Early the next day the explorers, boys and adults, resumed their investigation of the Lost City. The professor estimated that it would take some time before they had completed their work and collected relics, records and films of the various features of absorbing scientific interest to be found there.

Joe and Nat struck out in one direction, while the Professor, Ding-dong and Mr. Tubbs assumed another line of investigation. The path taken by the two boys led them down one of the crumbling streets to the lake front of the Lost City. On the way they entered several of the houses and collected some small relics and Joe, who had some talent that way, busied himself in making rough sketches of the buildings they examined.

At last, thoroughly tired out, the two lads sat themselves down on a raised pile of carefully fitted stones in the courtyard of a splendid white building with a pyramid-like cupola. They had brought some sandwiches and a flask of water with them and made a light meal while they rested.

“Seems like a sort of sacrilege to be eating corned beef sandwiches in what may have been a temple,” said Nat as he ate.

Joe laughed.

“From what we know of the folks that used to live here they used to make corned beef out of anyone they didn’t like, so don’t worry about that end of it, old fellow.”

“That’s so,” agreed Nat. “I wonder, for instance, if this business we’re sitting on at this moment isn’t an old altar of some kind. Looks as if it might have been.”

“It does that,” agreed Joe, “and see here, Nat, here’s a metal ring right here in this slab of stone. I wonder if they used to tie their poor victims to it?”

He indicated a big ring of dull, greenish metal which they had not noticed before. It was countersunk in one of the slabs of stone that formed the top of the altar.

Nat examined it.

“I guess more likely it was used to raise this stone,” he said. “Maybe the altar is hollow inside and contains relics of some sort.”

“Cracky! I’d like to raise it,” declared Joe; but, although he tugged and pulled till his ruddy face was redder than usual, Joe could make no impression on the stone.

“Let me try,” suggested Nat.

With what idea, he could not exactly say, the boy gave the ring a gentle twisting motion instead of tugging at it. Then an astonishing thing happened. The entire top of the altar tipped downward and the boys were shot, scrambling and struggling, into the interior of the altar, if such it had been. Before they knew just what had occurred they found themselves in total darkness, for, having tipped them off, the stone had swung into place again.

A thrill of fear crept icily through Nat’s veins as he realized that they were prisoners. But he put all the heart he could into his reply when Joe in a frightened voice gasped out:

“What on earth happened, Nat?”

“Why, just this,” was the reply. “That altar top was counterbalanced. Our weight was on one end of it. In some way, when I twisted that ring, a spring or catch must have been loosened and—and—we’re in the interior of the altar.”

“Can we get out again, do you think?”

“That’s just what we’ve got to find out, and quickly, too, Joe,” was the response. “Got any matches?”

“Yes; luckily I brought some. I’ve got a pocket lantern here, too, with a candle in it. Shall I light up?”

“Yes, do so as soon as you can,” rejoined Nat.

The next minute the interior of the altar was illumined by a yellow light. But so perfectly had the swinging top of the altar been fitted that not a crevice appeared and as for any lever or handle by which it might have been opened, none was revealed by the light.

But it was some minutes before the boys found out this fact. When they did, however, it came with a sense of stunning bitterness. If they could not find a means of egress from the altar, they were, in all human probability, doomed to die in that gloomy prison.

Although they both realized their situation, neither lad voiced his fears. There still remained one end of the altar to be examined, and Nat lost no time in proceeding to investigate the hitherto neglected portion of their prison. But its masonry appeared to be as solidly constructed as was the case in every other part of the altar. Nat, almost in despair, was turning away when Joe, who had been at his side, gave a sudden cry.

“Nat! Nat! There’s a stone loose here. I can move it with my foot. When I press down on it—Great-jumping-horned-toads!”

Joe’s exclamation was caused by the fact that as he pressed down on the loose stone a small door opened out before them in the end of the altar. It was impossible to say, however, whither it led, as beyond lay total darkness.

“What do you say, shall we try it?” asked Joe in a rather tremulous voice, for the darkness looked singularly mysterious and forbidding.

“We’ve got to try it,” said Nat gloomily. “It’s our only alternative, unless we want to stay here and starve to death.”

Joe had to agree that this was a true statement of the facts of the case, and not without quickened pulses the two lads made the plunge into the darkness beyond the door. The portal was square and so low that they had to bend to get through it. The rays of Joe’s candle-lantern showed the two youths that they were in a low-roofed passage, or tunnel, just wide enough for them to proceed in single file.

“You go first,” said Joe in a rather quivery tone, which showed better than anything else that the adventure was having its effect upon him, the usually unperturbed.

“All right, give me the lantern.”

“I wonder where this passage can lead to?”

“Haven’t the least idea. I think we are going south, but I’m not sure.”

“I’m all twisted up, too. I wish we’d left that old ring alone.”

“Maybe I don’t, too. If we ever get out of this place, I’ll leave all such devices severely to themselves in future.”

“Have you any idea of the purpose of this passage?”

“Not I. Maybe it was used as a means of escape. In that case——”

“In that case we will get out to daylight again,” Joe concluded.

“On the other hand, it may have been designed as a means of executing their criminals or enemies. I’ve heard of such things.”

Joe fairly shuddered.

“Oh, talk of something pleasant,” he said, with a groan.

No more was said for a time. The circumstances didn’t make the boys feel much inclined for conversation.

All at once they emerged into a vaulted chamber, seemingly cut out of the living rock. At the top of its arched roof was set a huge crystal, very like the one they had seen in the “telescope tower,” only much larger. Through this lens light was streaming into the place, the walls of which were painted and carved with all manner of strange-looking inscriptions and designs. Nat was so intent on gazing at these that he did not look as carefully where he was going as he had in his progress down the passage.

Suddenly his feet slipped from under him and he found himself falling downward. Joe uttered a cry as he saw his comrade vanish. He leaped forward, checking himself just in time to avoid sharing Nat’s plight. He found himself on the brink of a sort of well about ten feet deep. At the bottom of this was Nat. Joe uttered a cry of relief as Nat hailed him and assured him that, by a miracle, he was not hurt.

“But how are you going to get out of there?” demanded Joe the next instant.

How, indeed? The question certainly was a poser. The walls of the well were as smooth as glass almost and Joe noticed a peculiar feature. From its “curb” radiated long lines extending over the floor of the rocky chamber. These lines were cut in the rock and reminded Joe of lines he had seen cut on a sun dial.

But he gave little thought to this at the moment. His mind was centered on finding a means to get Nat out of his predicament. But, though he thought and thought, no solution of the problem occurred to him.

Joe was still wrapped in thought at the edge of the well when he felt a sudden blast of fearful heat on his back. He looked hastily round. His first thought was that some hidden fire must suddenly have burst into life behind him.

But, no, what he had felt had been the rays of the sun pouring through the crystal at the top of the cavern and striking down with tremendously magnified force upon him.

“Phew! That felt like an oven!” exclaimed Joe, moving away.

It was a moment later that he observed something that filled him with a vague sense of alarm, which swiftly crystallized into a sharp, livid pang of fear.

The sun was now striking down into the well. Like a thunderbolt the purpose of the pit and the reason of the crystal lens burst upon Joe.

The ancient dwellers of the Lost City had been Sun Worshippers. This chamber was a sacrificial one and the priests of the vanished race had offered up their victims’ lives by literally dedicating them to the Sun gods. As this alarming truth broke upon Joe a faint cry came from Nat, down in the pit.

“Joe, for gracious sake, do something to get me out of here! The sun is striking down into the pit. It is fearfully hot. If you don’t get me out soon I’ll be baked alive.”

Poor Joe cast his eyes about him despairingly. The sun was streaming through the lens at an angle now. What would happen when its direct rays poured down into the narrow well he could not bear to think.