"Very well. Now suppose then that the great engineer Dixon Wells invents a machine capable of traveling very fast, enormously fast, nine-tenths as fast as light. Do you follow? Good. You then fuel this miracle ship for a little jaunt of a half million miles, which, since mass (and with it inertia) increases according to the Einstein formula with increasing speed, takes all the fuel in the world. But you solve that. You use atomic energy. Then, since at nine-tenths light-speed, your ship weighs about as much as the sun, you disintegrate North America to give you sufficient motive power. You start off at that speed, a hundred and sixty-eight thousand miles per second, and you travel for two hundred and four thousand miles. The acceleration has now crushed you to death, but you have penetrated the future." He paused, grinning sardonically. "Haven't you?" "Yes." "And how far?" I hesitated. "Use your Einstein formula!" he screeched. "How far? I'll tell you. _One second!_" He grinned triumphantly. "That's how possible it is to travel into the future. And as for the past--in the first place, you'd have to exceed light-speed, which immediately entails the use of more than an infinite number of horsepowers. We'll assume that the great engineer Dixon Wells solves that little problem too, even though the energy out-put of the whole universe is not an infinite number of horsepowers. Then he applies this more than infinite power to travel at two hundred and four thousand miles per second for _ten_ seconds. He has then penetrated the past. How far?" Again I hesitated. "I'll tell you. _One second!_" He glared at me. "Now all you have to do is to design such a machine, and then van Manderpootz will admit the possibility of traveling into the future--for a limited number of seconds. As for the past, I have just explained that all the energy in the universe is insufficient for that." "But," I stammered, "you just said that you--" "I did _not_ say anything about traveling into either future or past, which I have just demonstrated to you to be impossible--a practical impossibility in the one case and an absolute one in the other." "Then how _do_ you travel in time?" "Not even van Manderpootz can perform the impossible," said the professor, now faintly jovial. He tapped a thick pad of typewriter paper on the table beside him. "See, Dick, this is the world, the universe." He swept a finger down it. "It is long in time, and"--sweeping his hand across it--"it is broad in space, but"--now jabbing his finger against its center--"it is very thin in the fourth dimension. Van Manderpootz takes always the shortest, the most logical course. I do not travel along time, into past or future. No. Me, I travel across time, sideways!" I gulped. "Sideways into time! What's there?"