"O," said Rollo, laughing, "I never thought of that. Go down, Nathan," he continued, "to the door, and I will go round and knock out the fid." So Nathan went down, and Rollo, meeting him there, knocked out the fid, and released him from his imprisonment. QUESTIONS. What was the first experiment with the bellows, described in this chapter? Why could not Nathan press the two sides of the bellows together, while the nose was stopped? What was the second experiment? What was the effect produced by turning the bellows bottom upwards, as in the third experiment? What was the fourth experiment? What was the use of the smoke of the paper? How were the experiments interrupted? What evidence did Rollo and Nathan have that the air was a real substance, when in the barn chamber? CHAPTER V. PRESSURE. One evening, just after tea, Rollo came to his father, who was sitting by the side of the fire, and said,-- "Father, I wish we could see the air, as we can the water, and then perhaps we could try experiments with it." "O, we can try experiments with the air as it is," said his father. "Can we?" said Rollo; "I don't see how." "We cannot see the air, it is true; but then we can see its effects, and so we can experiment upon it." "Well, at any rate," said Rollo, "we can't build a dam, and make it spout through a hole, like water." "No," said his father, "not exactly. In your dam, for instance, when it was full, you had water on one side of the board, and no water on the other; and then, by opening a hole in the board, the water spouted through; but we cannot very well get air on one side of a partition, and no air on the other; if we could, it would spout through very much as the water did." "Why can't we do that, sir?" said Rollo. "Because," replied his father, "we are all surrounded and enveloped with air. It spreads in every direction all around us, and rises many miles above us. Whereas, in respect to water, you had one little stream before you, which you could manage just as you pleased. If you were down at the bottom of the sea, then the water would be all around you and above you; and there, even if you could live there, you could not have a dam." "No, sir," said Rollo, "the water would be everywhere." "Yes," replied his father, "and the air is everywhere. If, however, we could get it away from any place, as, for instance, from this room, then bore a hole through the wall, the weight of the air outside would crowd a portion of it through the hole, exactly as the weight of the water above the board in your dam crowded a part through the hole in the board." "I wish we could try it," said Rollo. "We _can_ try it, in substance," said his father, "in this room; or--no, the china closet will be better."