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 af 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
