Carthoris of Helium had come all unannounced to the court of his father’s friend that day. He had come alone in a small flier, sure of the same welcome that always awaited him at Ptarth. As there had been no formality in his coming there was no need of formality in his going. To Thuvan Dihn he explained that he had been but testing an invention of his own with which his flier was equipped—a clever improvement of the ordinary Martian air compass, which, when set for a certain destination, will remain constantly fixed thereon, making it only necessary to keep a vessel’s prow always in the direction of the compass needle to reach any given point upon Barsoom by the shortest route. Carthoris’ improvement upon this consisted of an auxiliary device which steered the craft mechanically in the direction of the compass, and upon arrival directly over the point for which the compass was set, brought the craft to a standstill and lowered it, also automatically, to the ground. “You readily discern the advantages of this invention,” he was saying to Thuvan Dihn, who had accompanied him to the landing stage upon the palace roof to inspect the compass and bid his young friend farewell. A dozen officers of the court with several body servants were grouped behind the jeddak and his guest, eager listeners to the conversation—so eager on the part of one of the servants that he was twice rebuked by a noble for his forwardness in pushing himself ahead of his betters to view the intricate mechanism of the wonderful “controlling destination 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.