thing about what to do in case of danger. I guess I'd better go down and see what they said."
But as he swam down toward the floor his mind was busy in trying to account for the strange shock he had received at the center of the earth.
"The only possible explanation I can see," said William, "is that there must have been something in the tube in front of the car, because the shock threw me upward toward the ceiling. And now that I come to think of it, I see that the trouble must have been caused by some large stone that fell into the tube and remained at the center of the earth. It must have been a pretty big one, though, to give me such a shock as that!"
Our hero did not stop to reflect that he was traveling at such a frightful speed that, had it really been a stone that he had struck, the shock would have smashed the car to atoms.
What is known in physics as the living force of a body is equal to its mass times the square of its velocity. Consequently, if the car had struck even a small stone, the stone would in all probability have passed through it like a bullet. Hence in the present case the chances were that the shock experienced was due to the presence of some very light particles of matter that had accumulated at
