Arrived at one corner of the office building, young Benson, who was in the lead, signaled a stop.  Hal halted just behind him.

“It’s the submarine, all right, that the fellow’s after,” whispered Jack excitedly, as he peeped.  “Make him out over there, at the door?  Gracious!  He’s unlocking and throwing the padlock off.  And, blazes!  Can’t you make out who it is, Hal?”

“Josh Owen!  But he gave up his keys.”

“He had at least one duplicate, then,” declared Jack, in a tremulous whisper.  “There, he’s gone inside.  Come on, Hal—­soft-foot!  We’ll take a near look at what he’s doing.”

There was some distance to be traveled, and it had to be done with the utmost stealth.  Whatever Josh Owen—­if it was truly he—­was doing in the submarine shed, the young shadows did not wish to put him on his guard until they had caught him red-handed.

“Where’s the night watchman while all this is going on?” wondered Jack as he tip-toed forward.  It was afterwards discovered that the watchman, who sometimes drank liquor, was at this moment sound asleep in one of the sheds.  There was no time to be squandered in looking for him if Josh Owen was to be followed and foiled.

Creeping to the now open door of the submarine’s shed, Jack, who was in the lead, took a peep inside.

There was a dim light in there, though it came from the further side of the hull.  Benson signaled, and his friend followed him, stealthily, a step or two at a time, around to the stern of the “Pollard” as she lay on the stocks.

By this time a noise that plainly proceeded from the use of tools came to the ears of the boys.  Their nerves were on the keenest tension as they reached the stern of the propped-up hull.

Then they came in sight of the quarry.  Almost in the same flash they realized what the night’s mischief was.

Depending wholly on the light of a dark lantern that lay on the floor of the shed, Owen, with two or three tools, was swiftly, wickedly tampering with one of the sea-valves belonging to one of the forward water compartments of the submarine.

This valve, if leaking badly when the craft lay submerged, would let in enough water to cause the “Pollard” to lurch and then go, nose-first, to the bottom.  It was wholly possible, too, that a capable workman could tamper with the valve so that, on casual inspection, the damage would not be detected.

Hal Hastings’s heart beat fast as he viewed this dimly illumined piece of cowardly treachery.  His fingers itched to lay hold of Josh Owen, uneven though the fight might be with both boys for assailants.

But Jack Benson, though his first impulse was to let out a Comanche yell, and then dart forward into the fray, instantly conceived a plan that he thought would work better.

Gripping his chum’s arm for silence, Jack whispered in his ear: 

“Can you set the camera for universal focus, here in the shadow?”