"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."