The young inventor was idly handling some pieces of the very hard rock that had cropped out in the tunnel cut Tom had tested it, he had pulverized it (as well as he was able), he had examined it under the microscope, and he had taken great slabs of it and set off under it, or on top of it, charges of explosive of various power to note the effect. But the results had not been at all what he had hoped for.

"What's to be done, Tom?" repeated the contractor.

"Well, Mr. Titus," was the answer, "the only thing I see to do is to make a new explosive."

"Can you do it, Tom?"

The reply was characteristic.

"I can try."

And in the days that followed, Tom began work on a new line. He had brought from Shopton with him much of the needful apparatus, and he found he could obtain in Lima what he lacked.

A message to his father brought the reply that the new ingredients Tom needed would be shipped.

"The kind of explosive we need to rend that very hard rock," the young inventor explained to the Titus brothers, "is one that works slowly."

"I thought all explosions had to be as quick as a flash," said Walter.

"Well, in a sense, they do. Yet we have quick burning and slow-burning powders, the same as we have fuses. A quick- burning explosive is all right in soft rock, or in soil with rock and earth mingled. But in rock that is harder than flint if you use a quick explosive, only the outer surface of the rock will be scaled off.

"If you take a hammer and bring it down with all your force on a hard rock you may chip off a lot of little pieces, or you may crack the rock, but you won't, under ordinary circumstances, pulverize it as we want to do in the tunnel.

"On the other hand, if you take a smaller hammer, and keep tapping the rock with comparatively gentle blows, you will set up a series of vibrations, that, in time, will cause the hard rock to break up into any number of small pieces.

"Now that is the kind of explosive I want one that will deal a succession of constant blows at the hard rock instead of one great big blast."

"Can you make it, Tom?"

"Well, I don't know. I'll do the best I can."

From then on Tom was busy with his experiments.

Work on the tunnel did not cease while he was searching for a new explosive. There was plenty of the old explosive left and charges of this were set off as fast as holes could be drilled to receive it. But comparatively little was accomplished. Sometimes more rock would be loosed than at others, and the native laborers, now seemingly perfectly contented, would be kept busy. Again, when a heavy blast would be set off hardly a dozen dump cars could be filled.

But the work must go on. Already the time limit was getting perilously close, and the contractors did not doubt that their rivals were only waiting for a chance to step in and take their places.

Nothing more had been seen or heard of the bearded man, Waddington, or Blakeson & Grinder. But that the rival firm had not given up was evidenced by the efforts made in New York to cripple, financially, the firm in which Tom was interested. In fact, at one time the Titus brothers were so tied up that they could not get money enough to pay their men. But Tom cabled his father, who was quite wealthy, and Mr. Swift loaned the contractors enough to proceed with until they could dispose of some securities.

It might be mentioned that Tom was to get a large sum if the tunnel were completed on time, so it was to his interest and his father's, to bring this about if he could.

Tom kept on with his powder experiments. Mr. Damon helped him, for that gentleman had succeeded in putting the affairs of the wholesale drug business on a firm foundation, and there was no more trouble about getting the supplies of cinchona bark to market. The natives seemed to have taken kindly to the eccentric man, or perhaps it was the reputation of Tom Swift and his electric rifle that induced them to work hard.

It must not be supposed that Professor Bumper was idle all this while.

He came and went at odd times, accompanied by his little retinue of Indians, a guide and a native cook. He would come back to the tunnel camp, where he made his headquarters, travel stained, worn and weary, with disappointment showing on his face.

"No luck," he would report. "The hidden city of Pelone is still lost."

Then he would retire to his tent, to pour over his note- books, and make a new translation of the inscription on the golden plates. In a day or so, refreshed and rested, he would prepare for another start.

"I'll find it this time, surely!" he would exclaim, as he marched off up the mountain trail. "I have heard of a new valley, never before visited by a white man, in which there are some old ruins. I'm sure they must be those of Pelone."

But in a week or so he would come back, worn out and discouraged again.

"The ruins were only those of a native village," he would say. "No trace of an ancient civilization there."

The professor took little or no interest in the tunnel, though he expressed the hope that Tom and his friends would be successful. But industrial pursuits had no charm for the scientist. He only lived to find the hidden city which was to make him famous.

He heard the story of the queer shaft leading down into the bore under the mountain, and, for a time, hoped that might be some clue to the lost Pelone. But, after an examination, he decided it was but the shaft to some ancient mine which had not panned out, and so had been abandoned after having been fitted with a balanced rocky door, perhaps for some heathen religious rite.

There seemed to be no further trouble among the Indian tunnel workers. Those who had disappeared--who had, seemingly, gone willingly up the knotted rope to hide themselves in the valley--kept on with their work. If they told their fellows why and where they had gone, the others gave no sign. The evil spirits of the tunnel had been exorcised, and there was now peace, save for the blasts that were set off every so often.

Tom tried combination after combination, testing them inside and outside the tunnel, always seeking for an explosive that would give a slow, rending effect instead of a quick blow, the power of which was soon lost. And at last he announced:

"I think I have it!"

"Have you? Good!" cried Job Titus.

"Yes," Tom went on, "I've got a mixture here that seems to give just the effect I want. I tried it on some small pieces of rock, and now I want to test it on some large chunks. Have you brought any down lately?"

"Yes, we have some big slabs in there."

Some large pieces of the hard rock, which had been brought down in a recent blast, were taken outside the tunnel, and in them one afternoon Tom placed, in holes drilled to receive it, some of his new explosive. The rocks were set some distance away from the tunnel camp, and Tom attached the electric wires that were to detonate the charge

"Well, I guess we're ready," announced the young inventor, as he looked about him.

The tunnel workers had been allowed to go for the day, and in a log shack, where they would be safe from flying pieces of rock, were Tom, Mr. Damon and the two Titus brothers.

Tom held the electric switch in his hand, and was about to press it.

"This explosive works differently from any other," he explained. "When the charge is fired there is not instantly a detonation and a bursting. The powder burns slowly and generates an immense amount of gas. It is this gas, accumulating in the cracks and crevices of the rock, that I hope will burst and disintegrate it. Of course, an explosion eventually follows, as you will see. Here she goes!"

Tom pressed the switch and, as he did so, there was a cry of alarm from Mr. Damon.

"Bless my safety match, Tom!" cried the old man. "Look! Koku!"

For, as the charge was fired, the giant emerged from the woods and calmly took a seat on the rock that was about to be broken up into fragments by Tom's new explosive.