"Sh!" whispered Hastings. "Keep quiet and take a sly peep around the corner. Look up along the fence and see what you make out."
Slipping off his hat, Jack took a hasty look, exposing very little of his head, while Hal now crowded close to him from behind.
"Someone trying to scale the fence," whispered Jack. "By Jove, there he goes. He has a good hold, and is going—now he's over in the yard."
Such stealthy prowling could mean little else than mischief brewing.
To both the boys came instantly the same thought:
"The submarine boat!"
"Did you recognize him?" whispered Hal, quivering.
"No; too dark for that, and, besides, he was too quick. But we must hustle to alarm someone."
"There's a watchman in the yard," Hal replied. "He ought to be getting busy."
"I don't hear any hail, or any shot," Jack replied. "Hal, old fellow, we've got to do something ourselves."
"Well, we can climb the fence as well as that stranger did."