“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, who was searching as eagerly as the others, went back a little, flashing his lamp on every square of stone.  Suddenly he uttered a cry.

“Look yeah, Massa Tom!  Heah’s suffin’ dat looks laik a big door knob.  Maybe yo’ kin push it or pull it.”

They rushed to where he was standing in front of a niche similar to the one where he had found the golden image.  Sunken in the wall was a round black stone.  For a moment Tom looked at it, and then he said solemnly: 

“Well, here goes.  It may shut off the water, or it may make it rise higher and drown us all, or the whole tunnel may cave in, but I’m going to risk it.  Hold hard, everybody!”

Slowly Tom put forth his hand and pushed the knob of stone.  It did not move.  Then he pulled it.  The result was the same—­nothing.

“Guess it doesn’t work any more,” he said in a low tone.