The southern half of Virginia had been swept over. Operators, telegraph and telephone, staying at their posts had sent in constant messages that had terminated with an abruptness which told of the tragic sequel. Yet, at that distance from its source, the intensity of the gas had been to some extent dissipated. Poisonous beyond any gas known, so deadly as to make hydrocyanic gas innocuous in comparison, still as it was swept northward on the wings of the wind, there had been an increasing number of non-fatal casualties. The most northernly point reached by the gas was Richmond, and here some fifty per cent of those stricken had suffered paralysis instead of death. But a new element had been injected into the situation. Even the heroic courage shown by the populace in the beginning had had its limits. The morning after the news of the Invisible Death's advent was made public mobs had gathered in all the large cities of the East, demanding surrender. The submerged elements of crime and disorder had come to the surface at last. Committees were formed, with the avowed object of yielding to the Invisible Emperor, and averting further disaster. In Washington, a city of the dead, half the members of Congress and the Senators had gathered in the ruined Capitol, to debate the situation. There were rumors of an impending march on the White House, of a coup d'ètat. * * * * * The action of the Government was prompt. Five hundred loyalists were enrolled, armed, and posted round the White House: every avenue of approach was commanded by machine-guns. Meanwhile the news was spread by radio that the headquarters of the Invisible Emperor had been located, and that a strong bombing squadron was being dispatched to destroy it. The entire fleet was to follow, and it was confidently anticipated that within a little while the Terror would be at an end. Those at the white House were less sanguine. There was none but realized the diabolical strength of their antagonists. "Everything depends upon the outcome of the next forty-eight hours, and everything depends on you, Rennell," said Secretary Norris to Dick, as he stood beside his plane. Behind him his flight of a dozen airships was drawn up. "Find them," added the Secretary; "cover Abaco Island with the black gas, and the navy and the marines will wipe up the mess that you leave behind you. God help you--and all of us, Rennell!" He gripped Dick's hand and turned away. Dick was very sober-minded as he climbed into his cockpit. He knew to the full how much depended upon himself and Luke Evans. Already the shouts of the insurgents were to be heard at the ends of the barriers, commanded by the machine-guns, and patrolled by the enlisted volunteers.