"Are their motors made with sheet steel cylinders or with duralumin engine blocks?" "Sheet steel." "The devil! I hadn't foreseen this, although it was bound to happen if my theory was right. Tell them to climb! Climb all they know! Don't let them shut off their motors for any reason, unless they are about to crash. Turn this ship to the north and have the pilot climb--fast!" A nod from General Merton confirmed the doctor's orders. The line of planes kept on to the west, but the flagplane turned to the north and climbed at a sharp angle, her three motors roaring at full speed. With the aid of binoculars, the two ships in trouble could be picked out, falling gradually behind the line. They were flying so slowly that it seemed inevitable that they would lose flying speed and crash to the ground. "More speed!" cried the doctor. "We've elevation enough!" The altimeter stood at eight thousand feet when the pilot leveled out the flagplane and tore at full speed toward the laboring ships. The main fleet was twenty miles to the west. * * * * * They were almost above the point where the two planes had first began to slow down. As they winged along, the three motors of the flagplane took on a different note. It was a laboring note, pitched on a lower scale. Gradually the air-speed meter of the ship began to show a lower reading. "Locate us on the map, Carnes!" snapped Dr. Bird. Operative Carnes of the United States Secret Service bent over a large-scale map of Maryland, spread open on a table. With the aid of the navigating officer, he spotted on the map the point over which the plane was flying. "There goes Burleigh's ship!" cried the executive officer. There was a gasp from the occupants of the flagplane's cabin. Far below them, one of the crippled planes had slowed down until it had lost flying speed. Whirling like a leaf, it plunged toward the ground. Two small specks detached themselves from the falling mass. They hovered over the falling plane for an instant. Suddenly a patch of white appeared in the air, and then another. The two specks fell more slowly. "Good work!" exclaimed General Merton. "They took to their 'chutes just in time." "We'll be taking them in a few minutes if our motors don't pick up!" replied the executive officer. Far below them, the doomed plane crashed to the ground. As it struck there was a blinding flash followed by vivid flames as the gasoline from the bursted tank ignited. The two members of the crew were drifting to the east as they fell. It was evident that they were in no danger. "Where is Lightwood's plane?" asked General Merton anxiously. "It's still aloft and making its way slowly north. He intends to try for an emergency landing at the Aberdeen Proving Ground field," replied the executive officer.