"Let us stop up the key-hole," said Rollo, "and then it can't get in." "Yes," said his father, "there are a great many little crevices all around the closet, where the air can come in." "Couldn't we stop those up too?" said Rollo. "No," said his father, "not so as to make the closet air-tight. For, if the crevices could all be stopped exactly, the air would come in through the very wood itself." "How?" said Rollo. "Why, there are little pores in wood, that is, little channels that the sap flowed in when the wood was growing, and the air can pass through these." Here Rollo's father observed that Rollo was looking very intently at the table; and he asked him what he was doing: he said he was trying to find some of the pores. "You can't see them there," said his father. "St. Domingo mahogany is a very hard and close-grained kind of wood. If it was summer, and you could dig down and get a small piece of the root of the great elm-tree in the yard, you could see the pores and channels there." After some more conversation on this subject, Rollo asked his father if he could not think of some other experiments for them to try. His father said that he did not just then think of any experiment, but that, if Rollo and Nathan would come and sit down by the fire, he would give them some information on the subject. Rollo's mother said that she should like to hear too. They accordingly waited until she was ready, and then, when all were seated, Mr. Holiday began thus:-- "Air is in many respects much like water." "Yes," interrupted Rollo, "just like water, only thinner, because, you see----" "You must not interrupt me," said his father, "unless to ask some question, which is necessary to understand what I say. It is entirely irregular for a pupil, instead of listening to his teacher, to interrupt, in order to tell something that he knows himself." Rollo's father smiled, as he said this, but Rollo looked rather ashamed. Then his father proceeded:-- "There is one very remarkable difference between them. Water is not compressible by force; but air is." "What is the meaning of _compressible_?" said Nathan. "Compressible things," said his father, "are those that can be compressed, that is, pressed together, so as to take up less room than they did before. Sponge is compressible. A pillow is compressible. But iron is not compressible, and water is not compressible." "I should think it was," said Nathan; "it is very soft." "It is very _yielding_," replied his father, "when you press it, but it is not pressed into any smaller space. It only moves away. If you have a tumbler half full of water, and press a ball down into it, you could not crowd the water into any smaller space than it occupied at first; but, as fast as the ball went down, the water would come up around the sides of the ball." "But suppose," said Rollo, "that the ball was just big enough to fit the tumbler all around; then the water could not come up."