Sime burned his hand opening the narrow door. The paint was already blistering off it. The trouble was immediately apparent. One of the integrator chambers, in which atomic hydrogen was integrated to form atomic iron and calcium (sometimes called the Michelson effect), had sprung a leak. The heat escaping into the little room was not the comparatively negligible heat of burning hydrogen, but the cosmic energy of matter in creation. Sime slammed the door. The radiated light was so intense that it stung even his hardened skin. Looking through the rear range-finding periscope, he saw that they were about twenty miles from the fort. They had ceased firing. "Won't be long, Tolto," he said, taking over the controls himself again, "before our tail's going to drop off. Got to make time." It was, in fact, about ten minutes when, without warning, their nose dropped. "Tail's gone!" Sime announced. Their momentum, under the destructive rate of speed they had been making, was great, and as the levitators, with independent power supply, still held them up, Sime continued to steer a course for the twin cities of Tarog. He was aided by a light breeze, and the Sun was nearing the western horizon by the time their rate of motion had become negligible. "Might at well land," Sime decided. "Conserve fuel. If we get a favorable wind to-morrow we can go up and drift with it." But Tolto, who had been narrowly scanning the terrain, advised continuing a little longer. "I thought I saw a little smoke, a few miles ahead. Seems to be gone now. But we're still drifting slow." * * * * * Sime searched the indicated spot in the ground glass of the forward magnifying periscope. After a few minutes he discovered a blackened spot which might be the remains of a fire. It was surrounded by huge blocks of orange rock, the igneous rock which is the outstanding feature of the Martian desert landscape. "Looks like he built the fire around there so nobody on the same level would see him," he hazarded. He set the altitude control to fifty feet. There was part of the globular skeleton of a desert hog in the fire; whoever had built it had dined most satisfyingly not long before, and as the fugitives looked their stomachs contracted painfully. "I could eat a whole one of them myself," Tolto said wistfully. The urge to descend here was strong upon Sime too. He realized that the fire might have been made by some dangerous criminal--a fugitive from justice; but dangerous men are no novelty to the I. F. P. On the other hand, there was a possibility that it was just some political offender, driven into the desert by persecution. Or a prospector. At any rate, he would have food, or would know where it could be procured.