Jack and Hal were both worried.  The sudden break had a bad look for the Pollard boats, in the success of which these submarine boys were most vitally interested.

At last, from below, the suspected parts of the engine were reported to be cool enough for examination.  The naval officer in charge followed Jack and Hal below.

Taking off his uniform blouse and rolling up his sleeves, Hal sailed in vigorously to locate the fault.  Machinists and cadets stood about, passing him the tools he needed, and helping him when required.

At last, after disconnecting some parts, Hal drew out a long, slender brass piston.

As he held it up young Hastings’s face went as white as chalk.

“Do you see this?” he demanded, hoarsely.

“Filed, crazily, and it also looks as though the inner end had been heated and tampered with,” gasped Jack Benson.

“This, sir,” complained Hal, turning around to face the naval officer in charge, “looks like a direct attempt to tamper with and damage the engine.  Someone has done this deliberately, sir.  It only remains to find the culprit.”

“Then we’ll find out,” retorted the naval officer, “if it takes a court of inquiry and a court martial to do it.  But are you sure of your charge, Mr. Hastings?”

“Am I sure?” repeated Hal, all the soul of the young engineer swelling to the surface.  “Take this piston, sir, and examine it.  Could such a job have been done, unless by sheer design and intent?”

“Will the lieutenant permit me to speak?” asked the senior machinist, taking a step forward and saluting.

“Yes; go ahead.”

“Yesterday morning, sir,” continued the senior machinist, “we thought the engines needed some overhauling by someone more accustomed to them than we were.  We saw one of the machinists of the ‘Farnum,’ sir, hanging about on shore.  So we invited him aboard and asked him to look the engines over.”

“Describe the man,” begged Jack.

The senior machinist gave a description that instantly denoted Sam Truax as the man in question.

“Did you leave him alone in here, at any time?” demanded Hal.

“Let me see.  Why, yes, sir.  The man must have been alone in here some three-quarters of an hour.”

Jack and Hal exchanged swift glances.

There seemed, now, very little need of carrying the investigation further.

CHAPTER XV:  ANOTHER TURN AT HARD LUCK

When he could trust himself to speak Hal Hastings addressed the naval officer.

“I think Mr. Benson and myself understand, sir, how it happened that this damage was done.  There are extra parts in the repair kit.  In twenty minutes, sir, I think we can have the engines running smoothly once more.”

The naval officer was wise enough not to press the questioning further just then.  Instead, he went on deck.