.

that there were shipwrecked people in these caves. How could they help knowing it, if they had killed Davis and the others? But, cowardly hounds as they were, they had been afraid to attack the place until they knew how many people were in it, what arms they had, and in what way the place could best be assailed. This Mok had found out everything. If the boy could swim across the lake, that black man could do it, and he had gone out through the cleft, and was probably now making his report to the gang.

      All this flashed through the captain’s brain in a few seconds. He set his teeth together. He was ashamed that he had allowed himself to be so tricked. That African, probably one of the gang, and able to speak English, should have been kept a prisoner. What a fool he had been to treat the black-hearted and black-bodied wretch as one of themselves, and actually to put him on guard!

      Of course, it was of no use to go to look for him, and the captain had put down his gun, and was just about to turn to speak to the others, when Maka seized him by the coat. The negro seemed wildly excited and still unable to speak. But it was plain that he wanted the captain to follow him along the passage. There was no use in asking questions, and the captain followed, and behind him came Ralph, Edna, and Mrs. Cliff.

      Maka was about to climb over the rocky partition which divided the passage, but the captain stopped him. “Stay here,” said he, “and watch the passage. I will see what is the matter over there.” And then he and Ralph jumped over and hurried to the lake. As they came out on the little platform of rock, on which the evening light, coming through the great; cleft, still rendered objects visible, they saw Mok crouching on his heels, his eyes wide open as usual.

      The captain was stupefied. That African not gone! If it were not he, who had gone?

      Then the captain felt a tight clutch upon his arm, and Ralph pulled him around. Casting eyes outward, the captain saw that it was the lake that had gone!

      As he and Ralph stood there, stupefied and staring, they saw, by the dim light which came through the opening on the other side of the cavern, a great empty rocky basin. The bottom of this, some fifteen or twenty feet below them, wet and shining, with pools of water here and there, was plainly visible in the space between them and the open cleft, but farther on all was dark. There was every reason to suppose, however, that all the water had gone from the lake. Why or how this had happened, they did not even ask themselves. They simply stood and stared.

      In a few minutes they were joined by Edna, who had become so anxious at their absence and silence that she had clambered over the wall, and came running to them. By the time she reached them it was much darker than when they had arrived, but she could see that the lake had gone. That was enough.

      “What do you suppose it means?” she said presently. “Are we over some awful subterranean cavern in which things sink out of sight in an instant?”

      “It is absolutely unaccountable,” said the captain. “But we must go back to Mrs. Cliff. I hear her calling. And if Maka has come to his senses, perhaps he can tell us something.”

      But Maka had very little to tell. To the captain’s questions he could only say that a little while before, Mok had come running to him, and told him that, being thirsty, he had gone down to the edge of the lake to get a drink, and found that there was no water, only a great hole, and then he had run to tell Maka, and when Maka had gone back with him, so greatly surprised that he had deserted his post without thinking about it, he found that what Mok had said was true, and that there was nothing there but a great black hole. Mok must have been asleep when the water went away, but it was gone, and that was all he knew about it.

      There was something so weird and mysterious about this absolute and sudden disappearance of this great body of water that Mrs. Cliff became very nervous and frightened.

      “This is a temple of the devil,” she said, “and that is his face outside. You do not know what may happen next. This rocky floor on which we stand may give way, and we may all go down into unknown depths. I can’t think of staying here another minute. It is dark now. Let us slip away down to the beach, and take the boat, and row away from this horrible region where human devils and every other kind seem to own the country.”

      “Oh, no,” said the captain, “we can’t consider such wild schemes as that. I have been thinking that perhaps there may be some sort of a tide in this lake, and in the morning we may find the water just as it was. And, at any rate, it has not entirely deserted us, for in these pools at the bottom we can find water enough for us to drink.”

      “I suppose I would not mind such things so much,” said Mrs. Cliff, “if they happened out of doors. But being shut up in this cave with magical lakes, and expecting every minute to see a lot of bloodthirsty pirates bursting in upon us, is enough to shake the nerves of anybody.”

      “Captain,” said Ralph, “I suppose you will not now object to letting me go in the morning to explore that opening. I can walk across the bottom of the lake without any danger, you know.”

      “Don’t you try to do anything of the kind,” said the captain, “without my permission.”

      “No, indeed!” exclaimed Mrs. Cliff. “Supposing the water were to suddenly rise just as you were half-way across. Now that I think of it, there are springs and bodies of water which rise and fall this way, some of them in our own Western country, but none of them are as large as this. What if it should rise in the night and flood the cave while we are asleep?”

      “Why, dear Mrs. Cliff,” said Edna, “I am not afraid of the water’s rising or of the earth’s sinking. Don’t let us frighten ourselves with imaginations like that. Perhaps there may not even be any real thing to be afraid of, but if there should be, let us keep courage for that.”

      The disappearance of the lake gave the captain an uneasiness of which the others had not thought. He saw it would be comparatively easy for the Rackbirds to gain access to the place through the cleft in the eastern wall of the lake cavern. If they should discover that aperture, the cavern might be attacked from the rear and the front at the same time, and then the captain feared his guns would not much avail.

      Of course, during the darkness which would soon prevail there was no reason to expect a rear attack, and the captain satisfied himself with leaving Mok at his former post, with instructions to give the alarm if he heard the slightest sound, and put Maka, as before, in the outer passage. As for himself, he took an early nap in the evening, because at the very first break of dawn it would be necessary for him to be on the alert.

      He did not know how much he had depended upon the lake as a barrier of defence, but now that it had gone, he felt that the dangers which threatened them from the Rackbirds were doubled.

      CHAPTER VIII. THE ALARM

      It was still dark when the captain woke, and he struck a match to look at his watch. It was three o’clock.

      “Is that you, captain?” said a voice from the next room. “Is it time for you to begin watch again?”

      “Yes,” said the captain, “it is about time. How do you happen to be awake, Miss Markham? Ralph! I believe the boy is snoring.”

      “Of course he is,” said Edna, speaking in a low voice. “We cannot expect such a boy to keep awake, and so I have been on watch. It was easy enough for me to keep my eyes open.”

      “It is too bad,” said the captain, and then, listening for a moment, he said: “I truly believe that Maka is snoring, too, and as for that black fellow over there, I suspect that he sleeps all the time. Miss Markham, you have been the only person awake.”

      “Why shouldn’t I be?” said she. “I am sure that a woman is just as good as a man for keeping watch.”

      “If they should come,” thought the captain, as he again sat in the dark, “I must not try to fight them in the passage. That would have been my best chance, but now some of them might pick me off from behind. No, I must fight them in this chamber. I can put everybody else in the middle apartment. Perhaps before to-morrow night it might be well to bring some of those loose rocks here and build a barricade. I wish I had thought of that before.”