Under the impetus from the electric motors, the propeller shafts began to throb.  The next instant the submarine shot below, going down at so steep an angle that many of the middies were forced to reach for new footing.

“The gauge registers sixty feet below,” announced Jack.

In another moment, by the quick flooding of some of the compartments astern, the young skipper brought the boat to an even keel.

Having finished the prescribed distance under water, Captain Jack turned on the compressed air to expel the water from the compartments.  The conning tower soon rose above the water, and a moment later the “Pollard” also emerged.

Other cadets were transferred from the gunboat to the submarines, and the instruction proceeded.  The manoeuvers for the day were ended with a half-hour run under water.

“By the way, sir, did you question Truax to see what you could learn about his reasons for acting as he did on the ’Farnum’?” asked Jack Benson the next day.  Jack and Doctor McCrea were talking with Mr. Mayhew.

“I had him before me last night, and again this morning,” replied Mr. Mayhew.  “He said he hadn’t an idea what I meant, and that is all I could get out of him.”

Jack looked thoughtfully at Doctor McCrea for a moment before he exclaimed: 

“Doctor, if I had anything like your chance, I’d have Sam Truax talking!”

“How?” Doctor McCrea looked interested.

“Why, I’d—­” Jack hesitated, glancing toward the gunboat’s commanding officer.

“I’d better go and see how the midshipmen are doing,” laughed Mr. Mayhew, rising.

For some minutes Jack talked with Doctor McCrea.  As the medical officer listened, he grinned, then laughed unrestrainedly.

“Mr. Benson, you’re certainly ingenious!”

“Will you do what I’ve suggested?”

“Why, I—­er—­er—­” Doctor McCrea hesitated.  “I—­well, I’ll think it over.”  Again Doctor McCrea roared with laughter.

CHAPTER XX:  CONCLUSION

Sam Truax sat in the brig, between decks on the “Hudson,” his scowling face turned toward the barred door, when the marine guard, taking a turn, peered in.

“Good heavens, man!  What ails you?” demanded the marine.

“I’m all right,” growled the prisoner.

“I’ll be hanged if you look it.”

“What are you talking about!” demanded the prisoner angrily.

“Man alive, I wish you could see your face!”

Three minutes later a sailor halted at the door, looked at Truax, then wheeled about to the marine.

“Say, what ails that man?  What’s the matter with his face?”

“Don’t know.  Looks fearful, doesn’t he?”

“Awful!  Ought to have the doctor.”

Sam shifted uneasily.

Five minutes later a sailor wearing on one sleeve the Red Cross of the hospital squad came along.