his river isn't a natural one--I mean that it flows along
banks of smooth stone, just as if they were cut for it, a canal you
know."

"That's right," said Tom, as he looked at the edge of the channel of
the underground stream. "These stones are cut as cleanly as the rest
of the tunnel. Whoever built that must have made a regular channel
for this river to flow in. And it's square on the other side, too,"
he added, flashing his lamp across.

"Then don't you see," continued Ned, "that this river hasn't always
been here."

"Bless my gaiters!" gasped Mr. Damon, "what does he mean? The river
not always been here?"

"No," proceeded Tom's chum. "For the ancients couldn't have cut the
channel out of stone, or made it by cementing separate stones
together while the water was here. The channel must have been dry at
one time, and when it was finished they turned the water in it."

"But how is that going to help us?" asked Tom. "I grant you that the
river may not have been here at one time, but it's here NOW, which
makes it all the worse for us."

"But, Tom!" cried his chum, "if the river was turned aside from this
channel once it can be done again. My notion is that the ancients
could make the river flow here or not, just as they choose. Probably
they turned it into this channel to keep their enemies from crossing
to the city of gold, like the ancient moats. Now if we could only
find--"

"I see! I see!" cried Tom enthusiastically. "You mean there must be
some way of shutting off the water."

"Exactly," replied his chum. "We've got to shut that stream of water
off, or turn it into some other channel, then we can cross, and keep
on to the city of gold. And I think there must be some valve--some
lever, or handle or something similar to the one that moved the
altar-near here that does the trick. Let's all look for it."

"Bless my chopping block!" cried Mr. Damon. "That's the strangest
thing I ever heard of! But I believe you're right, Ned. We'll look
for the handle to the river," and he laughed gaily.

Every one was in better spirits, now that there seemed a way out of
the difficulty, and a moment later they were eagerly flashing their
lamps on the sides, floor and ceiling of the tunnel, to discover the
means of shutting off the water. At first they feared that, after
all, Ned's ingenious theory was not to be confirmed. The walls,
ceiling and floor were as smooth near the edge of the river as
elsewhere.

But Eradicate,