In the first place, she was able to remain under water indefinitely. It was not necessary for her to return to the surface every so often to replenish her air tanks, for she carried no tanks. The D-17 generated her air supply from the water, by means of a secret process known only to high officials of the British admiralty. Also, the D-17 was the last word in submarine craft in other respects. She had a speed of more than thirty knots when submerged, could move along even faster upon the surface of the water, and her bow, a solid piece of substantial glass, in which reposed a huge searchlight, made it possible for her to pick her course under the water--something that no other submarine craft was able to do. For this latter reason she could venture places where no other vessel would dare. The huge light flashing in front now, Jack took his stand and stood with eyes straining into the distance ahead. And while the D-17 is thus moving toward the strongest fortification of the enemy, a few words concerning the two subordinate officers aboard are necessary. Frank Chadwick, an American youth of some seventeen years of age, found himself in his present position as a result of a series of strange happenings. He had lost his father in Italy immediately after the outbreak of the great war, and while hunting him in Naples, had been shanghaied aboard an Italian schooner. Here, following a mutiny of the crew and the death of the captain, he found himself a prisoner, remaining so until released by Jack Templeton. The latter's appearance came about in this manner: Jack, the son of an Englishman, nevertheless had spent most of his life in a little African coast town. His father died there and Jack found himself sole owner of a little store, which had almost a native trade alone, though an occasional ship put in for supplies. One day a schooner hove in sight. A boat put over the side and its occupants obtained provisions at Jack's store. In the lad's absence, they departed without making payment. Jack hurried after them to collect and climbed aboard the schooner just as she raised anchor. A fight followed his demand for payment. By some resourcefulness and some good hard fighting, the lad succeeded in gaining the upper hand. Then he released Frank, whom he found a prisoner with another, a British secret service agent. A series of subsequent events threw the lads into the company of Lord Hastings, who, taking a liking to them, answered their pleas to be made sailors by securing them commissions and taking them aboard his own ship at the opening of the great world war.