it?"

"I fail to see why the necklace thieves should bother. They've got the
trinket they wanted, haven't they? It is the canal blowers we are facing
now."

"You know Ned's theory," whispered Harry. "Well, if the necklace thieves
have brought the bauble back to the Isthmus, they think we're hot after
them, and so may strike at us before we can get our guard up. See?"

"No, I don't see," replied Frank. "I'd like to believe they brought the
necklace over here, though. Then I might stand a chance to get it back.
You'll find that it is the men who are plotting against the big dam that
we are mixing with."

The motor car ran through Gatun without stopping, and finally drew up at a
rambling old structure which seemed to have been deserted ever since the
days of Balboa. The messenger explained that they were to wait there for
the lieutenant, and all entered the ancient ruin, the boys looking
carefully about as they stepped through the doorway.

The room which first received them was long and narrow, with walls showing
both age and neglect. They were met at the door by a tall gentleman of
military bearing and a dwarf whose mischievous black eyes stared fixedly
into their faces.

"The lieutenant is late," the military man explained. "If one of you is
Frank Shaw, however, a portion of the business of the day may be taken up
before his arrival."

Frank admitted his identity, and was invited into a smaller room opening
from the apartment in which the others waited.

CHAPTER VIII.

EXPLOSIVES FOR THE GATUN DAM.

Ned and Jimmie listened for some moments to the steady click-click of
metal which came, or appeared to come, from the ground directly underneath
their feet, and then Ned arose and crept forward.

"Where you goin'?" whispered Jimmie.

"Down there."

Ned pointed to the dark corner.

"You'd better come away," warned the boy.

"We are here to investigate," Ned replied, almost impatiently.

"Then investigate with a bomb, or with a cannon," advised Jimmie.

"No time for that," came the reply. "The conditions which exist now may
not exist in an hour's time. It is now or never."

Moving forward, Ned saw a faint finger of light cutting the shadows in the
corner Jimmie had pointed out. Jimmie saw it at the same instant.

"I'll bet they've got a blacksmith shop down there," he said.

There was no opening in the great stone slabs of the floor through which a
man might make his way--only the crevice thro