iful.

"Now, father," said Rollo, "tell me how you know that there is any wind
at all."

"I did not say that there was any _wind_. I said _motion of the air_."

"Why, father," replied Rollo, "I thought that wind was motion of the
air."

"So it is," said his father; "but all motion of the air is not wind.
Wind is a _current_ of air, that is, a _progressive_ motion;--and in
fact, there is, this morning, a slight current from the westward."

"How can you tell, father?" asked Rollo.

"By the smokes from the chimneys; don't you see that they all lean a
little from the west towards the east?"

"Not but a little, father;--and there's one, from that red house, which
goes up exactly straight."

"Yes," said his father, "there is one; but, in general, the columns of
smoke lean; which is proof that there is a gentle current of air to the
eastward."

"_Westward_, you said, father," rejoined Rollo.

"Yes, _from_ the westward, but _to_ the eastward.

"That is what is called a progressive motion," continued Rollo's father;
"that is, the whole body of air makes progress; it advances from west to
east. But there is another kind of motion, called a _vibratory_ motion."

"What kind of a motion is that, father?" asked Rollo.

"It is a very hard kind to describe, at any rate," said his father. "It
is a kind of quivering, which begins in one place and spreads in every
direction. Don't you hear a kind of a thumping sound?"

"Yes," said Rollo, "a great way off; what is it?"

"Look over across the pond there," said his father; "don't you see that
man cutting wood?"

"Yes," said Rollo; "that's what makes the noise.--No, father," he
continued, after a moment's pause, "that's not it. Look, father, and
you'll see that the thumping sound comes when his axe is lifted up."

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. Presentl