"Such a ball as that is called a _balloon_," said his father. "I wish I could see a balloon," said Nathan. QUESTIONS. What is the important difference between air and water, which was explained in the last chapter, and mentioned in this? Does the air tend to expand again after it is compressed? What is this property of the air called? Is the air around us already condensed, or is it in its natural state? What causes it to be condensed? Suppose a thin glass vessel were to be filled with air, and another with water, and the air suddenly removed from the room around them; what would be the effects? What effect does heat have upon the expansibility of air? How may this effect be made to appear over a lamp? In a chimney? What was the story which Rollo's father told Nathan? CHAPTER VII. PHILOSOPHICAL DISCUSSION. Some time after this, Rollo, and Nathan, and James, were playing in the shed, one pleasant morning in the spring. They were building with sticks of wood, which they piled in various ways, so as to make houses. They took care not to pile the wood, in any case, higher than their shoulders, for Jonas had told them that, if they piled the wood higher than that, there would be danger of its falling down upon them. After some time, Rollo went into the house a few minutes, and James and Nathan went to the open part of the shed, and began to look out of doors. The sun was shining pleasantly, but the ground was wet, being covered with streams and pools of water, and melting snow-banks. "What a pleasant day!" said James. "I wish it was dry, so that we could go out better." "I wish we could fly," said Nathan, "for it is very pleasant up in the air." "I wish we had a balloon," said James. "If we had a balloon, we could go up in the air, easier than to fly." "O James," said Nathan, "you could not get into a balloon if you had one." "Why not?" said James. "Because," said Nathan, "it would not be big enough." "Why, Nathan," said James, "a balloon is bigger than this house." "O James, it is not higher than my head." "It is," said James, "I know it is. I have read about balloons bigger than a house." "And my father," said Nathan, putting down his foot in a very positive air, "my father told me himself, that a balloon was about as high as my head." The boys disputed some time longer upon the subject, and finally, when Rollo came out of the house, they both appealed to him very eagerly to settle the dispute. "Isn't a balloon higher than Nathan's head?" said James. "Is it as high as a house?" said Nathan. "Why, I know," said Rollo, "that a man made a balloon once about as high as Nathan's head, because my father said so; but perhaps it was a little one." "Yes," said James, "I know it must be a little one; for balloons are big enough for men to go up in them." "O James," said Nathan, "I don't believe it. Besides, the fire would burn 'em." "What fire?" said James.