"Then I shall engage some adventurous fellow as assistant. First, we must set the rudder, which is both horizontal and vertical, so that the projectile can be steered up, down, or to either side. Having fixed it so as to be directed a little upward, I begin with the currents. Suppose the projectile weighs a ton, I gradually neutralize the positive current, which we are acquainted with as gravity. When it is exactly neutralized, the projectile weighs nothing, and the pressure of the air is enough to make it rise more rapidly than a balloon. When I have created a negative current, the projectile acquires a buoyancy equal to its previous weight. That is, it will now _fall up_ as rapidly as it would previously have fallen down. It will not do to put on the full negative current at once, for we should acquire a velocity that would simply burn us up by friction with the atmosphere. However, the air is soon passed; if in the ether beyond there is very little friction, or none at all, we shall go at full speed, which will be the constantly increasing velocity of a falling body. "Somewhere between the Earth and the nearest planet," he continued, "there is a place where the attraction of one is just equal to the attraction of the other; and if a body is stopped in that fatal spot it will be anchored there for ever, by the equally matched forces tugging in opposite directions. There is such a dead line between all the planets, and our principal danger lies in falling into one of these, for we should remain there a twinkling star throughout eternity! We must trust to our momentum to carry us past this point, and into space where the gravitational attraction of the other planet is paramount. Then we must promptly change our current from negative to positive, so that the other planet will attract us to her. Otherwise, she would repel us back to the dead line. "With a positive current we are now literally falling into the new planet. We need not land unless we wish, for as soon as we enter a resisting atmosphere we can steer a course lacking barely a quarter of being directly away from the planet, just as you can sail a boat three quarters against the wind." "But suppose you experiment at making a landing on this new planet?" I suggested. "Very well. Of course, as soon as we enter an atmosphere, it behoves us to travel slowly to avoid overheating. We can still safely travel several hundred miles an hour, however. We continue falling until rather near the planet; then, turning the rudder gently down, we can sail around and around the planet until we choose our landing place. Gently reversing currents, a mild negative one soon overcomes our momentum. Tempering our currents experimentally to the pressure of the air, we can, if we desire, float like a feather and be wafted with every breeze. Just a suspicion of a positive current brings us gently to the surface, and, when we have cooled, we unscrew the rear port-hole and crawl out to explore a new world."