“It is quite simple, being nothing more than a radium generator diffusing radio-activity in all directions to a distance of a hundred yards or so from the flier. Should this enveloping force be interrupted in any direction a delicate instrument immediately apprehends the irregularity, at the same time imparting an impulse to a magnetic device which in turn actuates the steering mechanism, diverting the bow of the flier away from the obstacle until the craft’s radio-activity sphere is no longer in contact with the obstruction, then she falls once more into her normal course. Should the disturbance approach from the rear, as in case of a faster-moving craft overhauling me, the mechanism actuates the speed control as well as the steering gear, and the flier shoots ahead and either up or down, as the oncoming vessel is upon a lower or higher plane than herself. “In aggravated cases, that is when the obstructions are many, or of such a nature as to deflect the bow more than forty-five degrees in any direction, or when the craft has reached its destination and dropped to within a hundred yards of the ground, the mechanism brings her to a full stop, at the same time sounding a loud alarm which will instantly awaken the pilot. You see I have anticipated almost every contingency.” Thuvan Dihn smiled his appreciation of the marvellous device. The forward servant pushed almost to the flier’s side. His eyes were narrowed to slits. “All but one,” he said. The nobles looked at him in astonishment, and one of them grasped the fellow none too gently by the shoulder to push him back to his proper place. Carthoris raised his hand. “Wait,” he urged. “Let us hear what the man has to say—no creation of mortal mind is perfect. Perchance he has detected a weakness that it will be well to know at once. Come, my good fellow, and what may be the one contingency I have overlooked?” As he spoke Carthoris observed the servant closely for the first time. He saw a man of giant stature and handsome, as are all those of the race of Martian red men; but the fellow’s lips were thin and cruel, and across one cheek was the faint, white line of a sword-cut from the right temple to the corner of the mouth. “Come,” urged the Prince of Helium. “Speak!” The man hesitated. It was evident that he regretted the temerity that had made him the centre of interested observation. But at last, seeing no alternative, he spoke. “It might be tampered with,” he said, “by an enemy.” Carthoris drew a small key from his leathern pocket-pouch. “Look at this,” he said, handing it to the man. “If you know aught of locks, you will know that the mechanism which this unlooses is beyond the cunning of a picker of locks. It guards the vitals of the instrument from crafty tampering. Without it an enemy must half wreck the device to reach its heart, leaving his handiwork apparent to the most casual observer.”