The Bardyle wondered audibly how the enemy could possibly maintain indefinitely an attack so profligate of energy, but he soon learned that there were at least four of the floating fortresses engaged in the undertaking. Occasionally the two creations then attacking were replaced by two precisely similar structures, presumably to return to Chlora in order to renew their supplies of the substance, whatever it was, from the atomic disintegration of which they derived their incomprehensible power. And slowly, contesting stubbornly and bitterly every foot of ground lost, the forces of Valeron were beaten back under the relentless, never-ceasing attack of the Chloran monstrosities--back and ever back toward their central dome as ring after ring of the outlying fortifications slagged down into that turbulently seething, that incandescently flaming lake of boiling lava. XX. Valeron was making her last stand. Her back was against the wall. The steadily contracting ring of Chloran force had been driven inward until only one thin line of fortified works lay between it and the great dome covering the city itself. Within a week at most, perhaps within days, that voracious flood of lava would lick into and would dissolve that last line of defense. Then what of Valeron? All the scientists of the planet had toiled and had studied, day and night, but to no avail. Each new device developed to halt the march of the encroaching constricting band of destruction had been nullified in the instant of its first trial. "They must know every move we make, to block us so promptly," Quedrin Radnor had mused one day. "Since they certainly have no visiray viewpoints of material substance within our dome, they must be able to operate a spy ray using only the narrow gravity band, a thing we have never been able to accomplish. If they can project such viewpoints of pure force through such a narrow band, may they not be able to project a full materialization and thus destroy us? But, no, that band is--_must_ be--altogether too narrow for that." Stirred by these thoughts he had built detectors to announce the appearance of any nongravitational forces in the gravity band and had learned that his fears were only too well founded. While the enemy could not project through the open band any forces sufficiently powerful to do any material damage, they were thus in position to forestall any move which the men of Valeron made to ward off their inexorably approaching doom. Far beneath the surface of the ground, in a room which was not only sealed but was surrounded with every possible safeguard, nine men sat at a long table, the Bardyle at its head. "--and nothing can be done?" the coördinator was asking. "There is no possible way of protecting the edges of the screens?"