"Something has happened!" cried Mr. Titus as he ran forward, followed by Tom, Mr. Damon and Koku. Professor Bumper started with them, but on the way he saw a curious bit of rock which he stopped to pick up and examine.

At the entrance of the tunnel, from which came rushing dirt-stained and powder-blackened men, Mr. Titus was met by a man who seemed to be in authority.

"Hello, Job!" he cried. "Glad you're back. We're in trouble!"

"What's the matter?" was the question. "This is my brother Walter," he said. "This is Tom Swift and Mr. Damon," thus hurriedly he introduced them. "What happened, Walter?"

"Premature blast. Third one this week. Somebody is working against us!"

"Never mind that now," cried Job Titus. "We must see to the poor fellows who are hurt." "I guess there aren't many," his brother said. "They were on their way out when the charge went off. Some more of Blakeson & Grinder's work, I'll wager!"

They were rushing in to the smoke-filled tunnel now, followed by Tom, Mr. Damon and Koku, who would follow his young master anywhere. Tom saw that the tunnel was lighted with incandescent lamps, suspended here and there from the rocky roof or sides. The electric lights were supplied with current from a dynamo run by a gasoline engine.

"Where is it, Serato? Where was the blast?" asked Walter Titus, of a tall Indian, who seemed to be in some authority.

"Back at second turn," was the answer, in fairly good English. "I go get beds."

"He means stretchers," translated Job. "That's our Peruvian foreman. A good fellow, but easily scared."

They ran on into the tunnel, Tom and Mr. Damon noticing that a small narrow-gage railroad was laid on the floor, mules being the motive power to bring out the small dump cars loaded with rock and dirt, excavated from the big hole.

"Mind the turn!" called Job Titus, who was ahead of Tom and Mr. Damon. "It's rough here."

Tom found it so, for he slipped over some pieces of rock, and would have fallen had not Koku held him up.

"Thanks," gasped Tom, as on he ran.

A little later he came to a place where a cluster of electric lights gave better illumination, and he could see it was there that the damage had been done.

A number of men were lying on the dirt and rock floor of the tunnel, and some of them were bleeding. Others were staggering about as though shocked or stunned.

"We must get the injured ones out of here!" cried Walter Titus. "Where are the men with stretchers?"

"I sint that Spalapeen Serato for thim!" broke in a voice, rich in Irish brogue. "But he's thot stupid he might think I was after sindin' him fer wather!"

"No, Tim. Serato is after the stretchers all right," said Walter. "We passed him on the way."

"That's Tim Sullivan, our Irish foreman, though he has only a few of his own kind to boss," explained Job Titus in a whisper.

Some of the workmen (all of whom save the few Irish referred to were Peruvian Indians) had now recovered from their shock, or fright, and began to help the Titus brothers, Tom, Mr. Damon and Koku in looking after the injured. Of these there were five, only two of whom were, seemingly, seriously hurt.

"Me take them out," said Koku, and placing one gently over his left shoulder, and the other over his right, out of the tunnel he stalked with them, not waiting for the stretchers.

And it was well he did so, for one man was in need of an immediate operation, which was performed at the rude hospital the contractors maintained at the tunnel mouth. The other man died as Koku was carrying him out, but the giant had saved one life.

Serato, the Indian foreman, with some of his men now came in, and the other injured were carried out on stretchers, being attended to by the two doctors who formed part of the tunnel force. Among a large body of men some were always falling ill or getting hurt, and in that wild country a doctor had to be kept near at hand.

When the excitement had died down, and it was found that one death would be the total toll of the accident and that the premature blast had done no damage to the tunnel, the two Titus brothers began to consider matters.

Tom, Mr. Damon and the two contractors sat in the main office and talked things over. Koku was eating supper, though the others had finished. but, naturally, it took Koku twice as long as any one else. Professor Bumper was busy transcribing material in his note-book.

"Well, I'm glad you've come back, Job," said his brother. "Things have been going at sixes and sevens here since you went to get some new kind of blasting powder. By the way, I hope you got it, for we are practically at a standstill."

"Oh, I got it all right--some of Tom Swift's best-- specially made for us. And, better still, I've brought Tom back with me."

"So I see. Well, I'm glad he's here."

"Now what about this accident to-day?" went on Job.

"Well, as I said, it's the third this week. All of them seemed to be premature blasts. But I've sent for some of the fuses used. I'm going to get at the bottom of this. Here is Sullivan with them now. Come in, Tim," he called, as the Irishman knocked at the door.

"Are they the fuses used in the blasts?" Walter asked.

"They are, sor. An' they mostly burn five minutes, which is plenty of time fer all th' min t' git out of danger. Only this time th' fuse didn't seem to burn more than a minute, an' I lit it meself."

"Let's see how long they burn now," suggested Job.

One of the longer fuses was lighted. It spluttered and smoked, while the contractors timed it with their watches.

"Four minutes!" exclaimed Job. "That's queer, and they're the regular ten minute length. I wonder what this means.

He took up another fuse, and examined it closely.

"Why!" he cried. "These aren't our fuses at all. They're another make, and much more rapid in burning. No wonder you've been having premature blasts. They go off in about half the time they should."

"I can't understhand thot!" said Tim, thoughtfully. "I keep all the fuses locked up, and only take thim out when I need thim."

"Then somebody has been at your box, Tim, and they took out our regular fuses and put in these quicker ones. It's a game to make trouble for us among our men, and to damage the tunnel."

"Bless my rubber boots!" cried Mr. Damon. "Who would do a thing like that?"

"Our rivals, perhaps, though I do not like to accuse any man on such small evidence," said Walter. "But we must adopt new measures."

"And be very careful of the fuses," said Job.

"Thot's what I will!" declared Tim. "I'll put th' supply in a new place. No wonder there was blasts before th' min could git out th' way! Bad cess t' th' imps thot did this!" and he banged his big fist down on the table.

Since the trouble began a guard had been always posted around the tunnel entrance and surrounding buildings, and this night the patrol was doubled. Tom, Mr. Damon and the two Titus brothers sat up quite late, talking over plans and ideas.

Professor Bumper went to bed early, as he said he was going to set off before sunrise to make a search for the lost city.

"I regard him as more or less of a visionary," said Mr. Job Titus; "but he seems a harmless gentleman, and we'll do all we can to help him."

"Surely," agreed his brother.

The night was not marked by any disturbance, and after breakfast, Tom, under the guidance of the Titus brothers, looked over the tunnel with a view to making his first experiment with the new explosive.

The tunnel was being driven straight into the face of one of the smaller ranges of the Andes Mountains. It was to be four miles in length, and when it emerged on the other side it would enable trains to make connections between the two railroads, thus tapping a rich and fertile country.

On the site of the tunnel, which was two days' mule travel east from Rimac, the Titus brothers had assembled their heavy machinery. They had brought some of their own men, including Tim Sullivan, with them, but the other labor was that of Peruvian Indians, with a native foreman, Serato, over them.

There were engines, boilers, dynamos, motors, diamond drills, steam shovels and a miniature railway, with mules as the motive power. A small village had sprung up at the tunnel mouth, and there was a general store, besides many buildings for the sleeping and eating quarters of the laborers, as well as places where the white men could live. Their quarters were some distance from the native section.

Powder, supplies, in fact everything save what game could be obtained in the forest, or what grains or fruits were brought in by natives living near by, had to be brought over the rough trail. But Titus Brothers had a large experience in engineering matters in wild and desolate countries, and they knew how to be as comfortable as possible.

Mr. Damon learned that one of the districts whence his company had been in the habit of getting quinine was distant a day's journey over the mountain, so he decided to make the trip, with a native guide, and see if he could get at the bottom of the difficulty in forwarding shipments.

This was a few days after the arrival of our friends. Meanwhile, Tom had been shown all through the tunnel by the Titus Brothers and had had his first sight of the hard cliff of rock which seemed to be a veritable stone wall in the way of progress--or at least such progress as was satisfactory to the contractors.

"Well, we'll try what some of my explosive will do," said Tom, when he had finished the examination. "I don't claim it will be as successful as the sample blast we set off at Shopton, but we'll do our best."

Holes were drilled in the face of the rock, and several charges of the new explosive tamped in. Wires were attached to the fuses, which were of a new kind, and warning was given to clear the tunnel. The wires ran out to the mouth of the horizontal shaft and Tom, holding the switch in his hand made ready to set off the blast.

"Are they all out?" he asked Tim Sullivan, who had emerged, herding the Indian laborers before him. Tim insisted on being the last man to seek safety when an explosion was to take place.

"All ready, sor," answered the foreman.

"Here she goes!" cried Tom, as his fingers closed the circuit.