“But,” interjected Robert, struck by a fresh disquieting thought, “if we are now rushing directly toward Mars, won’t we be left far behind by the time we reach its present location because of its rapid movement along its orbit?” “Oh, what’s a few million miles or so to us?” said Taggert with extravagant nonchalance. “Your deductions are partly correct, Robert,” answered Professor Palmer, smiling at their guest’s sally. “There is a curious thing about moving bodies in a void: they will continue in _one_ direction indefinitely until attracted or propelled, by some other force. In the case of a propelling force, unless it is in a directly opposite direction, it will simply result in the body going off at a tangent, still maintaining its original rate of velocity in the original direction in conjunction with its new direction. For example: Mars and the Earth moving in virtually parallel directions in their respective orbits at present, it was a comparatively simple matter to lay a straight course for Mars, as the Earth’s orbit velocity was imparted to the _Sphere_ when we left it. But since the Earth moves somewhat faster along its orbit than Mars, we would gradually forge _ahead_ of Mars if we had laid what at first appeared to be a direct course toward it, and would only have arrived finally by continually correcting our course, and having swung round in a vast curve. Instead, by calculating the difference in the known orbit velocities of the two planets, and accordingly laying a course which at first appeared to be toward a point already passed by Mars, we promptly found ourselves on very nearly a direct course toward the planet.” Notwithstanding his comparative ignorance of astronomy, Taggert unconsciously echoed Robert’s sigh of relief over this assurance that they were on the correct course. It was clear, even to him, that with no basis of stability they would be in a bad way should they pass Mars at a distance too great to establish gravitational contact with it. With their limited reserve of power and provisions they could not afford to knock around the universe at random. 10 Thee thirteenth “day” saw them nearing Mars rapidly. The ruddy-hued planet gleamed at them with magnificent brilliance, its cold glitter thrilling them and filling them with vague misgivings. At this distance it became apparent that Professor Palmer’s feverishly corrected course would carry the _Sphere_ at least several thousand miles ahead of the planet. This variation was to be expected, however; the miracle of it was that he had been able to judge direction so closely in those few nerve-racking and never-to-be-forgotten moments in the reeling _Sphere_.