cealment was imperative,
and the fellow proceeded at a swift pace for an hour.

At the end of that time he was in a lonely section of country, where
rounded knolls were surrounded by the dense growth of the jungle. In spite
of the wildness of the spot, however, Ned saw that civilization had at
some distant time made its mark there. Here and there low, broken walls of
brick lifted from the grass, and the vegetation was not quite so
luxuriant. In numerous places, as they advanced, the boys saw that the
ground had once been leveled off as if to make way for a building, the
ruins of which were still to be seen.

"One of the ruined cities of the Isthmus," Jimmie whispered. "If Peter
could see this he would know all about it."

"It wasn't a very large city," laughed Ned.

"There's the ruins of a temple over there," insisted the boy. "There's a
wall standing yet. And there's the man we want going into it."

As the boy spoke the man they were following disappeared behind the wall.
Before he could be restrained Jimmie wiggled forward to the foot of the
ruin. Nestor saw him peering around the end of the line of brick and
hastened forward.

The man they had followed was nowhere in sight when Ned turned the angle,
and Jimmie lay on the ground in the shadows, kicking up his heels.

"He went down through the earth," the boy giggled, regardless of the
danger of the situation. "He went right down through the ground. Say, but
he's a corker, to get out of sight like that."

Ned caught the lad by the arm, to silence him, and listened. A steady
click-click came from the ground beneath their feet. The sounds came
continuously, almost with the regularity of the ticking of a clock.

"Where was he when he disappeared?" asked Ned.

"Over there in the corner," was the reply. "He walked up to the wall and
stepped out of sight. What's that queer smell?" he added, sniffing the
air.

"There must be a fire down there in the vaults of the old temple," replied
Ned. "They must have a fire, for the smoke is coming out of a crevice at
the top of that wall, and they are working on metal."

"Yes," said Jimmie, "an' I'll bet they're makin' more bombs--bombs for the
dam."

CHAPTER VII.

WORKING ON NED'S THEORY.

At daybreak Frank Shaw stood in the screened porch facing west, watching
and waiting for the return of Nestor and Jimmie. It had been a long night
for him, but he had kept his vigil alone, knowing that his chums needed
all t