compass,” as the thing was called. 

 “For example,” continued Carthoris, “I have an all-night trip before me, as to-night. I set the pointer here upon the right-hand dial which represents the eastern hemisphere of Barsoom, so that the point rests upon the exact latitude and longitude of Helium. Then I start the engine, roll up in my sleeping silks and furs, and with lights burning, race through the air toward Helium, confident that at the appointed hour I shall drop gently toward the landing stage upon my own palace, whether I am still asleep or no.” 

 “Provided,” suggested Thuvan Dihn, “you do not chance to collide with some other night wanderer in the meanwhile.” 

 Carthoris smiled. “No danger of that,” he replied. “See here,” and he indicated a device at the right of the destination compass. “This is my ‘obstruction evader,’ as I call it. This visible device is the switch which throws the mechanism on or off. The instrument itself is below deck, geared both to the steering apparatus and the control levers. 

 “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. 

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