There was a dull, muffled report, a sort of rumbling that seemed to extend away down under the earth and then echo back again until the ground near the mouth of the tunnel, where the party was standing, appeared to rock and heave. There followed a cloud of yellow, heavy smoke which made one choke and gasp, and Tom, seeing it, cried:

"Down! Down, everybody! There's a back draft, and if you breathe any of that powder vapor you'll have a fearful headache! Get down, until the smoke rises!"

The tunnel contractors and their men understood the danger, for they had handled explosives before. It is a well-known fact that the fumes of dynamite and other giant powders will often produce severe headaches, and even illness. Tom's explosive contained a certain percentage of dynamite, and he knew its ill effects. Stretched prone, or crouching on the ground, there was little danger, as the fumes, being lighter than air, rose. The yellow haze soon drifted away, and it was safe to rise.

"Well, I wonder how much rock your explosive tore loose for us, Tom," observed Job Titus, as he looked at the thin, yellowish cloud of smoke that was still lazily drifting from the tunnel.

"Can't tell until we go in and take a look," replied the young inventor. "It won't be safe to go in for a while yet, though. That smoke will hang in there a long time. I didn't think there'd be a back draft."

"There is, for we've often had the same trouble with our shots," Walter Titus said. "I can't account for it unless there is some opening in the shaft, connecting with the outer air, which admits a wind that drives the smoke out of the mouth, instead of forward into the blast hole. It's a queer thing and we haven't been able to get at the bottom of it."

"That's right," agreed his brother. "We've looked for some opening, or natural shaft, but haven't been able to find it. Sometimes we shoot off a charge and everything goes well, the smoke disappears in a few minutes. Again it will all blow out this way and we lose half a day waiting for the air to clear. There's a hidden shaft, or natural chimney, I'm sure, but we can't find it."

"Thot blast didn't make much racket," commented Tim Sullivan. "I doubt thot much rock come down. An' thot's not sayin' anythin' ag'in yer powder, lad," he went on to Tom.

"Oh, that's all right," Tom Swift replied, with a laugh. "My explosive doesn't work by sound. It has lots of power, but it doesn't produce much concussion."

"We've often made more noise with our blasts," confirmed Job Titus, "but I can't say much for our results."

They were all anxious, Tom included, to hurry into the tunnel to see how much rock had been loosened by the blast, but it was not safe to venture in until the fumes had been allowed to disperse. In about an hour, however, Tim Sullivan, venturing part way in, sniffed the air and called:

"It's all right, byes! Air's clear. Now come on!"

They all hurried eagerly into the shaft, Mr. Damon stumbling along at Tom's side, as anxious as the lad himself. Before they reached the face of the cliff against which the bore had been driven, and which was as a solid wall of rock to further progress, they began to tread on fragments of stone.

"Well, it blew some as far back as here," said Walter Titus. "That's a good sign."

"I hope so," Tom remarked.

There were still some fumes noticeable in the tunnel, and Mr. Damon complained of a slight feeling of illness, while Koku, who kept at Tom's side, murmured that it made his eyes smart. But the sensations soon passed.

They came to a stop as the face of the cliff loomed into view in the glare of a searchlight which Job Titus switched on. Then a murmur of wonder came from every one, save from Tom Swift. He, modestly, kept silent.

"Bless my breakfast orange!" cried Mr. Damon. "What a big hole!"

There was a great gash blown in the hard rock which had acted as a bar to the further progress of the tunnel. A great heap of rock, broken into small fragments, was on the floor of the shaft, and there was a big hole filled with debris which would have to be removed before the extent of the blast could be seen.

"That's doing the work!" cried Job Titus.

"It beats any two blasts we ever set off," declared his brother.

"Much fine!" muttered the Peruvian foreman, Serato.

"It's a lalapaloosa, lad! Thot's what it is!" enthusiastically exclaimed Tim Sullivan. "Now the black beggars will have some rock to shovel! Come on there, Serato, git yer lazy imps t' work cartin' this stuff away. We've got a man on th' job now in this new powder of Tom Swift's. Git busy!"

"Um!" grunted the Indian, and he called to his men who were soon busy with picks and shovels, loading the loosened rock and earth into the mule-hauled dump cars which took it to the mouth of the tunnel, whence it was shunted off on another small railroad to fill in a big gulch to save bridging it.

Tom's first blast was very successful, and enough rock was loosed to keep the laborers busy for a week. The contractors were more than satisfied.

"At this rate we'll finish ahead of time, and earn a premium," said Job to his brother.

"That's right. You didn't make any mistake in appealing to Tom Swift. But I wonder if Blakeson & Grinder have given up trying to get the job away from us?"

"I don't know. I'd never trust them. We must watch out for Waddington. That bomb on the vessel had a funny look, even if it was not meant to kill Tom or me. I won't relax any."

"No, I guess it wouldn't be safe."

But a week went by without any manifestation having been made by the rival tunnel contractors. During that week more of Tom's explosive arrived, and he busied himself getting ready another blast which could be set off as soon as the debris from the first should have been cleared away.

Meanwhile, Professor Bumper, with his Indian guides and helpers, had made several trips into the mountain regions about Rimac, but each time that he returned to the tunnel camp to renew his supplies, he had only a story of failure to recite.

"But I am positive that somewhere in this vicinity is the lost Peruvian city of Pelone," he said. "Every indication points to this as the region, and the more I study the plates of gold, and read their message, the more I am convinced that this is the place spoken of.

"But we have been over many mountains, and in more valleys, without finding a trace of the ancient civilization I feel sure once flourished here. There are no relics of a lost race--not so much as an arrow or spear head. But, somehow or other, I feel that I shall find the lost city. And when I do I shall be famous!"

"Mr. Damon and I will help you all we can, Tom said. "As soon as I get ready the next blast I'll have a little time to myself, and we will go with you on a trip or two."

"I shall be very glad to have you," the bald-headed scientist remarked.

Tom's second blast was even more successful than the first, and enough of the hard rock was loosed and pulverized to give the Indian laborers ten days' work in removing it from the tunnel.

Then, as the services of the young inventor would not be needed for a week or more, he decided to go on a little trip with Professor Bumper.

"I'll come too," said Mr. Damon. "One of the sub- contractors whose men are gathering the cinchona bark for our firm has his headquarters in the region where you are going, and I can go over there and see why he isn't up to the mark."

Accordingly, preparations having been made to spend a week in camp in the forests of the Andes, Tom and his party set off one morning. Professor Bumper's Indian helpers would do the hard work, and, of course, Koku, who went wherever Tom went, would be on hand in case some feat of strength were needed.

It was a blind search, this hunt for a lost city, and as much luck might be expected going in one direction as in another; so the party had no fixed point toward which to travel. Only Mr. Damon stipulated that he wanted to reach a certain village, and they planned to include that on their route.

Tom Swift took his electric rifle with him, and with it he was able to bring down a couple of deer which formed a welcome addition to the camp fare.

The rifle was a source of great wonder to the Peruvians. They were familiar with ordinary firearms, and some of them possessed old-fashioned guns. But Tom's electric weapon, which made not a sound, but killed with the swiftness of light, was awesome to them. The interpreter accompanying Professor Bumper confided privately to Tom that the other Indians regarded the young inventor as a devil who could, if he wished, slay by the mere winking of an eye.

Mr. Damon located the quinine-gathering force he was anxious to see, and, through the interpreter, told the chief that more bark must be brought in to keep up to the terms of the contract.

But something seemed to be the matter. The Indian chief was indifferent to the interpreted demands of Mr. Damon, and that gentleman, though he blessed any number of animate and inanimate objects, seemed to make no impression.

"No got men to gather bark, him say," translated the interpreter.

"Hasn't got any men!" exclaimed Mr. Damon. "Why, look at all the lazy beggars around the village."

This was true enough, for there were any number of able- bodied Indians lolling in the shade.

"Him say him no got," repeated the translator, doggedly.

At that moment screams arose back of one the grass huts, and a child ran out into the open, followed by a savage dog which was snapping at the little one's bare legs.

"Bless my rat trap!" gasped Mr. Damon. "A mad dog!"

Shouts and cries arose from among the Indians. Women screamed, and those who had children gathered them up in their arms to run to shelter. The men threw all sorts of missiles at the infuriated animal, but seemed afraid to approach it to knock it over with a club, or to go to the relief of the frightened child which was now only a few feet ahead of the animal, running in a circle.

"Me git him!" cried Koku, jumping forward.

"No, Wait!" exclaimed Tom Swift. "You can kill the dog all right, Koku," he said, "but a scratch from his tooth might be fatal. I'll fix him!"

Snatching his electric rifle from the Indian bearer who carried it, Tom took quick aim. There was no flash, no report and no puff of smoke, but the dog suddenly crumpled up in a heap, and, with a dying yelp, rolled to one side. The child was saved.

The little one, aware that something had happened, turned and saw the stretched out form of its enemy. Then, sobbing and crying, it ran toward its mother who had just heard the news.

While the mothers gathered about the child, and while the older boys and girls made a ring at a respectful distance from the dog, there was activity noticed among the men of the village. They began hurrying out along the forest paths.

"Where are they going?" asked Tom. "Is there some trouble? Was that a sacred dog, and did I get in bad by killing it?"

The interpreter and the native chief conversed rapidly for a moment and then the former, turning to Tom, said:

"Men go git cinchona bark now. Plenty get for him," and he pointed to Mr. Damon. "They no like stay in village. T'ink yo' got lightning in yo' pocket," and he pointed to the electric rifle.

"Oh, I see!" laughed Tom. "They think I'm a sort of wizard. Well, so I am. Tell them if they don't get lots of quinine bark I'll have to stay here until all the mad dogs are shot."

The interpreter translated, and when the chief had ceased replying, Tom and the others were told:

"Plenty bark git. Plenty much. Yo' go away with yo' lightning. All right now."

"Well, it's a good thing I keeled over that dog," Tom said. "It was the best object lesson I could give them.~'

And from then on there was no more trouble in this district about getting a supply of the medicinal bark.

A week passed and Professor Bumper was no nearer finding the lost city than he had been at first. Reluctantly, he returned to the tunnel camp to get more provisions.

"And then I'll start out again," he said.

"We'll go with you some other time," promised Tom. "But now I expect I'll have to get another blast ready."

He found the debris brought down by the second one all removed, and in a few days, preparations for exploding more of the powder were under way.

Many holes had been drilled in the face of the cliff of hard rock, and the charges tamped in. Electric wires connected them, and they were run out to the tunnel mouth where the switch was located.

This was done late one afternoon, and it was planned to set off the blast at the close of the working day, to allow all night for the fumes to be blown away by the current of air in the tunnel.

"Get the men out, Tim," said Tom, when all was ready.

"All right, sor," was the answer, and the Irish foreman went back toward the far end of the bore to tell the last shift of laborers to come out so the blast could be set off.

But in a little while Tim came running back with a queer look on his face.

"What's the matter?" asked Tom. "Why didn't you bring the men with you?"

"Because, sor, they're not there!"

"Not in the tunnel? Why, they were working there a little while ago, when I made the last connection!"

"I know they were, but they've disappeared."

"Disappeared?"

"Yis sir. There's no way out except at this end an' you didn't see thim come out: did you?"

"Then they've disappeared! That's all there is to it! Bad goin's on, thot's what it is, sor! Bad!" and Tim shook his head mournfully.