center of the earth there would indeed be no attraction to cause it to fall any farther; but you forget that it would then be traveling at considerable speed, and so would not stop."
"Yes, I understand that," said Mr. Curtis; "but as soon as the stone passed the center of the earth, the attraction would begin to pull it from behind, and so would draw it back to the center."
"To be sure, it would pull from behind," laughed Dr. Giles; "but the trouble is, it would n't pull hard enough. If you harnessed a dog to the front of a carriage and a horse at the back, and made them both pull with all their strength, you would certainly not expect the carriage to go forward. No matter how hard the dog might pull, the horse would easily drag him backward."
"I must say that I don't see the connection," observed Mr. Curtis.
"No? Well, then I'll explain. You probably know that a body falling to the earth falls sixteen feet the first second, forty-eight feet the next second, and eighty feet the third."
"Yes, I know that. The speed of a falling body constantly increases, and the increase is about thirty-two feet per second."
"The increase is thirty-two feet per second at
