“We’ll rig up some sort of lamps,” Tom explained, “and come back to make a thorough examination of this place.  I think the scientific men and historians will be glad to know about this city, and I’m going to make some notes about it.”

They soon came 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, even the small pools of water in the depressions having evaporated.

The gas torches worked to perfection, and revealed the beauties and wonders of the city of gold to the astonished gaze of our friends.  It was even richer in the precious metal than they had at first supposed.