"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: