"Yes, fossils. They are the remains of plants and animals that lived a very long time ago. Many rocks are almost entirely made of fossils. Fish and sh.e.l.ls also have been covered with soft clay and left their imprints. Great beasts have walked in the mud, and we now find their footprints in the hard stone. Coral--you have seen coral?--is often found in limestone. It is made of the sh.e.l.ls of little animals, called the polyp, which live in the sea."

[Ill.u.s.tration: Coral]

"So you see that the firm ground under foot is made of rock, some of which has been ground up over and over again. But there is something else besides rock that makes the world"

"Water," said Donald promptly as he looked down upon the river.

"Yes, the water is just as much a part of our world as the solid rock and the soil. There is water in the soil and in the solid rock, too. It comes out to us in----"

"Springs," said Donald.

"Water fills hollows in the earth----"

"Ponds and lakes," said Frank.

"Water runs down the slopes--"

"Streams," said Frank.

"Rivers," said Donald.

"There is water in the air--mist, fogs, and clouds--and there is much water in the air which we can not see."

"Vapor?" asked Frank.

"Sometimes water is so thin we can not see it, and again it is so thick and hard that we may walk over it."

"Ice," said Susie.

"Tiny bits of vapor come together until they become so heavy that they fall to the ground."

"Raindrops," said Donald.

"Water is sometimes frozen in the clouds in beautiful white crystals, and then they sail down to the earth."

"Snowflakes," said Susie.

"Sometimes drops start from the clouds and go through very cold air. The cold air freezes them quickly, and then they rattle on the roof and dash on the ground. They cut the corn leaves and destroy the crops."

"Hailstones," said Donald.

"Oh," said Susie, "I saw a hailstone once as big as an egg."

"The lakes are hollows in the ground filled with water. There are many small hollows, and some big ones, but there is one so great that we may call it immense. It is the largest hollow in the world--so large that it occupies three-fourths of the earth's surface."

[Ill.u.s.tration: Ocean islands]

"The ocean," said Frank.

"Yes, the ocean is only a great big hollow filled with water."

"How deep is the ocean?" asked Frank.

"Very deep in some places--deeper than the height of the highest mountains. In others it is very shallow. In some places bits and ma.s.ses of land rise out of the ocean."

"Islands?" asked Donald.

"Four great ma.s.ses of land rise above the ocean level. These immense rock ma.s.ses are called--"

"Continents," said Frank.

"Yes," said Uncle Robert. "We live on one of them."

"The continent of North America," said Donald.

"Our island rises right out of the river," said Susie.

"Rock and water make only a part of our world. We live on the firm earth. But we live in something. Indeed, we live at the bottom of a great, deep ocean, deeper than the water ocean, and broader than the rock and water surface taken all together."

"We live at the bottom of an ocean!" said Donald in surprise.

"Now you are joking, Uncle Robert," said Susie. "If we lived on the bottom of an ocean we should all drown."

"Fish live in the ocean, and we live in an ocean, too--a very deep one, how deep no one really knows. It may be a hundred, or hundreds of miles deep. We see a part of the surface of the earth and of the water, but no one has ever seen the surface of the mighty ocean in which we live."

Susie and Donald were puzzled. Frank's face lighted up as he said:

"I think you mean the air, Uncle Robert."

"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.

[Ill.u.s.tration: Meteors.]