Jimmy Dodd's grin, seen in the half-light, was rather disconcerting. "How far do you think we dropped just now?" Dodd asked.
"Why, I'd say several hundred yards," replied Tommy. "What's your estimate?"
"Just about ten miles," answered Dodd.
"What? You're still crazy! Why, we slowed up!"
"Yeah," grinned Dodd, "we slowed up. We're inside the crust of the world. That's the long and short of it. The earth we've known is just a shell over our heads."
"Yeah? Walking head downward, are we? Then why don't we drop to the center of the earth, you damn fool?"
"Because, my dear fellow, you can swing a pailful of water round your head without spilling any of it. In other words, our old friend, centrifugal force. The speed with which the earth is rotating, keeps us on our feet, head downward. To be precise, the center of the earth's gravity lies in the middle of the hollow sphere, of course, but the counteraction of centrifugal force throws it outward to the middle of the ten-mile crust. That's why we slowed down after we were half-way through. We were moving against gravity."
"And what's up there, or down there, or whatever you call it?" asked Tommy, pointing to what ought to have been the sky.
"Nothing. It's the center of the tennis ball, though I imagine it's pretty near a vacuum when you get up a mile or so, owing to the speed of the earth's rotation, which forces the heat into the shell."
"You mean to say you actually believe that stuff you've been handing me?" asked Tommy, after a pause. "Then how did human beings get here, and those damn beetles? And why's the grass red?"