"Well," remarked Tom Swift, in mournful tones, "this looks as if we were up against it; doesn't it?"

"Bless my umbrella, it certainly does," agreed Mr. Damon.

"And it's all my fault," said Mr. Jenks. "I shouldn't have gone into the big cave. I might have known those men would come back any time."

The above conversation took place as our friends lay securely bound in a small cave, or recess, opening from the larger cavern, where, about an hour before, they had been captured and made prisoners by the diamond makers. Despite their struggles they had been overpowered and bound, being carried to the cave, where they were laid in a row on some old bags.

"It certainly is a most unpleasant situation, to say the least," observed Mr. Parker.

"And all my fault," repeated Mr. Jenks.

"Oh, no it isn't," declared Tom Swift, quickly. "We were just as ready to follow you into that cave as you were to go. No one could tell that the men would return so soon. It's nobody's fault. It's just our bad luck."

From where he lay, tied hand and foot, the young inventor could look out into the cave where he and the others had been caught. The diamond makers were busily engaged, apparently in getting ready to manufacture another batch of the precious stones. They paid little attention to their captives, save to warn them, when they had first been taken into the little cave, that it was useless to try to escape.

"They needn't have told us that," observed Tom, as he and the others were talking over their situation in low voices. "I don't believe any one could loosen these ropes."

"They certainly are pretty tight," agreed Mr. Damon. "I've been tugging and straining at mine for the last half hour, and all I've succeeded in doing is to make the cords cut into my flesh."

"Better give it up," advised Mr. Jenks.

"We'll just have to wait."

"For what?" the scientist wanted to know.

"To see what they'll do with us. They can't keep us here forever. They'll have to let us go some time." Following their capture, Folwell and Munson, the latter the stowaway of the airship, had been in earnest conversation regarding our friends, but what conclusion they had reached the adventurers could only guess.

"And we didn't have time to examine the diamond-making machinery close enough so that we could duplicate it if necessary," complained Tom, a little later.

"No," agreed Mr. Jenks. "There are certain things about it that are not clear to me. Well, I don't believe I'll have another chance to inspect it. They'll take good care of that, though they seem to be getting ready to make more diamonds."

"Perhaps they're going to manufacture a big batch, and then leave this place," suggested Mr. Damon. "They will probably go to some other secret cave, and leave us here."

"I hope they untie us before they leave, and give us something to eat," remarked the young inventor.

For two hours longer the captives lay there, in most uncomfortable positions. Then Folwell and Munson, leaving the group of diamond makers who were grouped about the machinery, approached the captives.

"Well," remarked Munson, "we got ahead of you after all; didn't we. You thought you had our secret, but it will be a long while before you ever make diamonds."

"What are you going to do with us?" asked Tom.

"Never mind. You came where you had no right to, and you must take the consequences."

"We did have a right to come here!" exclaimed Mr. Jenks. "I am entitled to know how the diamonds are made. I paid for the information, and you tricked me. If ever it's possible I'll have the whole gang arrested for swindling."

"You'll never get the chance!" declared Folwell. "You were given some diamonds for the money you invested, and that makes us square."

"No, it doesn't!" declared Mr. Jenks. "I invested the money to learn how to make diamonds, and you know it! You tricked me, and I had a right to try to discover your secret! I nearly have it, too, and I'll get it completely before I'm done with you!"

"No, you won't!" boasted Folwell. "But we didn't come here to tell you that. We came to give you something to eat. We're not savages and we'll treat you as well as we can in spite of the fact that you are trespassers. We're going to give you some grub, but I warn you that any attempt to escape will mean that some of you will get hurt."

He signalled to some of his confederates. These men unbound the captives' arms, and stood over them while they ate some coarse food that was brought into the small cave. They were given coffee to drink, and then, when the simple meal was over, they were securely bound again, and left to themselves, while the diamond makers went back to their machinery.

It was evident that they were going to attempt a big operation, for an unusually large quantity of the white stuff was prepared. The prisoners watched them idly. They could see some but not all of the operations. In this way several hours passed.

Gloom possessed the hearts of Tom and his friends. Not only had their expedition been almost a failure so far, but the young inventor was worried lest the gang might discover and wreck his airship. This would prove a serious loss. Lying there in the semi-darkness the lad imagined all sorts of unpleasant happenings.

At times he dozed off, as did the others. They had become somewhat used to the pain caused by the bonds, for their nerves were numb from the strain and pressure.

Once, as he was lightly sleeping, Tom was awakened by hearing loud voices in the main cave. He looked out, rolling over slightly to get a better view. He saw the man who, once before had run in to give news of an approaching electrical storm.

"Are you fellows all ready?" asked this same man again.

"Yes. Is there another storm coming?"

"Yes, and it's going to be a corker!" was the reply. "It's one of the worst I've ever seen. It's sweeping right up the valley. It'll be here in an hour."

"That's good. We need a big flash to make all the material we have prepared into diamonds. It's the biggest batch we ever tried. I hope it succeeds, for we're going to leave--" The rest was in so low a tone that Tom could not catch it.

The storm messenger departed. Folwell and Munson busied themselves about the machinery. Tom dozed off again, dimly wondering what had become of Bill Renshaw, and whether the former ghost knew of their plight. The others were asleep, as the young inventor saw by the dim light of a lantern in the cave. Then, he too, shut his eyes.

Tom was suddenly awakened by feeling some one's hands moving about his clothing. At first he thought it was one of the diamond-making gang, who had sneaked in to rob him. "Here! What are you up to?" exclaimed Tom.

"Quiet!" cautioned a voice. "Are you all here?"

"All of us--yes. But who are you?"

"Easy--keep quiet, Tom Swift! I'm Bill Renshaw! I've been searching all over for you, since I got back to your cave and found it empty. Now I'm going to free you. I got in here by a secret entrance. Wait, I'll cut your ropes." There was a slight sound, and an instant later Tom was freed from his bonds.