Teddy was thrown down by the concussion, and fell in a heap against the commandant. He leaped to his feet and rushed to the window, from which the glass had disappeared. He saw the remnants of the sheet of flame dying away and saw that the low-lying cloud of mist had been blown from the surface of the ice. A gaping orifice, five hundred feet across, showed itself where Teddy and the lieutenant had been working. Of the lieutenant and his men no trace could be seen. Two or three of the little red flags that had marked the path through the mist still remained, however, and a small sledge was lying, overturned, beside the sledge route. Four tiny black figures lay in twisted attitudes beside the sledge. As Teddy looked one of them began to struggle feebly.

Teddy stared, speechless. For a moment he was dazed by the suddenness and the overwhelming nature of the calamity that had befallen the young lieutenant and his detachment. Only accident had saved him from a similar fate. Then his professional instinct re-asserted itself, and he began to piece together what he knew of the bomb. In a moment the solution came to him.

"Varrhus planned this," he said unsteadily. "He filled up his hollow cold bombs with solid iron. The heat that would come in would first melt and then vaporize the interior until the pressure inside was more than the still-solid crust could stand. And all that vaporized iron would burst out. What a fiend that man must be!"

An hour later, baffled and discouraged, he was sitting in the laboratory with his head in his hands, trying desperately to grapple with this new problem. The new cold bombs apparently could not be assailed without destruction of those who attacked them. It was impossible to imagine that volunteers could be found to sacrifice their lives to destroy each new bomb as it was placed. The horror of being annihilated by a blast of metallic vapor would deter men who would not hesitate to face death in a less terrible form. And Varrhus was evidently able to place them again nearly as fast as they were blown up. Telegrams announcing the explosion of the Jacksonville and Charleston ice floes lay before Teddy, supplemented by a cablegram from Panama saying that the Miraflores Locks had been destroyed by the blast when the Panama cold bomb had burst. Teddy was nearly certain that the next morning would find the exploded bombs replaced. Varrhus' black flyer was evidently capable of carrying a great weight at an immense speed. It also seemed able to reach an almost incredible height, from the fact that the second cold bomb had been dropped in the Narrows in broad daylight without the flyer having been sighted.

Evelyn turned from the instruments with which she had been working. She had scraped off a small bit of the lacquerlike surface of the silver bracelet, and had been analyzing it in the hope of finding what element or combination had been used to produce the mystifying heat-inductive effect.

"Teddy," she said depressedly, "I can't find a thing. The lacquer effect seems to be simply the appearance of some way he has treated the metal. The surface gives just the same analysis as the filings from the inside of the metal. I took a spectro photo and it gives silver lines with a trace of lead. Analysis by arsenic reduction gives the same result."

"Perhaps those detectives will be able to trace Varrhus by the mailing box they took," said Teddy, without much hope. "It's not very likely, though. We've got to think of something!"

Silence fell in the laboratory again, broken only by the faint whistling sound of the flame Evelyn had used in her analytical work.

"The trouble is," said Teddy grimly, "that we've been trailing Varrhus, instead of anticipating him. If we could know where he was going to be——"

"He'll have to show up sooner or later," Evelyn commented. "We know, for instance, that he'll have to replace that bomb in the Narrows or let the harbor stay open. The use of these new explosive bombs means that he has to expose himself more than he'd have to with the old ones."

"There ought to be an aËrial patrol above the city——"

Teddy stood up sluggishly, discouragement in every line of his figure. A servant tapped on the door of the laboratory.

"Lieutenant Davis, of the military flying corps, sir."

"Show him in," said Teddy listlessly.

A slim young officer came in. His friendly, boyish face was full of a whimsical humor.

"This is rather an intrusion, I'm afraid," he said half apologetically, "but I thought you might be able to help me out."

"I've done nothing so far," said Teddy in a rather discouraged tone. "Miss Hawkins and I were just canvassing the situation. You're talking about the iceberg and Varrhus, aren't you?"

"Of course. No one talks about anything else nowadays. My taxi had a tough time getting through the crowds on the streets. They don't understand about the explosion in the Narrows yet."

Teddy introduced him to Evelyn.

"Pleasure, I'm sure," said Davis with a smile. Then his face sobered. "That was rotten hard luck about your father, Miss Hawkins. I'm not good at making speeches, but I hope you realize that every one is sympathizing with you and in a measure sharing your sorrow."

Evelyn shook hands.

"I will allow myself to grieve when Varrhus has been disposed of," she said quietly. "Until then I dare not let myself think."

Davis released her hand and turned to Teddy.

"Varrhus—or the chap in the black flyer, anyway—killed my best friend, Curtiss. He was driving the little Nieuport that attacked Varrhus the day you blew up the first bomb. I was the first man to reach the spot where Curtiss had crashed, and I swore I'd get Varrhus for that."

"I remember," said Teddy. "Frozen."

Davis nodded, his face grave.

"I have what is probably the fastest little machine in the United States, at the fort. A two-seater, with twin Liberty Motors that shoot her up to a hundred and fifty miles an hour without any trouble at all. I think I can get Varrhus with it. I came to you to learn what you think about Varrhus' weapons. It's only the part of wisdom to learn all you can about your opponent, you know."

Teddy found the young man impressing him very favorably.

"I haven't given the matter much thought," he confessed, "but you remember Varrhus' tactics?"

"He dropped like a tumbler pigeon," said Davis, "and Curtiss overshot him. There wasn't a sign of firing except from Curtiss. He simply overran the place where Varrhus had been three or four seconds before and then dropped. He was frozen stiff when I found him."

"I think," said Teddy carefully, "that Varrhus had shot up a jet of some liquified gas, probably hydrogen. It hung suspended in the air for a moment, and in that moment the biplane ran into it. A drop of liquid hydrogen placed in the palm of your hand would freeze your arm solidly up well past the elbow. It's something over five hundred degrees below zero. Your friend ran into what amounted to a shower of it."

Davis considered:

"Cheerful thing to fight against, isn't it?" he asked, with a smile. "Tactics, mustn't run above the black flyer and mustn't run below it. He can probably shoot it straight down, too."

"And almost certainly from the sides," said Teddy. "The man must have been working on this thing for years, and even if he's insane he'd be a fool not to make his weapon as efficient as possible."

Davis' expression became rueful.

"And so I'm supposed to keep my distance," he remarked, "and take pot shots at him while dancing merrily around in mid-air. Can't we do anything about that stuff to nullify it?"

"Burn it," suggested Evelyn. "Liquid hydrogen burns just as readily as the same gas at normal temperatures."

The three of them were silent for a moment.

"Would rockets set it afire?" asked Davis presently. "I could keep a stream of fire balls shooting out before my machine."

"They ought to." Teddy was losing his discouragement in this new prospect of coming to grips with Varrhus. "I say, will your machine burn readily?"

"Only the gas tank. The wings and struts are fireproof. New process."

Davis stood up suddenly.

"Would it bother you to come over and look at my machine? We could probably figure out the thing better then."

Teddy rose almost enthusiastically.

"We'll go over now if you say so."

The taxicab bearing Teddy and the young aviator down to the fort was forced to travel slowly amid the throngs of apprehensive people that overflowed the sidewalks and made the streets almost impassable. The launch took them swiftly to the fort, and in a few moments they had arrived at the small aviation field behind the fortifications on Staten Island. Davis led Teddy directly to the shed that contained the swift machine of which he was so proud. It was a splendid product of the aircraft maker's art. Twin Liberty Motors developed nearly eight hundred horse power between them, and two great shining propellers pulled the machine through the air with irresistible force.

"You see," said Davis, with some enthusiasm, "the motors aren't in the fusilage, so the gunner sits up here in the bow and can fire freely in any direction. The one-man planes with synchronized machine guns firing through the propeller aren't in it with these for real fighting. They're splendid little machines—I drove one in France—but I honestly believe this is better than they are. This one responds to the controls every bit as readily, and with a good gunner——"

"Machine gunner in France myself," said Teddy, touching his breast. "Would you take a chance on letting me sit up front to-night?"

"To-night?" asked Davis.

"I believe Varrhus will appear to drop another cold bomb to-night. It will probably be dropped inside the harbor so the ice cake will touch the Battery. That will set the people frantic, and make them beg the government to enter into a parley with Varrhus. It's paid no official attention to him so far, you know."

Davis' expression became keen and rather stern.

"We've four hours before dark. We'll have to set to work."

Teddy went over and stepped up the ladder that leaned against the cockpit.

"I want to see your gasoline supply," he remarked. In a moment he came down, looking a trifle dubious. "If I'm right about Varrhus using liquid hydrogen for a weapon, and we can set it afire, we'll dive through half a dozen sheets of flame to-night. Something will have to be done to protect that gas tank from catching fire, and some protection for the carburetors, too."

"We'll fix that in a hurry," said Davis briskly. "Oh, Simpson! Come here!"

In twenty minutes there were half a dozen mechanicians at work, and Teddy was carefully inspecting the machine gun at the bow of the fusilage.

Teddy telephoned back to Evelyn what he anticipated would occur that night and his own share in it.

"Of course there's some risk in it," he finished, "but I guess we'll come out."

Evelyn's voice was more anxious than Teddy had expected.

"Do be careful, Teddy," she said in a worried tone. "Please be very careful. Varrhus has so many fiendish weapons. I'm terribly afraid."

Teddy's voice was grim.

"With the kind assistance of the German government," he remarked, "we have a few fiendish inventions, too. I'm using explosive bullets only to-night. Varrhus is outlawed."

Evelyn spoke almost faintly.

"But take good care of yourself, please, Teddy," she urged. "It were better that Varrhus got away this once than that you should be killed for nothing."

Teddy smiled. "I've no intention of being killed, Evelyn, but I have some intention that Varrhus shall be."

There was a curious sound from the other end of the wire.

"But—but——" Evelyn's voice died away. "I'm—I'm going to be praying, Teddy. Good-by."

The last was very faint. Teddy turned from the instrument and went out to where the aËroplane had been rolled from its shed. The sun was sinking and dusk was falling. Time passed and darkness settled down upon the earth. Stars twinkled into being. A long searchlight poked a tentative finger of light into the sky.

"We'd better be going," said Davis thoughtfully. "We want to be well up before he appears."

Teddy clambered up to his seat and adjusted the straps that would hold him in place. He pulled down the helmet and fitted the telephone receivers securely over his ears. A telephone was necessary for communication with Davis, four feet behind him, because of the tremendous roar of the engines. He took the machine-gun butt and found the trigger, then made sure the first of a belt of cartridges was in place. He settled back in his seat as the mechanics began to twirl the propellers. He was going out to fight the black flyer, but most incongruously he was not thinking of Varrhus at all. His thoughts dwelt with strange intensity upon Evelyn.