Then he sent the submarine forward, and, through the teleview, examined what his four weapons had done.

Huge chunks of rock had been tumbled down, completely closing the tunnel.

"Well," said Graham, "it's over! Finished! They'll never get through that!"

A full-throated cheer burst from the men below, a cheer that rang for minutes as they realized they were free forever of the octopi, of the cold underwater city, of the clutching tentacles. Graham grinned broadly.

"Sound happy--eh?" he chuckled. "Say, Keith, it's good we've got those two octopi our fighting cook killed. Knapp would never believe our story without them!"

He stared curiously at his commander. Wells was standing quite still, facing the teleview screen. A strange, far-away look was in his eyes.

"What's the matter, old man?" the first officer asked, smiling straight at him. "Aren't you glad we won through?"

"Of course," answered Keith with a tired smile in return.

"But why did you look that way?" Graham persisted. And Keith Wells told him:

"I was just wondering if Hemmy told the truth."

The Black Lamp

_By Captain S. P. Meek_

[Ill.u.s.tration: _"Look out!" He leaped to one side as he spoke._]

[Sidenote: Dr. Bird and his friend Carnes unravel another criminal web of scientific mystery.]

"The clue, Carnes," said Dr. Bird slowly, "lies in those windows."

Operative Carnes of the United States Secret Service shook his head before he glanced at the windows of the famous scientist's private laboratory on the top floor of the Bureau of Standards.

"I usually defer to your knowledge, Doctor," he said, "but this time I think you are off on the wrong foot. If the thieves came in through the windows, what was their object in cutting that hole through the roof? The marks are very plain and they indicate that the hole was cut in some manner from the inside."

Dr. Bird smiled enigmatically.

"That is too evident for discussion," he replied. "I grant you that the thieves entered from the roof through that hole. After they had secured their booty they left by the same route. I presume that you have noticed the marks on the roof where an aircraft of some sort, probably a helicopter, landed and took off. A question of much greater moment is that of what they did before they landed and cut the hole."

"I don't follow your reasoning, Doctor."

"Carnes, that hole was cut through the roof with a heavy saw. In cutting it, the workers dislodged quite a little plaster which fell to the floor and must have made a great deal of noise. Why wasn't that noise heard?"

"It was heard. The watchman heard it, but knew that Lieutenant Breslau was working here and he thought that he made the noise."

"Surely, but why didn't Breslau hear it?"

"How do we know that he didn't? He was taken to Walter Reed Hospital this morning with his mind an absolute blank and with his tongue paralyzed. He must have seen the thieves and they treated him in some way to ensure his silence. When he is able to talk, if he ever is, he'll probably give us a good description of them."

Dr. Bird shook his head.

"Too thin, Carney, old dear," he said. "Breslau is a very intelligent young man. He was perfectly normal when I left him shortly after midnight last night. He was working alone in here on a device of the utmost military importance. On the desk is a push b.u.t.ton which sets ringing a dozen gongs in the building. Surely a man of that type would have had sense enough when he heard and saw intruders cutting a hole through the roof to sound an alarm which would have brought every watchman on the grounds to his a.s.sistance. He must have been knocked out before the hole was started, probably before the helicopter's landing."

"How? Gas of some sort?"

"The windows were all closed and locked and I have already ascertained that the gas and water lines have not been tampered with. Gas won't penetrate through a solid roof in sufficient concentration to knock out a man like that. It was something more subtle than gas."

"What was it?"

"I don't know yet. The clue to what it was lies, as I told you, in those windows."

Carnes moved over and surveyed the windows closely.

"I see nothing unusual about them except that they need washing rather badly."

"They were washed last Friday, but they do look rather dirty, don't they? Suppose you take a rag and some scouring soap and clean up a pane."

The detective took the proffered articles and started his task. He wet a pane of gla.s.s, rubbed up a thick lather of scouring soap and applied it and rubbed vigorously. With clear water he washed the gla.s.s and then gave an exclamation of astonishment and examined it more closely.

"That isn't dirt, Doctor," he cried. "The gla.s.s seems to be fogged."

Dr. Bird chuckled.

"So it seems," he admitted. "Now look at the rest of the gla.s.s around the laboratory."

Carnes looked around and then walked to a table littered with apparatus and examined a dozen pieces carefully.

"It's all fogged in exactly the same way, Doctor," he said. "The only piece of clear gla.s.s in the room is that piece of plate gla.s.s on your desk."

Dr. Bird picked up a hammer and struck the plate on his desk a sharp blow. Carnes ducked instinctively, but the hammer rebounded harmlessly from the plate.

"That isn't gla.s.s, Carnes," said the doctor. "That plate is made of vitrilene, a new product which I have developed. It looks like gla.s.s, but it has entirely different properties. It is of enormous strength and is quite insensitive to shock. It has one most peculiar property.

While ultra-violet and longer rays will penetrate it quite readily, it is a perfect screen for X-rays and other rays of shorter wave length.

It appears to be the only piece of transparent substance in my laboratory which has not been fogged, as you call it."

"Do short waves fog gla.s.s, Doctor?"

"Not so far as I know at present, but you must remember that very little work has been done with the short wave-lengths. In the vast range of waves whose lengths lie between zero and that of the X-ray, only a few points have been investigated and definitely plotted. There may be in that range a wave-length which will fog gla.s.s."

"Then your theory is that some sort of a ray machine was put in operation before the helicopter landed?"