They all looked, and found that it was as Rollo had said. The strokes of the axe kept time, pretty well, with the sound of blows, which they heard, only the sounds did not correspond with the descent of the axe. When the axe appeared to strike the wood, they did not hear any sound, but they did hear one every time the axe was lifted up. "So, you see," said Rollo, "it is not that man that we hear. There must be some other man cutting wood." "We will wait a minute," said his father, "until he gets the log cut off, and then he will stop cutting; and we will see whether we cease to hear the sound." So they sat still, and watched the man for a minute. Presently he stopped cutting,--and, to Rollo's great surprise, the sound stopped too. "That's strange," said Rollo. In a moment more, the man had rolled the log over, and commenced cutting upon the other side; and in an instant after he began to cut, Rollo began to hear the sound of strokes again. "Yes," said Rollo, "it must be his cutting that we hear; but it is very strange that he makes a noise when he lifts up his axe, and no noise when it goes down." "I'll tell you how it is," said his father. "He makes the noise when his axe goes down; but, then, it takes some little time for the sound to get here; and by the time the sound gets here, his axe is up." "O," said Rollo, "is that it?" "Yes," replied his father, "that is it." Rollo watched the motion of the axe several minutes longer in silence, and then his attention was attracted by the singing of a bird upon a tree in his father's garden, at a short distance below him. Pretty soon, however, his mother said that it was time for her to return; and they all, accordingly, arose from their seats, and rambled along together a short distance upon the brow of the hill, but towards home. "Then the sound moves along through the air," said Rollo, "from the man to us." "Yes," said his father; "that is, there is a vibratory motion of the air,--a kind of quivering,--which begins where the man is, and spreads all around in every direction, until it reaches us. But there is no _progressive_ motion; that is, none of the air itself, where the man is at work, leaves him, and comes to us." "But, husband," said Rollo's mother, "I don't see how anything can come from where the man is, to us, unless it is the air itself." "It is rather hard to understand," said his father. "But I can make an experiment with a string, when we get home, that will show you something about it." They rambled about among the rocks for a short time longer, and then they descended by a steep and crooked path, in a different place from where they had ascended. When they had got nearly home, Rollo said that he would run forward and get his father's ball of twine and bring it out; and so have it all ready for the experiment.