He fell silent, driving the viewpoint hither and thither, like a hound in quest of a scent. "I thought so!" Another ruined city lay beneath them; a city whose buildings, works, and streets had been fused together into one vast agglomerate of glaringly glassy slag, through which could be seen unmelted fragments of strangely designed structural members. "Those ruins are fresh--that was done with a heat ray, Mart. But who did it, and why? I've got a hunch--wonder if we're too late--if they've killed them all off already?" Hard-faced now and grim, Seaton combed the continent, finding at last what he sought. "Ah, I thought so!" he exclaimed, his voice low but deadly. "I'll bet my shirt that the chlorins are wiping out the civilization of that planet--probably people more or less like us. What d'you say, folks--do we declare ourselves in on this, or not?" "I'll tell the cockeyed world--I believe that we should--By all means--" came simultaneously from Dorothy, Margaret, and Crane. "I knew you'd back me up. Humanity _über alles_--_homo sapiens_ against all the vermin of the universe! Let's go, _Two_--do your stuff!" * * * * * As _Two_ hurtled toward the unfortunate planet with her every iota of driving power, Seaton settled down to observe the strife and to see what he could do. That which lay beneath the viewpoint had not been a city, in the strict sense of the word. It had been an immense system of concentric fortifications, of which the outer circles had long since gone down under the irresistible attack of the two huge structures of metal which hung poised in the air above. Where those outer rings had been there was now an annular lake of boiling, seething lava. Lava from which arose gouts and slender pillars of smoke and fume; lava being volatilized by the terrific heat of the offensive beams and being hurled away in flaming cascades by the almost constant detonations of high-explosive shells; lava into which from time to time another portion of the immense fortress slagged down--put out of action, riddled, and finally fused by the awful forces of the invaders. Even as the four Terrestrials stared in speechless awe, an intolerable blast of flame burst out above one of the flying forts and down it plunged into the raging pool, throwing molten slag far and wide as it disappeared beneath the raging surface. "Hurray!" shrieked Dorothy, who had instinctively taken sides with the defenders. "One down, anyway!" But her jubilation was premature. The squat and monstrous fabrication burst upward through that flaming surface and, white-hot lava streaming from it in incandescent torrents, it was again in action, apparently uninjured. [Illustration: _But the squat and monstrous flying fort burst upward through the seething surface and was again in full action._]