"Most of them are probably ground up into bits of dust before they reach the ground. Some of them, indeed, do strike the ground, and very large ones bury themselves deep in the earth. When we go to the Field Columbian Museum, in Chicago, we shall see these visitors from other worlds. They are called meteoric stones, or meteorites. When they are in the air we call them meteors." "I am going to watch the next one I see," said Susie. "They fly so fast that you hardly see them before they are gone," said Donald. "Men who study the heavens tell of the depth of the atmosphere by the angle the meteor makes in falling, but perhaps you can not understand that now. So you see, children, we live on the bottom of a great ocean of air, and that air, or atmosphere, is a part of our world--the outside part." "How plain it all is," said Mrs. Leonard, "when we think of it this way!" "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. "A little over three quarters of a mile," said Mr. Leonard. "The farther they go down into the crust of the earth, the warmer they find it. I have been down in a mine thirty-two hundred feet, and it was very hot. No one could have lived there if cool air had not been brought down from the surface. "Some people have thought that inside the crust of the earth the rock is all a molten mass, like melted iron. You have read about volcanoes, and of the lava that is thrown out of them?" "Does that come out of the inside of the earth?" asked Donald. [Illustration: Down in a Gold Mine] "It comes from somewhere in the earth. Some men give their whole lives to the study of these questions, but you know they can not see beneath the crust of the earth. It is thought by some that the weight of the crust would keep the center of the earth a solid mass. So you see there are still many questions unsettled. We know that the crust is moving up and down all the time." "Oh, I hope the land won't rise here!" said Susie. "You wouldn't know it, Susie, if it did," said Uncle Robert, laughing. "Unless there was an earthquake," said Frank. "Or a volcano," said Donald. "I'd like to see one."