held his course.

The alarm bell awakened him, and he pointed the craft down under the great red disk of Big Jupe, toward the low range of purple cliffs indicated on his map.

A few minutes later he was knocking at the door of the dome-shaped laboratory.

Ann North was twice as beautiful in the flesh as she had seemed on the visi-screen. Attired in the modish shorts and tunic that had become universal garb for Earth-women, she looked like a figure from a Grecian frieze. She led him to the library.

"Dad's asleep at last," she said. "I persuaded him to rest for a few hours—on the strength of my argument that he'd accomplish more in the long run if he kept his brain clear."

Roger Kay nodded understandingly. "I just had a bit of sleep myself en route. Nobody at headquarters has slept much the last few days. By the way, I'm woefully in the dark about a lot of things. Will you tell me just what your father's trying to re-discover? If you can enlighten me, I'll not have to ask him so many darn-fool questions."

"You know, of course," said Ann North when they were comfortably seated, "that it's a ray that will explode any explosive at a distance. Or perhaps I shouldn't have said a ray—it's really a sound wave, in the ultra-sonic belt, traveling on a beam. It disrupts any unstable chemical compound."

Roger Kay nodded. "That much I know. I've examined one of the projectors. We've installed them at all the outposts. They're all ready, except—"

"Except for the catalyst. The part of the discovery that's lost in the chemical compound that produces the catalytic gas. The ultra-sonic waves, passing through the gas, change their vibration in some way."

"I see now," said Kay, "why it is directional. The ultra-sonic waves go in all directions, of course, but only those passing through the gas are disruptive. Right?"

The girl nodded her beautiful blond head. "It's all very simple, and it's all in the hands of the government, except for the formula for that catalyst. Fortunately my father has a reputation as a scientist. That's why the government was willing to take a chance on having those projectors set up, even though—"

Roger Kay smiled wryly. "Your father is the outstanding scientist of the System, Miss North. But even if he wasn't, we might have taken 

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