"You are right, Frank. The great ocean in which we live is the air, or, as it is called, the atmosphere. The atmosphere is just as much a part of our world as the rock and the water. The rock we may call solid, the water fluid, and the air gaseous. Solid, fluid, gas." "How do we know that the atmosphere is so deep?" asked Frank. "We do not know exactly, but there are ways of proving that it is very, very deep. When people began to study the atmosphere they thought it extended about fifty miles from the surface of the earth. Now they are sure that it is much deeper. We know that air has weight, like soil and water. It presses on us and everything else--" "Fifteen pounds to the square inch," said Donald. "We weigh the air with the---" "Barometer," said Susie. "It is heavier at the ocean level than it is on the tops of mountains. We are sure that the higher we go up---" "The less the air weighs," said Frank. "At the height of fifty miles it is thought to have little or no weight, and so people believed that was as far as it extended. But in time they discovered another way of measuring the atmosphere. You have seen falling stars, haven't you?" asked Uncle Robert. [Illustration: Meteors.] "Oh, yes," said the three children together. "I saw a star fall, so fast--just like a rocket. Then the light went out, and I wondered where it went," said Susie. "Falling stars are not stars at all, though they look like them. They are pieces of rock that break off from other worlds and whiz through space." "Oh!" said Susie. "Outside of our atmosphere there may be nothing for these masses of rock to strike against, but just as soon as they come into the air, it tries to stop them. The air is not strong enough to stop them, but it grinds them up." "Grinds them up!" exclaimed Donald. "Isn't that wonderful? But, uncle, what makes them look just like fire?" "If you put an axe or scythe on a dry grindstone and turn the crank, what do you see?" "Sparks of fire," said Frank. "Why do you put grease or oil upon the axles of your buggy?" "To keep them from becoming hot and dry," said Frank. "One time when father and I were on a train there was a hot box, and we had to stop to cool it." "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.