It was really a wonderful, even if a very simple, revolution in the handling of submarine boats that Jack Benson had thought out.

Up to that time many scores of lives had been lost, in different parts of the world, when the crews of submarine boats had found, for one reason or another, that they could not raise their craft from the bottom of the depths.  Formerly, when crews found themselves placed in that predicament, death followed.

Jack’s solution was wonderfully simple.  In brief, when the “Pollard” lay on the bottom of the little harbor at Dunhaven, the young captain had crawled into the long tube through which torpedoes were to be discharged in war time.

One end of this torpedo tube projects slightly into the water, at the bow of the submarine boat.  The other end of the tube is well inside the craft.  Two doors, or “ports,” as they are called, close the tube at the ends.  Ordinarily the forward port is closed, to keep water from entering the boat.  When a torpedo is placed in the tube for firing, the outer or forward port is opened automatically just at the instant of discharging the torpedo.  Enough compressed air is turned into the tube to force the torpedo out, after which the torpedo goes on its deadly journey propelled by its own motor.  The presence of the air thus turned into the tube at the instant of firing keeps out the water until the tube’s forward port is once more closed.  Then the rear port of the tube, inside the submarine boat, may be opened whenever it is desired.

Captain Jack Benson, when he reached bottom with the “Pollard,” and had donned his bathing suit, crawled into the tube through the rear port.  This port was then closed.  Hal Hastings simultaneously opened the outer port and discharged compressed air into the tube.  Thus Jack forced his way out into the water, and, with the aid of his natural buoyancy, made a quick swim for the surface.

In returning, he had dived down, close to the anchor cable.  Nearer the bottom he seized the cable, thus hauling himself down to the outer port of the torpedo tube.  He had quickly crawled into the tube, where the presence of air still kept the water out.  As he knocked heavily at the rear port with both hands, Hal swiftly turned in a moderate discharge of compressed air, while Eph, controlling mechanism inside, swung the forward port shut.  Then the rear port was swung back, Captain Jack crawling back into the forward compartment of the boat.

“The whole trick is rather easy,” Jack informed Mr. Farnum, as they walked that night in the village and discussed the matter in undertones.

“But you were in not more than seventy feet of water there,” suggested the builder.  “You couldn’t do it at much greater depth.”

“At eighty feet of water I could do it,” replied Benson, thoughtfully.

“But at a greater depth than eighty feet—?”

“Of course, the deeper one gets, the more tremendous the pressure of the water is,” answered the young captain.  “At a depth of a hundred feet, say, the pressure of the water would be enough to crowd me back into the tube, crushing my body.”