the rocket field, its boundaries invisible in the darkness.

"We're in," breathed Kane, "and there's the ship!"

Out on the field, far from all structures, it towered upright, its blunt nose five hundred feet above the blackened earth. Even though no light shone on or from it, they could recognize its lines as those of the vessel whose stolen plans they had gone over point by point—Siegfried, the dust ship.

"They must have raised it to launching position only tonight," said Kane harshly. "Otherwise we could have seen it from across the river. So—it must be loaded and ready to go!"

A thousand feet of open and empty field separated them from the space ship. With straining eyes they could see tiny human figures scurrying about its base in the moonlight, forming a protective circle. Then floodlights went on all over the field and left not a shadow anywhere. The Germans knew, or feared, that the invisible attackers had slipped inside their citadel.

The rain of steel on the gateway had stopped; instead came dull thudding concussions, and a creeping haze obscured the entrance. Gas.

They had prepared for that. But now a more formidable threat made itself known; from near the buildings came a frenzied barking of dogs.

"We've got to get across the open," snapped Kane. "Better stick together and run for it. If we can get among that gang around the ship—neither dogs nor instruments can tell the difference between visible and invisible men!"

They rose from their cover and pelted grimly across the endless-seeming field. To the right, parties of men with dogs were fanning out, too slowly to intercept the raiders. But they were only halfway to the ship when the lights suddenly snapped off—for a moment they stumbled, blinded, in darkness, and the lights flashed dazzlingly on again. A couple of seconds later the puzzling action was repeated—

And from the cordon about the ship, so near now, a voice screamed hysterically, "Da lauft einer!" On its heels came a thunder of shooting, and bullets snapped past the hurrying Americans.

They flung themselves flat on the scorched ground. The lights flashed again and again as if an insane hand were at the master switch. "What's happened?" gasped Manning. "Did they see us?"

"They've learned or guessed one of our 

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