Then Frank walked quickly back to the spot where the periscope protruded from below. Opening his coat he took the explosive out and, drawing a handkerchief from his pocket, tied it to the diminutive mine and hung the latter on the tube.
"Now for this German," he said to himself. "It wouldn't do for him to see that before I am ready."
He approached the man once more and asked several questions.
"Well," he said finally, "I guess I shall have to say good-night."
The German's reply was choked in his throat. Frank sprang forward, flung one arm around the man's, neck, and with the other clutched him by the throat, to prevent an outcry.
Then he freed one arm and struck out heavily. The German fell without a murmur. Frank ran across the deck to where he had left Jack.
"Into the boat quick!" he exclaimed.
Jack needed no further urging. Frank dropped lightly in after him, and soon they were rowing rapidly away.
"Give me that gun," said Frank after they had pulled some distance from the submarine.
"What are you going to do with it?" asked Jack.