Outside a rudely-constructed shack, in the middle of a large field, about a mile away from the nearest of the buildings owned by Tom Swift and his father, were gathered a group of figures one morning. From the shack, trailing over the ground, were two insulated wires, which led to a pile of rocks and earth some distance off. Out of the temporary building came Koku, the giant, bearing in his arms a big rock, of peculiar formation.

"That's it, Koku!" exclaimed Tom Swift. "Now don't drop it on your toes."

"No, Master, me no drop," the giant said, as he strode off with the heavy load as easily as a boy might carry a stone for his sling-shot.

Koku placed the big rock on top of the pile of dirt and stones and came back to the hut, just as Eradicate, the colored man-of-all-work, emerged. Koku was not looking ahead, and ran into Eradicate with such force that the latter would have fallen had not the giant clasped his big arms about him.

"Heah now! Whut yo' all doin' t' me?" angrily demanded Eradicate. "Yo' done gone an' knocked de breff outen me, dat's whut yo' all done! I'll bash yo' wif a rock, dat's what I'll do!"

Koku, laughing, tried to explain that it was all an accident, but Eradicate would not listen. He looked about for a stone to throw at the giant, though it was doubtful, with his feeble strength, and considering the great frame of the big man, if any damage would have been done. But Eradicate saw no rocks nearer than the pile in which ended the two insulated wires, and, with mutterings, the negro set off in that direction, shuffling along on his rheumatic legs.

From the shack Tom Swift hailed:

"Hi there, Rad! Come back! Where are you going?"

"I'se gwine t' git a rock, Massa Tom, an' bash de haid ob dat big lummox ob a giant! He done knocked de breff outen me, so he did."

"You come back from that stone pile!" Tom ordered. "I'm going to blow it up in a minute, and if you get too near you'll have the breath knocked out of you worse than Koku did it. Come back, I say!"

But Eradicate was obstinate and kept on. Tom, who was adjusting a firing battery in the shack, laughed, and then in exasperation cried:

"Koku, go and get him and bring him back. Carry him if he won't come any other way. I don't want the dear old chump to get the fright of his life, and he sure will if he goes too close. Bring him back!"

"Koku bring, Master," was the giant's answer.

He ran toward Eradicate, who, seeing his tormentor approaching, redoubled his shuffling pace toward the stone pile. But he was no match for the giant, who, ignoring his struggles, picked up Eradicate, and, flinging him over his shoulder like a sack of meal, brought him to the shack.

"There him be, Master!" said the giant.

"So I see," laughed Torn. "Now you stay here, Rad."

"No, sah! No, sah, Massa Tom! I--I'se gwine t' git a rock an'--an' bash his haid--dat's what I'se gwine t' do!" and the colored man tried to struggle to his feet.

"Look out now!" cried Tom, suddenly. "If things go right there won't be a rock left for you to 'bash' anybody's head with, Rad. Look out!"

The three cowered inside the shack, which, though it was rudely made, was built of beavy logs and planks, with a fronting of sod and bags of sand.

Tom turned a switch. There was a loud report, and where the stone pile had been there was a big hole in the ground, while the air was filled with fragments of rock and dirt. These came down in a shower on the roof of the shack, and Eradicate covered his ears with his trembling hands.

"Am--am de world comin' to de end, Massa Tom?" he asked. "Am dat Gabriel's trump I done heah?"

"No, you dear old goose!" laughed the young inventor. "That was just a charge of my new explosive--a small charge, too. But it seems to have done the work."

He ran from the shack to the place where the rock pile had been, and picked up several small fragments.

"Busted all to pieces!" exulted Tom Swift. "Not a piece left as big as a hickory nut. That's going some! I've got the right mixture at last. If an ounce did that, a few hundred pounds ought to knock that Andes tunnel through the mountain in no time. I'll telegraph to Mr. Titus."

Leaving Koku and Rad to collect the wires and firing apparatus, there being no danger now, as no explosive was left in the shack, Tom made his way back to the house. His father met him.

"Well, Tom," he asked, "another failure?"

"No, Dad! Success! This time I turned the trick. I seem to have gotten just the right mixture. Look, these are some of the pieces left from the big rock--one of the samples Mr. Titus sent me. It was all cracked up as small as this," and he held out the fragments he had picked up in the field.

Mr. Swift regarded them for a few moments.

"That's better, Tom," he said. "I didn't think you could get an explosive that would successfully shatter that hard rock, but you seem to have done it. Have you the formula all worked out?"

"All worked out, Dad. I only made a small quantity, but the same proportions will hold good for the larger amounts. I'm going to start in and make it now. And then--Ho! for Peru!"

Tom struck an attitude, such as some old discoverer might have assumed, and then he hurried into the house to telephone a telegram to the Shop' ton office. The message was to Mr. Titus, and read:

"Explosive success. Start making it at once. Ready for Peru in month's time."

"Thirteen words," repeated Tom, as the operator called them back to him. "I hope that doesn't mean bad luck."

The experiment which Tom Swift had just brought to a successful conclusion was one of many he had conducted, extending over several wearying weeks.

As soon as Tom had received the samples of the rock he had begun to experiment. First he tried some of the explosive that was so successful in the giant cannon, As he had feared, it was not what was needed. It cracked the rock, but did not disintegrate it, and that was what was needed. The hard rock must be broken up into fragments that could be easily handled. Merely to crack it necessitated further explosions, which would only serve to split it more and perhaps wedge it fast in the tunnel.

So Tom tried different mixtures, using various chemicals, but none seemed to be just right. The trials were not without danger, either. Once, in mixing some ingredients, there was an explosion that injured one man, and blew Tom some distance away. Fortunately for him, there was an open window in the direction in which he was propelled, and he went through that, escaping with only some cuts and bruises.

Another time there was a hang-fire, and the explosive burned instead of detonating, so that one of the shops caught, and there was no little work in subduing the flames.

But Tom would not give up, and finally, after many trials, he hit on what he felt to be the right mixture. This he took out to the big lot, and having made a miniature tunnel with some of the sample rock, and having put some of the explosive in a hole bored in the big chunk Koku carried, Tom fired the charge. The result we have seen, It was a success.

A day after receiving Tom's message Mr. Titus came on and a demonstration was given of the powerful explosive.

"Tom, that's great!" cried the tunnel contractor. "Our troubles are at an end now."

But, had he known it, new ones were only just beginning.

Tom at once began preparations for making the explosive on a large scale, as much of it would be needed in the Andes tunnel. Then, having turned the manufacturing end of it over to his men, Tom began his preparations for going to Peru.

Mr. Damon was also getting ready, and it was arranged that he, with Tom and Mr. Titus, should take a vessel from San Francisco, crossing the continent by train. The supply of explosive would follow them by special freight.

"We might have gone by Panama except for the slide in the canal," Tom said. "And I suppose I could take you across the continent in my airship, Mr. Titus, if you object to railroad travel."

"No, thank you, Tom. If it's just the same to you, I'd rather stay on the ground," the contractor said. "I'm more used to it."

A day or so before the start for San Francisco was to be made, Tom, passing a store in Shopton, saw something in the window he thought Mary Nestor would like. It was a mahogany work-box, of unique design, beautifully decorated, and Tom purchased it.

"Shall I have it sent?" asked the clerk.

"No, thank you," Tom answered.

He knew the young lady who had waited on him, and, for reasons of his own, he did not want her to know that Mary was to get the box.

Carrying the present to his laboratory, Tom prepared to wrap it up suitably to send to Mary, with a note. Just, however, as he was looking for a box suitable to contain the gift, he received a summons to the telephone. Mr. Titus, in New York, wanted to speak to him.

"Here, Rad!" Tom called. "Just box this up for me, like a good fellow, and then take it to Miss Nestor at this address; will you?" and Tom handed his man the addressed letter he had written to Mary. "Be careful of it," Tom cautioned.

"Oh, I'll be careful, Massa Tom," was the reply. "I'll shore be careful."

And Eradicate was--all too careful.