That slow steady bumping continued.

"Anything bumping against the bull of a boat at anchor, in that fashion
may be wrong," concluded the man, swiftly.

His mind made up to this much, the rest was not difficult to decide. The
cause of that bumping required instant investigation. Williamson caught
up the tool that came quickest to hand, a pair of nippers, thrust them
into his jumper and raced up to the deck.

"If it's any real mischief," he muttered, "I hope I won't be too
slow--too late!"

With that he dived overboard, at the starboard rail, the side nearest
the gunboat. There was a splash--then the waters closed over the
machinist.

He came up at about the point he had planned, where he had heard the
bumping.

Held below water as he was by the under-hull of the submarine, he could
move with certainty, though but slowly.

Groping, the machinist encountered the metal cylinder. Quickly he felt
for its connections which, like a flash, he knew must exist. He found
the wire, but reached for another. It all had to be done swiftly, for
his reserve "wind" was fast giving out. Not finding a second wire, he
fastened his nippers against the first wire--then cut. Now, steering
the metal cylinder, he pushed it out from under the hull. Cylinder and