“It does seem a mystery how it could have been taken in open daylight, while we were about camp together,” said Tom.  “But is the loss such a grave one, Professor Bumper?”

“Very grave.  In fact I may say it is impossible to proceed with the excavating without the map.”

“Then what are we to do?” asked Ned.

“We must get it back!” declared Tom.

“Yes,” agreed the scientist, “we can not work without it.  As soon as I make a little further search, to make sure it could not have dropped in some out-of-the-way place, I shall go over to Professor Beecher’s camp and demand that he give me back my property.”

“Suppose he says he hasn’t taken it?” asked Tom.

“Well, I’m sure he either took it personally, or one of his party did.  And yet I can’t understand how they could have come here without our seeing them,” and the professor shook his head in puzzled despair.

A more detailed search did not reveal the missing map, and Mr. Damon and his friend the scientist were on the point of departing for the camp of their rivals, less than a mile away, when Tom had what really amounted to an inspiration.

“Look here, Professor!” he cried.  “Can you remember any of the details of your map—­say, for instance, where we ought to begin excavating to get at the wonders of the underground city?”

“Well, Tom, I did intend to compare my map with the configuration of the country about here.  There is a certain mountain which serves as a landmark and a guide for a starting point.  I think that is it over there,” and the scientist pointed to a distant snow-capped peak.

The party had left the low and marshy land of the true jungle, and were among the foothills, though all about them was dense forest and underbush, which, in reality, was as much a jungle as the lower plains, but was less wet.

“The point where I believe we should start to dig,” said the professor, “is near the spot where the top of the mountain casts a shadow when the sun is one hour high.  At least that is the direction given in the old manuscripts.  So, though we can do little without the map, we might make a start by digging there.”

“No, not there!” exclaimed Tom.

“Why not?”

“Because we don’t want to let Beecher’s crowd know that we are on the track of the idol of gold.”

“But they know anyhow, for they have the map,” commented Ned, puzzled by his chum’s words.

“Maybe not,” said Tom slowly.  “I think this is a time for a big bluff.  It may work and it may not.  Beecher’s crowd either has the map or they have not.  If they have it they will lose no time in trying to find the right place to start digging and then they’ll begin excavating.

“Very good!  If they do that we have a right to dig near the same place.  But if they have not the map, which is possible, and if we start to dig where the professor’s memory tells him is the right spot, we’ll only give them the tip, and they’ll dig there also.”