Teddy threw himself out of the machine and rushed up the steps. Evelyn opened the door before he could ring, and his beaming face told her the news he had to give even without his enthusiastic, "It worked!"

"The steam plume has stopped?" asked the professor anxiously.

"Absolutely," said Teddy cheerfully. "Not a sign of steam except from two or three puddles of hot water that were cooling off when we left to get back to the fort. The commandant was setting his men to work with the navy-yard men when I started here."

"Tell me about this, won't you?" said the reporter briskly. "I'll catch the devil from the city editor for missing out on that part of it, but if you'll give me the full story——"

"What's your paper?"

The reporter told him.

"That's all right," said Teddy easily. "They were calling extras of that paper as I came uptown. The professor has told you the theory of the thing?"

"No," said Evelyn. "He was starting to, but the black flyer appeared and shot down the other aËroplane, and father was so much upset that he couldn't go into details. Was the pilot of the aËroplane killed?"

Teddy nodded.

"Frozen, poor chap. He never knew what struck him."

"What did happen?" asked the reporter again. "You people seem to take this so much as a matter of course, and no one else can do anything but guess."

"The professor knows more about low temperatures than any other man in the world," explained Teddy. "It's only natural that he should be fairly certain of his facts."

He smiled at the professor as the old man made a deprecating gesture.

"Father is much upset," said Evelyn. "I think it would be best if Teddy explained. Will that be all right?"

"Only, in your account of the matter," said Teddy decidedly, "the professor must be given credit for the whole thing. It's his work, and he's entitled to it."

"No, no," protested the professor. "Teddy did a great deal."

Evelyn pressed his arm, and he obediently was quiet. The two young people smiled at him.

"You see how I am ruled," said the professor in mock tragedy. "My daughter——"

"Is going to see that you rest a while," said Evelyn, with a twinkle in her eyes. "Teddy, you go and explain the whole thing while I take father out and discipline him."

With a laugh, she led the old man away. Teddy smiled.

"We aren't accustomed to reporters," he said, "or I suspect we'd act differently. Miss Hawkins is a most capable physicist, and helps her father immensely. The three of us work together so much that——Well, come along to the laboratory."

The two went to the rear of the house. On the way they passed through a long room full of glass cabinets in which odd bits of metal work glittered brightly.

"The professor's hobby," said Teddy, with a nod toward the cases. "Antique jewelry and ancient metal work. He's probably better informed on low temperatures than any one else I know of, but I really believe he's as much of an authority on that, too. This is Phoenician, and that's early Greek. These are Egyptian in this case. This way."

He opened a small door and they were in the laboratory.

"I'm afraid I'll have to lecture a bit," said Teddy. "Here's how the professor used to work out what was taking place out in the harbor."

He showed an intricate combination of silvered globes, tubes, and half a dozen thermometers.

"You see," Teddy began, "the water in the harbor was at a certain temperature. At this time of the year it would be around 52° Fahrenheit. The professor knew that fact, and then the fact that a huge mass of it was turned into ice. When you turn water into ice you have to take a lot of heat out of it, and that heat has to go somewhere. When water freezes normally in winter that heat goes into the air, which is cold. In this case the air was considerably warmer than the ice, and was as a matter of fact, undoubtedly radiating heat into the ice, instead of taking it away. The heat that would have to be taken from say ten pounds of water at 52° to make it freeze, if put into another smaller quantity of water would turn the smaller quantity of water into steam. You see?"

"The steam plume!" exclaimed the reporter.

"Of course," said Teddy. "We measure heat by calories usually. That's the amount of heat required to raise a pound of water one degree Fahrenheit. Suppose you have a mass of water. To make it freeze you have to take twenty thousand calories of heat out of it. Suppose you take that heat out. You've got to do something with it. Suppose you put it into another smaller mass of water. It will make that second mass of water hot, so hot that it will turn into steam at a high temperature."

"Then Varrhus," said the reporter thoughtfully, "was taking the heat from a big bunch of water and putting it into a small bunch, and the small bunch went up in steam. Is that right?"

"Precisely." Teddy turned to a file on which hung a number of sheets of paper covered with figures. "Here are the professor's calculations. We could only figure approximately, but we knew the size and depth of the ice cake, very nearly the temperature of the water that had been frozen, and naturally it was not hard to estimate the number of calories that had had to be taken out of the harbor water to make the ice cake. To check up, we figured out how much water that number of calories would turn into steam. The professor appealed to the government scientists who had watched the cake from the first. He found that from the size of the plume and the other means of checking its volume, he had come within ten per cent of calculating the amount of water that had actually poured out in the shape of steam."

"But—but that's amazing!" said the reporter.

"It was good work," Teddy said in some satisfaction. "Then we knew what Varrhus had done, and it remained to find out how he'd done it. Nothing like that had ever happened before. He couldn't very well have an engine working there in the water. The professor took to his mathematics again. Assume that I have a stove here that will make it just so warm at a distance of five feet. I'm leaving warm air out of consideration now and only thinking of radiated heat. If I put my thermometer ten feet away how much heat will I get?"

"Half as much?" asked the reporter.

"One-quarter as much," said Teddy. "Or three times away I'll get one-ninth as much, or four times away I'll get one-sixteenth as much. You see? If I want to make the ends of an iron bar hot, and I can only heat the middle, the middle has to be red-hot or white-hot to make the ends even warm. If I have to make the middle of a bar red-hot to have the ends warm, you see in order to make the ends cold the middle would have to be very cold indeed."

"Y-yes, I understand."

"Well, the professor worked on that principle. He knew the temperature of the edges, and he knew the size of the ice cake. It was easy to figure what the temperature must be in the middle. It worked out to within two degrees of absolute zero!"

"What's that?"

"There isn't any limit to high temperatures. You can go up two thousand degrees, three thousand, four, or five. Some things almost certainly produce a temperature of as much as eight thousand degrees. But high temperatures are produced by putting more heat in—by stuffing the thing with calories. I make an iron bar red-hot by putting calories in. I make it cold by taking calories out."

"Well?"

"If you keep that up you reach the point where there aren't any more calories left to take out. When you get to that point you have a temperature of 425° Centigrade, or one thousand and seventy-eight degrees Fahrenheit below zero. That's absolute zero."

Teddy spoke quite casually, but the reporter blinked.

"Rather chilly, then."

"Rather," Teddy agreed. "But our calculations told us that Varrhus had reached and was using a temperature within two degrees of that in the center of his ice cake. And right next to that temperature he had a very high one, as evidenced by the plume of steam."

"I can't see how you got anywhere," said the reporter hopelessly. "I'm all mixed up."

"It's very simple," said Teddy cheerfully. "On one side of a wall the man had what amounted to a thousand and some odd degrees below zero. On the other he had probably as much above zero. Evelyn—Miss Hawkins, you know—made the suggestion that solved the problem. She showed us this."

Teddy picked up what seemed to be a square bit of opaque glass.

"Smoked glass?"

"Yes, and no." Teddy smiled. "You can't see through it, can you?"

"No."

"Come around to this side and look."

The reporter made an exclamation of astonishment.

"It's clear glass!"

"It's a piece of glass on which a thin film of platinum has been deposited. It lets light through in one direction, but not in the other. Evelyn suggested that Varrhus had something which did the same thing with heat. It would let heat through in one direction, but not in the other. Of course if it would take all the heat from the air on one side and wouldn't let any come back from the other——"

"It would be cold?"

"On one side. The glass looks black because it lets the light go through and lets none come back. The surface, we have assumed, would be almost infinitely cold because it would let heat go through and would let none come back. We decided that Varrhus had made a hollow bomb of some shape or other, composed of this hypothetical material. Heat from the outside would be radiated into the interior because the surface absorbed heat like this glass absorbs light. It would act as a surface at more than a thousand below zero. Because something had to be done with the heat that would come in, Varrhus made the bomb hollow and left two openings in it. The inside of the bomb is intensely hot from the heat that has been taken out of the surrounding water. The hole at the bottom radiates a beam of heat straight downward which melts a very small quantity of ice and lets the water flow into the bomb, where it is turned into steam. Naturally, it flows out of the other hole at the top. There you have the whole thing."

"And you stopped it——"

"By dropping a T. N. T. bomb down the steam shaft. It went off and blew the cold bomb to bits. The iceberg will break up and melt now."

The reporter stood up.

"I'd like to thank you for this, but it's too big," he said feverishly. "Man, just wait till I wave this before the city editor's eyes!" He rushed out of the house.

The newspapers that afternoon had frantic headlines announcing the destruction of the steam plume and the fact that noticeable signs of melting had begun to show themselves on the ice cake. Smaller captions told of the dynamiting that had begun and of the destruction of the Yokohama and Folkestone bergs by soldiers acting on cabled instructions. The Straits of Gibraltar were cleared by salvos fired from the heavy guns on the Rock at the three great plumes of steam. The world congratulated itself on the speedy nullification of the menace to its democratic governments. It did not neglect, however, to rush detachments of men with trench mortars and hand bombs to its reservoirs, prepared to destroy any possible cold bombs on their first appearance. The aviation forces, too, made themselves ready to fight the black flyer on its next appearance, despite the mysterious means by which it had killed the American pilot.

This state of affairs lasted for possibly a week, when, within three hours of each other, the papers found two occasions to issue extras. The first extra announced the death by heart failure of Professor Hawkins, who had been found by his daughter, dead in his laboratory, holding in his hands an antique silver bracelet he had just opened at the clasp. The second, three hours later, announced the formation of an ice cake in the Narrows which grew in size even more rapidly than the original one, and was entirely unattended by the steam plume which gave Teddy Gerrod an opportunity to destroy the first. Within three hours the Narrows were closed, and the ice floe was creeping up toward New York.

In rapid succession came the news that Norfolk harbor was frozen over and Hampton Roads closed, that Charleston was blocked, then Jacksonville. The next morning delayed cablegrams declared that the Panama Canal was a mass of ice, and almost simultaneously the Straits of Gibraltar were again admitted to be firmly locked.