y 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.

Accordingly, when Rollo's father and mother arrived at the front door,
they found Rollo ready there with a small ball of twine in his hand,
about as large as an apple.

"Now, Rollo," said his father, "you may take hold of the end of the
twine, and walk along out into t