"All fourth-order stuff, Mart," Seaton, who had been frantically busy at his keyboard and instruments, reported to Crane. "Can't find a trace of anything on the fifth or sixth, and that gives us a break. I don't know what we can do yet, but we'll do something, believe me!" "Fourth order? Are you sure?" Crane doubted. "A fourth-order screen would be a zone of force, opaque and impervious to gravitation, whereas those screens are transparent and are not affecting gravity." "Yeah, but they're doing something that we never tried, since we never used fourth-order stuff in fighting. They've both left the gravity band open--it's probably too narrow for them to work through, at least with anything very heavy--and that gives us the edge." "Why? Do you know more about it than they do?" queried Dorothy. "Who and what are they, Dick?" asked Margaret. "Sure I know more about it than they do. I understand the fifth and sixth orders, and you can't get the full benefit of any order until you know all about the next one. Just like mathematics--nobody can really handle trigonometry until after he has had calculus. And as to who they are, the folks in that fort are of course natives of the planet, and they may well be people more or less like us. It's dollars to doughnuts, though, that those vessels are manned by the inhabitants of that interloping planet--that form of life I was telling you about--and it's up to us to pull their corks if we can. There, I'm ready to go, I think. We'll visit the ship first." The visible projection disappeared and, their images now invisible patterns of force, they stood inside the control room of one of the invaders. The air bore the faint, greenish-yellow tinge of chlorin; the walls were banked and tiered with controlling dials, meters, and tubes; and sprawling, lying, standing, or hanging before those controls were denizens of the chlorin planet. No two of them were alike in form. If one of them was using eyes he had eyes everywhere; if hands, hands by the dozen, all differently fingered, sprouted from one, two, or a dozen supple and snaky arms. But the inspection was only momentary. Scarcely had the unseen visitors glanced about the interior when the visibeam was cut off sharply. The peculiar beings had snapped on a full-coverage screen and their vessel, now surrounded by the opaque spherical mirror of a zone of force, was darting upward and away--unaffected by gravity, unable to use any of her weapons, but impervious to any form of matter or to any ether-borne wave. "Huh! 'We didn't come over here to get peeked at,' says they." Seaton snorted. "Amœbic! Must be handy, though, at that, to sprout eyes, arms, ears, and so on whenever and wherever you want to--and when you want to rest, to pull in all such impedimenta and subside into a senseless green blob. Well, we've seen the attackers, now let's see what the natives look like. They can't cut us off without sending their whole works sky-hooting off into space."