The ingenious little water wheel Whistler had seen at the foot of the dam had probably furnished power for some machine that had been fixed on the face of the dam with a charge of dynamite.  This invention had been rigged to explode the dynamite after a certain length of time—­time enough, without doubt, to enable the inventor to get well away from the vicinity of the dam.

“If Linder is his name,” Whistler said, when the boys were afterward talking it over among themselves, “I hope I’ll see him again some time.  He was never blown up with the dam, that is sure.”

“You don’t think he was ’hoist with his own petard, then?” suggested Torry.

“Hear the high-brow!” sniffed Frenchy.

“Oi, oi!” cried Ikey.  “He means was he blown up, too?  I bet not!”

“I ought to have told somebody about him before,” sighed Whistler.  “I had a feeling he wasn’t using his real name.”

“Say! why should you worry?  That Mr. Santley didn’t think anything wrong of him until he found the letter in German in Blake’s locker.  And we did set Mr. MacMasters and the S. P. Eight-eighty-eight after him and the oil boat, didn’t we?”

“By the way,” Whistler suddenly observed, drawing an official looking letter from his pocket.  “Did I tell you I got this?”

“No,” said Torry.  “What is it?”

“Hurray!” yelled Frenchy, the quick-witted.  “It’s our assignment to the Kennebunk, I bet you!”

“Is that right, Whistler?” asked Torry.

“That’s what it is,” admitted Morgan.  “We’re to report, however, to Mr. MacMasters at Rivermouth day after to-morrow.  But our ultimate destination is the Kennebunk, superdreadnaught, just built and fitted out for her first cruise.  You know, she was only christened a month ago.”

Even the Elmvale disaster and the mystery regarding the German spy, Franz Linder, were at once ousted from the minds of the Navy boys.  Their first cruise in a superdreadnaught was of much greater importance.

CHAPTER XI

UNEXPECTED PERIL

The four apprentice seamen went down to Rivermouth in great spirits.  The home folks were not actually glad to see them go, but they were a little relieved; for the chums had managed to keep things very lively about Seacove during their shore leave.

The terrible disaster at Elmvale, however, had sobered the four friends a good bit at the last.  Seven Knott had gone away before it happened, so he had had no part in their later adventures.  They were not even sure that he had gone to join the crew of the Kennebunk, the new superdreadnaught to which they were assigned for a brief cruise.

They had heard nothing from Ensign MacMasters, so the Navy boys did not know when or how they were to meet him; but they went to Rivermouth on the early train and had plenty of time to look about the port and see all of the shipping in the harbor.