"But how?"

It was Don Ernesto who asked.

"Very simply," said Jack. "Let me explain so that Prince Huaca can comprehend. This outfit, sir"--and, rising and walking to the table, Jack indicated the radio outfit reposing there--"can receive messages sent from very far away, but it cannot send messages except for a comparatively short distance, 150 miles at most. It was that which my father had in mind.

"However, at the Andine Monastery of the Cross of the Snows, Don Ernesto, you will remember that we built a sending station by utilizing the water power in the falls and the turbines of the power plant. I cannot explain more clearly to you, Prince Huaca," he added, addressing the latter, "without going into too great detail. But this will make it clearer to you. We can send the voice-through-the-air to another station, which in turn, will send it farther, just as one runner carries a message which he transfers to another."

Prince Huaca nodded, his eyes bright and expectant.

"And from the monastery, Jack?" suggested his father.

"Why, Father, you yourself told me that _La Prensa_, the great newspaper of Buenos Ayres, doubtless had established a radio station at its branch office in Santiago, the Chilian capital. Although when we were in Santiago," added Jack, "we were so busy with other matters I did not hear of it, or go to investigate."

"True, Jack," said Mr. Hampton. "Don Ernesto has told me _La Prensa_ had installed a radio station at Santiago. Of course, too, there is a commercial station at Valparaiso."

"But the one at Santiago can reach the President more quickly," said Jack.

CHAPTER XVII--THE FIGHT ON THE PARAPET

So it was decided to set up the field radio and attempt to raise the monastery. Prince Huaca had had the party brought to his room by way of the secret pa.s.sage, in order to avoid having them appear among the young n.o.bles in waiting in his anteroom. As the boys would have to be taken to the roof to set up the aerial, he first dismissed those in the anteroom, then called servants to carry the outfit to the battlements.

Don Ernesto, however, begged permission that Pedro and Carlos be summoned to a.s.sist, instead of servants who could not understand them.

Prince Huaca acquiesced, and sent the jailer for the two faithful huachos.

He, himself, was eager to observe every preparatory step. Self-contained though he was, and despite his matter-of-fact acceptance of the phenomenon of radio, yet it was plain to be seen that he was highly excited over the matter. Everything had to be explained to him.

For his field outfit, Mr. Hampton carried both batteries and a quarter-kilowatt generator, such as is in use in army operations. In fact, the outfit paralleled an army field outfit in a number of respects, including the umbrella type of aerial. This consisted of only one pole of hollow steel, and constructed in collapsible sections that made transportation an easy matter. From the top of the pole, the wires of the aerial were carried to the ground at some distance from the base, where they were attached to porcelain insulators. Thus, the wires served the double function of aerial and guy wires.

While the boys busied themselves erecting the aerial, a difficult matter because the battlement was all of stone and at first glance there appeared to be nothing to which the insulators could be fastened, Mr.

Hampton conversed with Prince Huaca, explaining this, that and the other about the outfit and about the reasons for doing certain things.

The prince pointed to what Jack and Frank were doing, and asked the reason for it. The boys were forcing wedge-shaped wooden blocks or pegs, to which insulators were fastened, into cracks between stones of the turret floor. Originally, these pegs were so made to be driven into the ground, thus affording anchorage for the aerial-guy wires. Had it not been for the cracks, they would have been unable to erect the aerial, as all about them was stone.

When this work was completed, the boys, working furiously, set up the generator on a pair of legs sufficiently high to give clearance for the handles by which it was to be turned. Wires were then run to the transformer, tuner attached, the headphone wired on, and the aerial and ground connections made.

Part of the outfit was not yet in use, and Prince Huaca pointed to the box and batteries questioningly.

"Are these objects not employed?" he said.

Mr. Hampton explained he had brought both batteries and generator to serve as sources of energy. They had been packed separately upon mules, so that in case one was lost the other might still remain. When the batteries were used, it was necessary also to use the tube transformer, he said, indicating the oblong box in which the tubes were mounted on springs. But when the generator was used, only the transformer and key were necessary.

"And why is this used rather than the other?" Prince Huaca wanted to know.

"The generator supplies more power," said Mr. Hampton, simplifying his explanation as much as possible. "It is a little man with a big voice that carries far, while the batteries represent a big man with only a medium voice."

Fast though the boys went about their preparations, in the light of torches held by servants, the time sped by more rapidly than they had expected. All the time there came up to them the shouts and laughter of those in the great square far below, where the festivities of the Feast of Raymi continued unabated.

Several times one or the other would wander to the parapet and stare at the scene below, where great fires burned, casting grotesque dancing shadows on the fronts of the Temple and the palaces surrounding the square, with the merry-making crowds surrounding poets and singers here and there, or dancing to the music of the minstrels who played queer stringed instruments.

As big Bob turned away from the parapet on one of these trips, to rejoin his comrades, he believed he discerned the shadowy form of a skulker in a nearby embrasure. He could not be certain, however, because his eyes were dazzled from staring at the scene below. All about him was starlit darkness, the moon had not yet risen. His friends, surrounded by the ring of torchlights, were some distance off.

What could a skulker be doing here? That was the question that leaped to mind. No sentries were posted, at least none had been seen so far. Nor was any other member of the party absent, as he could see in a quick glance to estimate their number.

The perilous situation in which Prince Huaca was placed recurred to his mind. Perhaps, after all, the prince was over-optimistic when he said that all within the fortress were loyal to him. Perhaps, in the loosening of the restraints of discipline, bound to come with the advent of the festival season, the soldiers below had permitted, altogether unawares, of course, some a.s.sa.s.sin intent on taking Prince Huaca's life, to enter the fortress, to slip by them unseen.

Bob stood, pressed against the parapet, his eyes on the spot, some yards distant, where he believed he had seen the skulking form. He was thinking. Not a sign of movement. Could he have been mistaken? Should he investigate? If someone lurked there, with evil intentions against Prince Huaca's life, he would be armed. Bob was without weapons. On the other hand, he realized he would not have to face firearms, but only a knife thrust or sword. And he was confident in his ability to take care of himself in a rough or tumble combat, a confidence bred of victories in the past, not only in school and college, but against ruffians in the surprising adventures into which they seemed fated perpetually to fall.

"I'll have a look," he muttered to himself. "No harm in making sure."

Stealthily, he removed his shoes, set them against the parapet where they could easily be found later, and began creeping noiselessly along the low wall toward the embrasure.

With beating heart, and muscles taut and ready for a spring, he reached the spot. Should he peer around the edge or get on top of the parapet and stare down? Either way held danger, supposing the embrasure occupied. Then he had an idea. As he had stolen along the parapet he had come across a broken lance b.u.t.t, some two feet in length, discarded by a sentry. This he had carried with him as a club. Now he took off his cap, put it on the end of the stick, and cautiously thrust it ahead of him around the edge of the embrasure.

Nothing happened. Bob was disappointed. Could it be he was mistaken? Had his eyes played him tricks? No, he felt certain he had seen a dark form skulking there. Perhaps he had the wrong embrasure. No, he felt certain this was the one. Casting caution aside, he thrust his head forward and took a quick look at the interior. It was empty.

As he stood, staring, uncomprehending, something soft and thick descended over him, a club came down on his head, a body fell upon him from above, and strong hands gripped his throat to prevent outcry. Like a flash of lightning, the truth was borne in upon him. He had not been mistaken. He had seen a form skulking there. And this man, seeing him come spying, had slipped to the top of the parapet and had leaped upon him.

Bob's first thought was to cry out; but a fold of the enveloping bag was in his mouth, and he felt certain the m.u.f.fled sound he made could not be heard. He realized, as in a flash, that whoever had attacked him, here in the center of Prince Huaca's stronghold, would be intent on silencing his lips and would have no mercy on him.

These thoughts sped through Bob's mind with lightning speed. The big fellow, on the other hand, reacted physically to the attack. He began fighting at once, and in a way that must have been totally unexpected by his antagonist. Instead of plucking at the other's hands, which were clutched about his throat, he crumpled up as if overcome and sank to the stones.

The other retained his grip on Bob's throat, a cruel pressure that set the blood to pounding in the boy's temples. Nevertheless, he was thrown off his balance, his body followed Bob's, bent above him.

The moment he touched the stones, Bob sank to the ground, drew up his legs with a convulsive effort, and then shot his feet upward with a tremendous thrust. He felt his bare feet strike a lightly-clad body.

There was a grunt. Then the hands about Bob's throat were torn loose from their grip, and the attacker went hurtling backward.

There was a thud, a dull groan, as the other struck against the parapet.

Bob was tearing frantically at the covering over his head, which was a thick woolen sack. Meantime, he was emitting roar after roar of purest rage.

"Bob, Bob. What is it? Oh."

CHAPTER XVIII--ARMED AGAIN

It was Frank's voice, and the exclamation was elicited by Frank catching sight of the figure against the parapet, now struggling to its feet, knife clutched in hand. Frank had been the first to reach his comrade's side. He did not pause but, unarmed though he was, sprang forward.

Bob pulled the sack from his head, just in time to see Frank's rush bear the other to his knees. Then the others were on the scene, soldiers with torches, Prince Huaca, Jack and the rest. It was all over in a trice.

The man was disarmed and in the hands of two soldiers, each holding him firmly by an arm. He was a stout rascal, with an evil face.

Prince Huaca looked at him keenly.