Corvo North shook his gray head. "The theory is sound; it should work. But three days! Man, we're working against a deadly deadline!" He grabbed a pad and pencil. "Here, I'll show you what to do and you can start on the headpiece that connects to the machine here."

And thus started the busiest, dizziest hours of Roger Kay's life. Sleep was a chimera that haunted every leaden-eyed hour, a mirage that beckoned and pleaded in vain.

And the hands of the laboratory clock crept inexorably onward. At three in the morning on Friday, Terran time, with nine hours left before the invaders would strike, Kay staggered to the televis and dialed Wargan.

"I think we'll finish in time," he reported. "We'll be ready for the first test in a couple of hours. Have you made the preparations we suggested?"

The S.B.I. chief nodded. "At the base of each projector we've installed practically a chemical warehouse. There is at least a small quantity of every available known chemical. And expert chemists waiting at each."

"Good. Then within fifteen minutes after I send you the formula, the projectors can be in operation?"

"Ten minutes, unless the formula is more complex than you believe. You say that Corvo North believes there are but six or seven ingredients?"

Roger Kay nodded wearily. "And the communications?"

"Open constantly. An operator on duty at each projector at all times. Test messages going through every fifteen minutes. Incidentally, latest reports still confirm early ones. The deadline is still noon today."

Roger Kay saluted, then snapped the switch. Back to work at the little box in the laboratory.

During those last hours, as well as the ones preceding them, Ann North had been a ministering angel. Sleeping almost as little as the two men, she was ever ready with encouragement—and hot coffee. At times, almost by force, she would pry one or the other of them away from their work for a brief period of rest.

On her own initiative she had called in Dr. Dane. Once he understood the situation, the doctor was invaluable. He took no part in the work on the machine, but he watched over Corvo North constantly and kept him at the highest point of efficiency under the circumstances.

Ten o'clock came—and 

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