip to Lieutenant Scope, who smiled happily and said to me, “The captain of the tug reports that all four shots were hits and all four torpedoes have been safely recovered.” I was too astonished to congratulate him on his marksmanship, as I should have done. “How in the name of miracles!” I gasped. “Can you receive a wireless telegram under the sea?” “By the Fessenden oscillator,” he replied, and added to the wireless man, “Take this gentleman below and show him how it works.” “Did you ever have another chap knock two stone together under water when you were taking a dive?” asked the operator. I nodded in vivid recollection. “Then you have some idea how sounds are magnified under water. It is an old idea to put submarine bells down under lighthouses and fit ships with some kind of receiver so that the bells can be heard and warning given when it is too foggy to see the light. The advantage over the old-style bell-buoy lies in the fact that sound travels about four times as fast through water as through air,[16] and goes further and straighter because it isn’t deflected by winds or what the aviators call ‘air-pockets.’ The man who knows most about these things is Professor Fessenden, of the Submarine Signal Company of Boston, who first realized the possibility of telegraphing through water.[17] [Illustration: Courtesy of the American Magazine. Fessenden oscillator outside the hull of a ship. The “ear” of a modern vessel.]