"The heat and the sparks of fire are caused by one body rubbing against another. The faster they move, the greater the heat. This rubbing is called friction."
"There was a time," said Mr. Leonard, "when fires were started by rubbing two pieces of wood together. Some Indians do so now."
"Then the great pieces of rock rub against the air when they whiz through it, and that makes the sparks?" asked Frank.
"You are right. We can see the blaze of fire caused by the friction."
"I should think the rocks would fall on us and kill us," said Donald.
"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!"