"It must be their food--human blood!" Norman repeated. "They may have thousands on thousands of humans penned up like this, like so many herds of cows, and perhaps they live entirely on the life-blood they milk from them. Human cows--God!" "Norman--look--they're calling to us!" * * * * * The two stiffened. All the others in the room had taken their turn at the blood-sucking mechanism and now the frog-men croaked their order to the two fliers. They had forgotten their own predicament in the horror of the scene, but now it became real to them. They backed against the room's wall, quivering, dangerous. The frog-guards came forward to drag them to the machine. A webbed paw was outstretched but Hackett with a wild blow drove the frog-man back and downward. The frog-guards leaped, and Norman and Hackett struck them back with all the greater strength the lesser gravitation gave them. The room was in an uproar, the green men shouting hoarsely and seeming on the point of rushing to their aid. But the menacing force-pistols of the other frog-guards held back the shouting men and in moments the two fliers were overpowered by sheer weight of frog-bodies. Norman felt himself dragged to the machine. Pain needled his upper arm as an incision was made. He felt the sucking-disk attached; then the machine hummed, and a sickening nausea swept him as the blood drained from his body. Held tightly by the guards he went dizzy, weak, but at last felt the sucker removed and a metal disk fastened over the incision. He was jerked aside and Hackett, his face deathly white, was dragged into his place. In a moment some of the latter's blood had been pumped from him also. The machine was withdrawn, Norman and Hackett were released, and the frog-men, with their black force-pistols watchfully raised, withdrew, the door clanging. The room settled back to quietness, the green men stretching in lassitude on the metal bunks around it. The two fliers crouched down near the door, shuddering nausea and weakness still holding them. Norman found that Hackett was laughing weakly. "To think that twenty-four hours ago I was in New York," he half-laughed, half-sobbed. "On Earth--Earth--" The other gripped his arm. "It's horrible, Hackett, I know. But it isn't instant death, and we've still a chance to escape. Hell, can damn frog-men keep us here? Where's your nerve, man?" A voice beside them made them turn in amazement. "You are men from Earth?" it asked, in queerly accented English. "From Earth?" * * * * * Astonishment held them as they saw who spoke. It was one of the green men in the room, who had settled down by their side. A tall figure with superb muscles and frank, clean countenance, his dark eyes afire with eagerness. "English?" Norman exclaimed. "You know English--you understand me?"