Working like beavers, and with the assistance of others standing about, Jack and Hal had the piston replaced and all the other parts in place within fifteen minutes.  Then, once more, Hal turned on the gasoline, set the ignition, and watched.

The engine ran as smoothly as ever.

“There won’t be any more trouble, unless someone is turned loose here with files and a blast lamp,” pronounced Hal.  Then he and his chum sought the deck, to report to the officer in charge.

“You think we’re in running order, now?” asked that officer.

“If you give the speed-ahead signal, sir, I think you’ll feel as though you had a live engine under your deck,” Hal assured him.

The signal was given, the “Pollard” immediately responding.  She cut a wide circle, at good speed, returning to her former position, where the propellers were stopped.

“You suspect your own machinist, who was aboard?” asked the naval officer, in a low tone, of the submarine boys.

“If you’ll pardon our not answering directly, sir,” Captain Jack replied, “we want to have more than suspicions before we make a very energetic report on this strange accident.  But we shall not be asleep, sir, in the matter of finding out.  Then we shall make a full report to Mr. Mayhew.”

“Success to you—­and vigilance!” muttered the naval officer.

The gunboat’s cutter came alongside, transferring Jack and Hal back to the “Farnum.”

Hal went directly below to the engine room.

“You fixed the trouble with the ’Pollard’?” demanded Eph Somers, eagerly.

“Yes,” Hal admitted.

“What was wrong?”

“Why, I don’t know as I’d want to commit myself in too offhand a way,” replied Hal, slowly, as though thinking.

“What appeared to be at the bottom of the trouble?”

“Why, it may have been that one of the naval machinists, not understanding our engines any too well, allowed one of the pistons to get overheated, and then resorted to filing,” Hal replied.

“What?  Overheat a piston, and then try to correct it with a file?” cried young Somers, disgustedly.  “The crazy blacksmith!  He ought to be set to shoeing snails—­that’s all he’s fit for.”

“It looks that way,” Hal assented, smiling.

Artful, clever Hal!  He had carried it all off so coolly and naturally that Sam Truax, who had been closely studying Hastings’s face from the background, was wholly deceived.

“This fellow, Hastings, isn’t as smart as I had thought him,” muttered Truax, to himself.

The interrupted cruise now proceeded, the parent vessel signaling for a temporary speed of sixteen knots in order to make up for lost time.

Twenty minutes later came the signal from the “Hudson:” 

“At the command, the submarines will dash ahead at full speed, each making its best time.  During this trial, which will end at the firing of a gun from the parent vessel, all cadets will be on deck.”