"Yes," said his father, "you had an opportunity to see the effects of air in motion then." "And now _air at rest_," replied Rollo. "Pretty nearly," said his father. "Yes, sir, _entirely_," said Rollo; "there is no wind at all, this morning: hold up your hand, and you can feel." So Rollo stopped a moment upon the grass, and held up his hand to see whether there was any wind. "I know there is not any wind that you can perceive in that way," said his father. "How can we perceive it, then?" said Rollo. "I'll tell you," replied his father, "when we get to the top of the hill." They reached the top of the hill soon after this, and sat down upon a smooth stone. There was a very wide prospect spread out before them,--fields, forests, hamlets, streams,--and here and there, scattered over the landscape, a little patch of snow. The sun was just up, and the whole scene was very bright and beautiful. "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."