"No," said his father, "I suppose you do not. And, in fact, I do not understand it very perfectly myself. I only know that the philosophers say, that, when a man strikes a blow with an axe upon a log of wood, it produces a little quivering motion of the air, which spreads all around, darting off in every direction very swiftly. If a boy strikes a tin pail with a drum-stick, it makes another kind of quivering or vibration, which is different from that which is made by the axe; but I don't know precisely how it differs. So, when the air is full of sounds, on a still morning, it is full of these little vibrations, like a string which trembles from end to end, though its ends are fastened so that it cannot move away." "Then the air is never at rest," said Rollo's mother. "No; certainly not, when any sound is to be heard; and it is never perfectly silent." "There is one thing very extraordinary," said Mrs. Holiday. "What is it?" asked Rollo's father. "Why, that, when a great many sounds are made at the same time," she replied,--"as, for example, when we are upon the top of a hill, on a still morning, and hear a great many separate sounds, as a man cutting wood, birds singing, a bell ringing, and perhaps a man shouting to his oxen,--all those tremblings or vibrations, being in the air together, do not interfere with one another." "Yes," said Mr. Holiday, "it is very extraordinary indeed. They do not seem to interfere at all. When there are too many sounds, or if there is a wind with them, they do interfere; but, in a calm morning, like this, when the air is at rest, you can hear a great many distant sounds very distinctly." "Yes," said Rollo, "and I mean to go up to the top of the rocks again after breakfast, and listen." QUESTIONS. What time of the year was it when Rollo took this walk? How did Rollo satisfy himself that there was no wind at all? How did his father prove that there was a little wind? Is all motion of the air wind? What two kinds of motion are mentioned? What sound did they hear? What made Rollo think the sound was not made by the man whom they saw cutting wood? How did his father explain this phenomenon? What experiment did they try with the string? Were the vibrations transmitted slowly or rapidly through the string? Did Rollo think that he understood perfectly the nature of the vibrations? What extraordinary circumstance did Rollo's mother mention at the breakfast table? End of Project Gutenberg's Rollo's Philosophy. [Air], by Jacob Abbott