“Well, I hope you do, Tom, but I doubt it.  However, I suppose you will hurry your preparations for going away, now that you have all the information you can get.”

“Right, dad.  I must send word to Mr. Damon and Ned at once.  A few more days’ work, and my balloon will be in shape for a trial flight, and then I can take it apart, pack it up, and ship it.  Then ho! for the city of gold!”

Mr. Swift smiled at his son’s enthusiasm, but he did not check it.  He knew Tom too well for that.

Naturally Mr. Damon and Ned were delighted with the additional information the missionary had sent, and Ned agreed with Tom that it was a mere matter of diligent search to find the underground city.

“Bless my collar button!” cried Mr. Damon.  “It may not be as easy as all that, but Tom Swift isn’t the kind that gives up!  We’ll get there!”

Meanwhile Tom worked diligently on his balloon.  He sent a letter of thanks to Mr. Illingway, at the same time requesting that if any more information was obtained within the next three weeks to cable it, as there would not be time for a letter to reach Shopton ere Tom planned to leave for Mexico.

The following days were busy ones for all.  There was much to be done, and Tom worked night and day.  They had to get rifles ready, for they might meet hostile natives.  Then, too, they had to arrange for the proper clothing, and other supplies.

To take apart and ship the balloon was no small task, and then there were the passages to engage on a steamer that would land them at the nearest point to strike into the interior, the question of transportation after reaching Mexico, and many other matters to consider.

But gradually things began to shape themselves and it looked as though the expedition could start for the city of gold in about two weeks after the receipt of the second letter from the missionary.

“I think I’ll give the balloon a trial to-morrow,” said Tom one night, after a hard day’s work, “It’s all ready, and it ought to work pretty good.  It will be just what we need to sail over some dense jungle and land down on the plain by the great temple.”

“Bless my slipers!” exclaimed Mr. Damon.  “I must think up some way of telling my wife that I’m going.”

“Haven’t you told her yet?” asked Ned.

The eccentric man shook his head.

“I haven’t had a good chance,” he said, “but I think I’ll tell her to-morrow, and promise her one of the gold images.  Then she won’t mind.”

Tom was just a little bit nervous when he got ready for a trial flight in the new dirigible balloon.  To tell the truth he much preferred aeroplanes to balloons, but he realized that in a country where the jungle growth prevailed, and where there might be no level places to get a “take off,” or a starting place for an aeroplane, the balloon was more feasible.

But he need have had no fears, for the balloon worked perfectly.  In the bag Tom used a new gas, much more powerful even than hydrogen, and which he could make from chemicals that could easily be carried on their trip.