"Quite right. Let us come back to Earth, and return to facts. My studies in physics led me to believe that all natural forces--gravity, centrifugal force, and even capillary attraction--are, like electricity and magnetism, both positive and negative in their action. If they do not normally alternate between a positive and negative current, as electricity does, they can be made to do so. Gravity and capillary attraction, as we know them, always act positively; that is, they always _attract_. On the other hand, centrifugal force always acts negatively; that is, it always _repels_. But each of these forces, I believe, can temporarily be made to act opposite to its usual manner. I know this to be the case with gravity, for I have caused its positive and negative currents to alternate; that is, I have made it repel and then attract, and so on, at will, by changing the polarity of the body which it acts upon." "Now that I remember it," I added, "our original ideas of magnetism were that it simply attracted. We knew the lodestone drew the steel, but only on better acquaintance did we learn of its alternating currents, attractive and repellant." "I have positively demonstrated with my working model that I can reverse the force of gravity acting upon the model, and make it sail away into space. I will show you this whenever you like. It is so arranged that the polarizing action ceases in three minutes, after which the positive current controls, and the model falls to the Earth again." "But have you ever attempted a trip yet?" I inquired. "Oh, no. The model was not built to carry me, but it has demonstrated all the important facts, and I now need ten thousand dollars to build one large enough to carry several persons, and to equip it with everything necessary to make a trip to one of the planets. With a man inside to control the currents, it will be far more easily managed than the experimental model has been." "Suppose you had the projectile built, and everything was ready for a start," I said, "what would be the method of working it?" "I should enter the forward compartment," began the doctor. "But would you make the trial trip yourself?" "I certainly would not trust the secret of operating the currents to any one else," he remarked, with emphasis. "And will you accompany me in the rear compartment?" "No, indeed; unless you will promise to return in time for the following day's market," I replied.