"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." [Illustration: 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 masses of land rise out of the ocean." "Islands?" asked Donald. "Four great masses of land rise above the ocean level. These immense rock masses 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."