In an hour Tom knew his balloon would not be visible to the Mexicans, and at the end of that time he pointed for the west.  And then, flying low so as to use the trees as a screen, but going at good speed.  Tom and his friends were well on their way to the city of gold.

“We must keep a good lookout down below,” said Tom, when everything was in working order.  “We don’t want to fly over the plain of the ruined temple.”

“We may in the night,” suggested Ned.

“No night flying this time,” said his chum.  “We’ll only move along daytimes.  We’ll camp at night.”

For three days they sailed along, sometimes over vast level plains on which grazed wild cattle, again over impenetrable jungles which they could never have gotten through in their ox carts.  They crossed rivers and many small lakes, stopping each night on the ground, the airship securely anchored to trees.  Tom could make the lifting gas on board so what was wasted by each descent was not missed.

One day it rained, and they did not fly, spending rather a lonely and miserable twelve hours in the car.  Another time a powerful wind blew them many miles out of their course.  But they got back on it, and kept flying to the west.

“We must strike it soon,” murmured Tom one day.

“Maybe we’re too far to the north or south,” suggested Ned.

“Then we’ll have to beat back and forth until we get right,” was Tom’s reply.  “For I’m going to locate that ruined temple.”

They ate breakfast and dinner high in the air, Eradicate preparing the meals in the tiny kitchen.  Ever did they keep looking downward for a sight of a great plain, with a ruined temple in the midst of it.

In this way a week passed, the balloon beating back and forth to the North or South, and they were beginning to weary of the search, and even Tom, optimistic as he was, began to think he would never find what he sought.

It was toward the close of day, and the young inventor was looking for a good place to land.  He was flying over a range of low hills, hoping the thick forest would soon come to an end when, as he crossed the last of the range of small mountains, he gave a cry, that drew the attention of Ned and Mr. Damon.

“What is it?” demanded his chum.

“Look!” said Tom.  “There is the great plain!”

Ned gazed, and saw, spread out below them a vast level plateau.  But this was not all he saw, for there, about in the centre, was a mass of something—­something that showed white in the rays of the setting sun.

“Bless my chimney!” cried Mr. Damon.  “That’s some sort of a building.”

“The ruined temple,” said Tom softly.  “We’ve found it at last,” and he headed the balloon for it and put on full speed.

CHAPTER XVIII

FINDING THE TUNNEL

In silence, broken only by the noise of the motor, did the gold-seekers approach the temple.  As they neared it they could see its vast proportions, and they noted that it was made of some white stone, something like marble.  Then, too, as they drew closer, they could see the desolate ruin into which it had fallen.