The planes circled around again and again, quartering the air like a pair of well-trained bird dogs will quarter a hunting field. First high and then low they swooped back and forth, the tanks lumbering slowly along in the same direction. Presently the occupants of the leading tank saw one of the planes bank sharply and swing around. It dropped to an altitude of only a few hundred feet and turned and went back over the ground it had just crossed. "I believe that fellow sees something!" exclaimed Carnes. As he spoke, three green Very lights came from the cockpit of the plane. The tank driver gave a grunt of satisfaction and turned the nose of his vehicle in that direction. The second tank followed. Hardly had they turned in the new direction before the ground began to get soft under their tracks and the heavy vehicles began to sink. The driver of the Doctor's tank forced it ahead, but the tank sank deeper in the mire until water flowed in around the feet of the occupants. "I reckon we'll have to get out and walk pretty soon, Doctor," said the driver. * * * * * Dr. Bird grunted in acquiescence. The tank made its way forward a few yards before the engine sputtered and died. The second tank stopped when the first one did, fifty yards behind it. Donning vitrilene helmets and taking vitrilene shields in their hands, the crews of both tanks climbed out into the waist-deep water and gathered around the Doctor for orders. "Form a skirmish line at ten-pace intervals and cross the swamp," he directed. "We may meet with no opposition, but if there is, the more scattered we are, the safer we will be. You all have hand grenades as well as your rifles?" A murmur of assent answered him and the line formed and started across the swamp. They had gone perhaps a hundred yards when three red lights came from one of the planes circling overhead. "Down!" cried the doctor, dropping to his knees in the muck. Four hundred yards ahead of them a concrete platform emerged from the marsh and rose slowly into the air. It was roofed with a dome of what looked like plate glass, but which the doctor shrewdly suspected was vitrilene. When the base of the platform was two-feet above the level of the water the dome slid silently aside disclosing two men bending over a tiny gun. Dr. Bird leveled his binoculars. "That's the Breslau gun model that was stolen as sure as I'm a foot high!" he cried. "They must have made some miniature shells and be planning to fire it."