"Now we have the land and the water," said Uncle Robert.
"And the atmosphere," put in Donald.
"And they are all right here close to us. Here is the land with its hollows, and there," pointing to the river glistening in the moonlight, "is the water, and—"
"You can't see the air," said Donald.
"We can feel it, anyway," said Susie.
"How large is the earth, uncle?" asked Frank.
"Eight thousand miles through it and twenty-five thousand miles around it," answered Uncle Robert.
"But, uncle, is it all solid rock for eight thousand miles?"
"No one knows. The rocky outside of the ball is called the crust of the earth. Miners have dug down nearly four thousand feet, and makers of artesian wells have bored still farther. They always find rock."
"I wonder how far four thousand feet would be," said Donald.