Improved as they were by Standish, the machines immediately placed the Sirians in a mental state where they were receptive to all commands. Yet they retained full control of their mental faculties. The work began. Frameworks for twenty space destroyers were laid. Like automatons the Sirians toiled, worked side by side with the men of Lyra. The twenty hulls were completed, and the atomic motors were being installed when Standish called Ga-Marr aside. "I'm going to leave you in charge," the Earthman said, "while I take the _Phantom_ out again. The more prisoners, the quicker we'll have a fleet. Besides the Sirians will have grown careless again by now." This time, however, Standish steadfastly refused to take Thalia along. "I'm going to skirt the very stratosphere of Earth," he told her, "and it'll be too dangerous. But I'll be back soon." Thalia pouted, but Standish was firm. With another Lyrian, Dar-ley, as his lieutenant, Standish took off. He headed at full speed for the interplanetary lane between Sirius and Earth. As he went on, suspicion assailed him. Not a single Sirian ship did he see. Once a slow-moving freighter from far off Protorus crossed his path. The freighter clapped on all speed in a frantic attempt to escape. But Standish viewed it without interest. He was drawing close to Earth. Alert, Standish kept the moon between him and his home planet, advancing cautiously. But there was no sign of trouble. The spaceways were empty. Now the cold expanse of the moon opened before him. The _Phantom_ soared over Tycho, Aristotle and Petavius, dipped downward and came to a rest on a barren lava plain. Standish took down a space suit, and a small magno telescope and went out through the air lock. Pacing slowly across the frigid flat, he tried to fathom the growing puzzle. A hundred yards from the ship he trained his scope on Earth, staring long and intently. But the range was too great and the scope too weak for detailed observation. And then abruptly he stiffened. Through the powerful retinite lens a tiny dot focused his vision. A rocket ship! He adjusted the glass and studied her lines. Unquestionably she was Sirian and heading toward the moon on an oblique angle. Standish ran for the _Phantom_. The air lock closed; he threw over the control lever, and the big ship headed with a lurch for the enemy. In the pilot cuddy Dar-Ley watched the cosmoscope and intoned the distance measurements. "Thirty thousand miles. Enemy still following same course." "Twenty thousand. No change." "Eight hundred." A frown crossed Standish's face. The Sirian ship must have seen them by now. Alone and without convoy, it should have turned and fled. Puzzled, the Earthman ordered a shot across the enemy's bows. The Sirian did not change her course. And then Dar-Ley gave a frantic cry. "Behind us. Look!" * * * * *