e again to the place of the underground river and found
no water there. Ned wanted to turn the stream back into the channel
again, but Tom said they might not be able to work the ancient
mechanism, so they left the black knob as it was, and hurried on.
They decided that the knob must have worked some counter-balance, or
great weight that let down a gate and cut off the river from one
channel, to turn it into another.

When they emerged at the top of the steps, and came out at the
opening which had been revealed by the rolling back of the great
altar, they saw there that counter weights, delicately balanced, had
moved the big stone.

"We might close that opening," said Tom, "and then if any one SHOULD
come along and surprise us, they wouldn't know how to get to the
underground city." This was done, the altar rolling back over the
staircase.

"Now to get the balloon in the temple, make the lamps, and go back,"
suggested Tom, and, storing the gold they had secured in a safe
place in the temple, they went back to move the airship.

This was an easy matter, and soon they had floated the big gas bag
and car in through one of the immense doorways and so into the great
middle part of the temple where the big stone altar was located.

"Now we're prepared for emergencies," remarked Tom, as he looked up
at the yawning hole in the dome-like roof. "If worst comes to worst,
and we have to run, we can float right up here, out of the temple,
and skip."

"Do you think anything is going to happen?" asked Mr. Damon
anxiously.

"You never can tell," replied Tom. "Now to make some lamps. I think
I'll use gas, as I've got plenty of the chemicals."

It took two days to construct them, and Tom ingeniously made them
out of some empty tins that had contained meat and other foods. The
tins were converted into tanks, and from each one rose a short piece
of pipe that ended in a gas tip. On board the dirigible were plenty
of tools and materials. Into the cans were put certain chemicals
that generated a gas which, when lighted, gave a brilliant glow,
almost like calcium carbide.

"Now, I guess we can see to make our way about," remarked Tom, on
the morning of the third day, when they prepared to go back to the
city of gold. "And we'll take plenty of lunch along, for we may stay
until nearly night."

It did not take them long to roll back the altar, descend into the
tunnel, and reach the underground city. The river channel was now
dry,