"Not very much. Our figures show that with this four-hundred-pound bar"--pointing to the copper cylinder in the exact center of the inner sphere--"we could develop not only the velocity of light, but an acceleration equal to that velocity, were it not for the increase in mass at high velocities, as shown by Einstein and others. We can't go very fast near the earth, of course, as the friction of the air would melt the whole works in a few minutes. Until we get out of the atmosphere our speed will be limited by the ability of steel to withstand melting by the friction of the air to somewhere in the neighborhood of four or five thousand miles per hour, but out in space we can develop any speed we wish, up to that of light as a limit." "I studied physics a little in my youth. Wouldn't the mere force of such an acceleration as you mention flatten you on the floor and hold you there? And any sudden jar would certainly kill you." * * * * * "There can't be any sudden jar. This is a special floor, you notice. It is mounted on long, extremely heavy springs, to take up any possible jar. Also, whenever we are putting on power we won't try to stand up, our legs would crimple up like strings. We will ride securely strapped into those special seats, which are mounted the same as the floor, only a whole lot more so. As to the acceleration...." "That word means picking up speed, doesn't it?" interrupted Dorothy. "The rate of picking up speed," corrected Seaton. "That is, if you were going forty miles per hour one minute, and fifty the next minute, your acceleration would be ten miles per hour per minute. See? It's acceleration that makes you feel funny when you start up or down in an elevator." "Then riding in this thing will be like starting up in an elevator so that your heart sinks into your boots and you can't breathe?" "Yes, only worse. We will pick up speed faster and keep on doing it...." "Seriously," interrupted the lawyer, "do you think that the human body can stand any such acceleration as that?" "I don't know. We are going to find out, by starting out slowly and increasing our acceleration to as much as we can stand." "I see," Vaneman replied. "But how are you going to steer her? How do you keep permanent reference points, since there are no directions in space?"