Read CHAPTER XII - AIR AT REST of Rollo's Philosophy, free online book, by Jacob Abbott, on ReadCentral.com.

A few days after the adventure described in the preceding chapter, Rollo heard his father proposing to his mother that they should take a walk the next morning before breakfast.  Rollo wanted to go too.  His father said that they should be very glad to have his company; and he promised to wake him in season.

Rollo felt rather sleepy, when his father called him the next morning; but he jumped up and dressed himself, and was ready first of all.  It was a cool, but a very pleasant morning.  The sun was just coming up.  The ground in the path before the door was frozen a little, and the air seemed very still.

When Rollo’s mother came out to the door, she said, ­

“Well, husband, which way shall we go?”

“Up on the rocks,” said Rollo; “let’s go up on the rocks, mother.  It will be beautiful there this morning.”

“Well,” replied his mother; “we’ll go up on the rocks.”

The place which Rollo called the rocks, was the summit of a rocky hill, which had a grassy slope upon one side, by which they could ascend, and a precipice of ragged rocks upon the other.  There was a very pleasant prospect from the top of the rocks.

As they walked along, Rollo said that it was very different weather that still morning, from what it was the day that he and Jonas were out upon the pond.

“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.”

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.

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 the street, while I hold the ball, and let the string unwind.”

Rollo did so.  He drew out a long piece of twine, as long as the whole front of the house, and then he stopped to ask his father if that was enough.

“No,” said his father; “walk along.”

So Rollo walked on for some distance farther, until, at last, the ball was entirely unwound.  Rollo had one end of it, and was standing at some distance down the road, while his father, with the other end, stood at the gate of the front yard.  The middle of the string hung down pretty near to the ground.

“Draw tight, Rollo,” said his father.

So Rollo pulled a little harder, and by that means drew the line straighter.

“Now,” said his father, “walk along slowly.”

So Rollo walked along, drawing the end of the line with him.  His father followed with the other end.  Thus they advanced several steps along the side of the road.

“There,” said his father.  “Stop.  That, you see, was a progressive motion.”

“Yes, sir,” replied Rollo.

“The whole string advanced along the road,” added his father.  “It made progress, and so it was a progressive motion.  Now, fasten your end of the string, Rollo, to that tree directly behind you.”

Rollo looked behind him, and saw that he was standing near a small maple-tree, which had been planted, a few years before, by the side of the road.

“Tie it right around the stem of the tree,” said his father, “about as high as your shoulder.”

Rollo fastened the string as his father had directed.  Then his father fastened his end, in the same way, to another tree, which was growing near where he was standing.

“Now,” said he, “there can be no more progressive motion, but there can be a vibratory one.  Take hold of the string near where it is fastened to the tree.”

Rollo took hold of it, as his father had directed, and then his father told him to shut his eyes.  When his eyes were shut, so that he could not see, his father said that he was going to strike the string, at his end of it, with his pencil-case, and he asked Rollo to observe whether he could feel any motion.

Rollo held very still, while his father struck the string; and immediately afterwards he called out, “Yes, sir.”  Then his father struck the string again, several times, and every time Rollo could feel a distinct vibratory or quivering motion, which was transmitted very rapidly through the string, from one end to the other; although, as the string was fastened by both ends to the trees, it was evident that there could be no progressive motion.

Rollo’s mother had been standing all this time at the step of the door, watching the progress of the experiment; and, when she saw the expression of satisfaction upon Rollo’s countenance, while he was standing, with his eyes shut, holding the end of the string, she wanted to come and take hold of it herself, so as to see what sort of a sensation the vibratory motion of the string produced.

So she came out through the gate, and asked Mr. Holiday to wait a moment while she went to where Rollo was standing, and took hold of the string.  But he said that it would not be necessary for her to go there, as she could take hold of his end of the line just as well, and let Rollo strike the other end.

They accordingly performed the experiment in that way, and Rollo’s mother could feel the vibrations very distinctly.

“One thing you must observe,” said Mr. Holiday; “and that is, that the vibrations pass along from one end of the line to the other very quick indeed.  We feel them at one end almost at the same instant that the other end is struck.”

Exactly at the same instant, sir,” said Rollo.

“No,” replied his father, “not exactly at the same instant, though it is very nearly the same.”

“I did not see any difference,” said Rollo.

“No,” replied his father, “you cannot perceive any difference in so short a string, but if we had a string, or a wire, a mile long, I presume that we should find that it would require a sensible period of time to transmit the vibrations from one end to the other.”

“What do you mean by a sensible period of time, father?” asked Rollo.

“Why, a length of time that you could perceive,” said his father; “just as it was with the man cutting wood.  We could see that some time elapsed between the striking of the blow, and our hearing the sound.”

“Yes,” said Rollo, “just as long as it took him to lift up his axe.”

“That is not certain,” replied his father, “because the sound that we heard might have belonged to a blow made before.  That is, it might be that, when he had struck one blow, he had time to raise his axe and strike another, and then raise his axe again, before the sound of the first blow came to us.”

“Yes, sir,” said Rollo, “I understand.”

Mr. Holiday then told Rollo that he might unfasten the string from the trees, and wind it up again into a ball, and bring it in.  Then he and Rollo’s mother went into the house, to see if breakfast was not almost ready.

That morning, after they were all seated at the breakfast table, Rollo said to his father that he did not exactly understand what sort of a motion the vibratory motion of the air was, after all.

“No,” said his father, “I suppose you do not.  And, in fact, I do not understand it very perfectly myself.  I only know that the philosophers say, that, when a man strikes a blow with an axe upon a log of wood, it produces a little quivering motion of the air, which spreads all around, darting off in every direction very swiftly.  If a boy strikes a tin pail with a drum-stick, it makes another kind of quivering or vibration, which is different from that which is made by the axe; but I don’t know precisely how it differs.  So, when the air is full of sounds, on a still morning, it is full of these little vibrations, like a string which trembles from end to end, though its ends are fastened so that it cannot move away.”

“Then the air is never at rest,” said Rollo’s mother.

“No; certainly not, when any sound is to be heard; and it is never perfectly silent.”

“There is one thing very extraordinary,” said Mrs. Holiday.

“What is it?” asked Rollo’s father.

“Why, that, when a great many sounds are made at the same time,” she replied, ­“as, for example, when we are upon the top of a hill, on a still morning, and hear a great many separate sounds, as a man cutting wood, birds singing, a bell ringing, and perhaps a man shouting to his oxen, ­all those tremblings or vibrations, being in the air together, do not interfere with one another.”

“Yes,” said Mr. Holiday, “it is very extraordinary indeed.  They do not seem to interfere at all.  When there are too many sounds, or if there is a wind with them, they do interfere; but, in a calm morning, like this, when the air is at rest, you can hear a great many distant sounds very distinctly.”

“Yes,” said Rollo, “and I mean to go up to the top of the rocks again after breakfast, and listen.”