Abbott Jacob
Rollo's Experiments
“Now you see, from all this,” added Rollo’s father, “that when a magnet touches a piece of steel, like a needle, it immediately makes it a magnet itself; that is, it makes the two ends magnetic, one having one kind of magnetism, and the other the other kind; and then, if you take two magnets, and bring those two poles which have the same magnetism together, they repel one another; and if you bring those together which have different magnetisms, they attract each other.”
“How do you know that they are the same magnetisms that repel, and different that attract?” said Rollo’s mother.
“I will show you,” said his father.
Then he took the needles that he had used off from their floats, and laid them away. He took next two new needles, exactly of a size, and he held them together between his thumb and finger, with the eyes projecting together. Then he rubbed them once or twice upon the end of Jonas’s magnet, saying,
“There, you see I use both of these needles alike. Of course the eyes have both the same magnetisms. Now you will find that when I put one of them upon the float, and then bring the eyes together, they will repel each other; but an eye and a point will attract. So two points will repel.”
“But you have not magnetized the points,” said Rollo’s mother.
“Yes,” said his father. “When we magnetize one end, the other end becomes magnetized, itself, in the contrary way.”
So he put one of the needles upon the float, and then brought the eye of the other down very near to its eye. It was repelled, as he had said it would be. He then brought the two points together, and they were repelled. But if he brought an eye towards a point, or a point towards an eye, they were attracted.
“This is the end of my lecture,” said he, “for to-night.”
“O, father,” said Rollo, “a little more.”
“No more to-night, only to recapitulate,” said he.
“Recapitulate? what is that?”
“Why, tell you, briefly, the substance of what I have explained, so that you may remember it.”
“Well, father,” said Rollo.
“In the first place, a magnet has a peculiar and mysterious attractive power for iron, residing in its two extremities, which are called its poles; and the power which resides in one extremity is, in some way or other, opposite in its nature to that of the other extremity. Each of these poles repels a pole like itself, and attracts one different from itself, in any other magnet.”
Poor Nathan could not understand this grave, philosophical disquisition very well, and he began to get pretty sleepy. He had, however, been somewhat amused, during the greater part of the time, in seeing the corks float about upon the water, with the needles upon them. So his father took the needles off, and let him have the two floats in one of the saucers to play with, a few minutes, while Dorothy put the other things away. He asked her to put all the things away together, so that they could get them ready the next evening, and then he said that perhaps he would give them another lecture.
INTELLECTUAL PHILOSOPHY
Rollo’s father gave one or two other lectures upon magnetism, in the course of which Rollo found out a good deal about the subject; and, having learned from his father’s explanations that any magnet, when balanced freely, would point to the north and south, that is, one end to the north and the other to the south, he determined to try the experiment. He accordingly poised a needle carefully upon a cork, as his father had done in his lecture, and put it in a basin of water upon the platform. But he did not succeed very well. The needle would always swing round, and turn its point towards the garden gate; but Rollo knew very well that the garden gate was not north from the platform. He remembered that the North Star was over the barn, for he and Jonas had noticed it particularly when they had made the dial. The needle, therefore, ought to have pointed towards the barn, according to his father’s lecture; but it would not. Rollo took up a straw, and pushed the point of the needle round, and said, “Point there! point there, I tell you!” But all in vain. The needle would not heed either his pushing or his commands; but, as soon as he let it go, it would immediately swing back into its old position, where it pointed towards the garden gate.
Rollo was just about giving up in despair, when he saw his sister Mary coming in from the garden gate, with a book under her arm.
“O Mary,” said he, “what shall I do? My needle won’t point right.”
“Why, what is the matter with it?” said Mary.
“It will point over towards the garden,” said Rollo; “look.”
So Mary came up, and looked at his needle. She saw that it was pointing towards the garden gate.
“Now I’ll push it away,” said Rollo, “and you will see that it comes directly back again.”
So he took up his straw, and pushed the point of the needle away. The cork moved, turning round rapidly, until at length it swung away towards one side of the basin, and then suddenly drifted up against the side, and stuck there.
“That’s another plague,” said Rollo. “It will run up to the side of the basin, and stick there.”
“What makes it?” said Mary.
“I don’t know,” said Rollo.
Mary sat down upon the platform, and examined the needle and the surface of the water very carefully. She observed that the water was heaped up a little against the side of the basin, all around. She asked Rollo to observe it.
“Yes,” said he, “and the needle and cork run right up that ridge of water.”
“And the bubbles too,” said Mary.
Mary pointed, when she said this, to several little bubbles which were adhering closely to the side of the basin, in another place.
She took up a little straw, and pushed away some of the bubbles from the side of the basin, and then gently moved them back again until they were pretty near, and observed that they would immediately rush up against the side again. She did not understand this phenomenon, especially as the water was raised a little along the edge by the side of the basin, so that the bubbles and the needle actually appeared to rush up hill.
After examining this for some time, Mary moved the cork float, with the needle upon it, back into the middle of the basin, and then left it to itself. It slowly moved around until it pointed to the garden gate, as it had done before.
“Now what is the reason?” said Rollo; “that isn’t north.”
Mary looked upon it very attentively for a few minutes in silence, and then said, suddenly,
“O, I see.”
“What?” said Rollo.
She did not answer, but pointed down to the platform by the side of the basin.
Rollo looked where she pointed, and saw the hammer lying there. He had had it to play with a short time before, and, when he brought the basin of water, he had laid it down by his side.
“What?” said Rollo.
“The hammer attracts the needle,” replied Mary.
“The hammer?” said Rollo.
“Yes,” replied Mary. “Don’t you know that iron attracts the needle, and it will not point to the north if there is any iron near to draw it away?”
Rollo was just going to take the hammer up, but Mary stopped him, saying,
“Wait a moment. Let me take it away slowly, and see the effect.”
So Mary told Rollo to watch the needle, while she carefully drew the hammer away.
Rollo did so. He and Mary both watched the needle. It was pointing pretty nearly toward the hammer, and when Mary gently moved the hammer away, the needle, released from the influence which the iron exerted upon it, slowly moved back towards the direction of the barn, that is, the direction of a north and south line, which is called the meridian.
“It’s going back! it’s going back!” said Rollo.
Mary said nothing, but watched it carefully. The needle swung beyond the direction of the meridian a little way, and then came slowly back again. So it continued vibrating from one side to the other, though to a less and less distance every time. Finally, it came to a state of rest; but it was not then, however, exactly in the meridian.
“What makes it swing so, back and forth?” said Rollo.
“I don’t know exactly,” said Mary. “I suppose the force that it moves with, carries it a little beyond, and then it is drawn back again, and that makes the oscillations.”
“Oscillations?” said Rollo, inquiringly.
“Yes. They call this swinging back and forth, oscillating; and each movement is an oscillation.”
“Is that the name of it?” said Rollo.
“Yes,” said Mary. “When you tie a little stone to a string, and hold the upper end of the string still, and let the stone swing back and forth, it makes oscillations.”
“I mean to try it,” said Rollo.
“Yes,” said Mary; “and I will help you by and by, after I have studied my philosophy lesson.”
“Your philosophy lesson?” asked Rollo. “Have you got a philosophy lesson to get?”
“Yes,” said Mary, “in that great book.”
So Rollo took up Mary’s book, which she had laid down upon the platform near Rollo’s apparatus. He found that the title of it was “Intellectual Philosophy.”
“Intellectual Philosophy?” said Rollo; “and what sort of philosophy is intellectual philosophy?”
“It is the philosophy of the mind,” replied Mary. “It explains to us about the thoughts and feelings of our minds.”
“Are there any experiments in intellectual philosophy?” asked Rollo.
“Yes,” said Mary, “we can try experiments in intellectual philosophy.”
“What experiments?” said Rollo.
“Why, there is a question whether we always dream when we are asleep.”
“I do,” said Rollo, “every night.”
“Yes, but perhaps not all night long.”
“Yes, I do,” said Rollo. “I have good long dreams.”
“But,” replied Mary, “you may dream several hours in the night, so as to remember good long dreams in the morning, and yet perhaps you might have been, for some time, perfectly sound asleep, so as not to have any dreams in your mind at all. Some persons think we dream all the time when we are asleep, and others think we don’t dream all the time. Now we might contrive some experiments to decide the question.”
“How?” said Rollo.
“Why, you and I might agree to wake each other up several times, from a sound sleep, and then, if we were dreaming at that time, we should probably remember it.”
“Well,” said Rollo, “let us try it.”
“That would be an experiment in intellectual philosophy,” said Mary.
Rollo determined to try the experiment; and then he took Mary’s book, and asked her where her lesson was that day. She found the place, and Rollo read a little. He could not understand it very well, and so he concluded that he would rather have Mary go and study her lesson, and then come down and help him make the experiments of oscillation.
Mary accordingly took her book and went in, and left Rollo at his play.
OSCILLATIONS
In about an hour, Mary came down into the yard in pursuit of Rollo, in order to try the experiments which she had proposed.
When Rollo saw her coming, he left his play, and ran to meet her.
“Well, Mary,” said he, “have you come to make the oscillations?”
“Yes,” said Mary. “I have brought some thread for strings, and I want you to get some pebble stones—some large, and some small ones.”
Rollo went for the pebble stones, while Mary looked about for a suitable place for making the experiments. In a corner of the yard there was a bench under a tree, and the branches came down pretty low. Mary thought that this would be a good place, for she could tie her strings to these branches with the pebbles hanging down below; and she and Rollo could watch the oscillations, while seated upon the bench.
Mary took her station here, and Rollo presently appeared, with the crown of his cap half filled with pebble stones. Mary said they would do finely. She poured them out upon the bench by her side, and Rollo put his cap upon his head again.
“Now, Rollo,” said she, “we will study the art of experimenting.”
“No,” said Rollo, “we are going to study oscillation.”
“Yes,” replied Mary; “the experiments are to be on oscillations; but what I want principally to teach you, is, the proper way to make experiments.”
“Well,” said Rollo.
Mary said no more, but she proceeded to tie a small pebble to the end of one of the long threads which she had brought out with her. Then she tied the other end of the thread to the branch of the tree, which was over her head. The pebble then hung down before them, so that both Rollo and herself could plainly see all its motions.
“The first thing,” said Mary, “is to get a clear idea of the nature of the oscillation, for we must know what we are experimenting about.”
So saying, Mary carefully took hold of the suspended pebble stone, and began to draw it off towards one side. She showed Rollo that, as it was confined by its string above, it must move in a curved line when she drew it away from its place, rising higher and higher the farther it was drawn away. And when she had drawn it out to a considerable distance, to one side, it was at a much higher level, than when it hung down freely in its natural position.
“Now,” said Mary, “you see that if I let it go, it will descend of course as much as it can, for the earth draws it downwards.”
“The earth draws it?” said Rollo.
“Yes,” said Mary. “The reason why things fall is that they are attracted, or drawn down, by the earth. Now the earth draws the pebble. It would go straight towards it, if it could; but the string confines it, and so it can only go down in the same way that it came up; that is, by the curved line.”
Mary then held one of her hands open at the place where the pebble had hung when it had been at liberty, and let go the pebble, which she had been holding with the other. It fell down in the curved line, or arc, as Mary had said it would, until it struck her hand, and there it stopped and remained at rest.
“What did you stop it for?” said Rollo.
“So that we could see and attend to one part of the phenomenon at a time,” said Mary; “that is, the descent of the pebble. You see the attraction of the earth causes the pebble to go down if it can, and the confinement of the string prevents its going down in any other way than in that curve or arc. For the string keeps it always just its own length from the branch, and so that makes the curved line the arc of a circle.”
“Yes,” said Rollo, “I understand.”
Then Mary drew up the pebble once or twice more, and let Rollo see it fall against her hand. Rollo observed that it was a very regular arc.
“Now we see,” continued Mary, “that I hold my hand so as to stop the pebble stone at the lowest point to which it can go; for I hold it exactly under the point where the upper end of the string is fastened to the tree. Now I will take my hand away, and then let the pebble fall, and we will see what takes place.”
So Mary took her hand away, and let the pebble fall freely. It descended as before through the arc, and then, by the force which it acquired in moving so far, it was propelled beyond the lowest point, and ascended in another curve, upon the other side, similar to the first. When the force was expended, it came back again; and thus it swung to and fro, several times, and at length came almost to a state of rest.
“There,” said Mary, “those are the oscillations we are going to experiment upon.”
“Yes,” said Rollo.
“And first,” said Mary, “we notice that they are regular.”
So she swung the pebble again; and as it moved to and fro, she counted the oscillations aloud, beating time with her hand, down and up, thus,—
“One,—two,—three,—four,” &c. Rollo perceived that they were very regular.
“Now, first we will endeavor to ascertain by our experiments,” said Mary, “what the time of the vibrations depends upon.”
“Well,” said Rollo.
“You see,” continued Mary, “it swings back and forth with a certain degree of rapidity. Now we want to know what this rapidity depends upon, and then we could make a pendulum so that it would oscillate faster or slower, just as we pleased.”
“A pendulum?” asked Rollo.
“Yes,” said Mary, “we call it a pendulum. Any heavy body hung in this manner, so as to swing back and forth by its weight, is called a pendulum. So that we are experimenting upon the oscillations of a pendulum.”
“Yes,” said Rollo, “I understand.”
“Now the question which we are going to examine,” said Mary, “is, what the rapidity of the vibrations depends upon.”
“O, it depends upon the bigness of the pebble,” said Rollo.
“How do you know?” said Mary.
“Why, of course, a bigger pebble will be heavier, and will fall quicker, and that will make it vibrate faster.”
“That is reasoning about it,” said Mary, “and what we want to do, now, is to experiment. Now, in order to decide it by experiment, we must try two pendulums, one with a small pebble, and the other with a large one.”
“Very well,” said Rollo, “we will; and then we shall see that the big one will vibrate the quickest.”
“Let us think, first, what other circumstances there are, that it may depend upon.”
“I can’t think of any thing else,” said Rollo.
“Why, there is the nature of the body which we suspend. A piece of cork may oscillate differently from a piece of stone.”
“Yes,” said Rollo, “it will oscillate slower.”
“We must not decide,” said Mary, “in our own minds, before we try the experiment. We must leave our minds free to observe the facts, and wait until we make the experiment, before we come to any conclusion, or else we shall not be good experimenters.”
“Why not?” said Rollo.
“Because,” said Mary, “when persons make up their minds beforehand what the facts will be, they are very apt not to observe fairly. So good observers or experimenters always take care to keep their minds free and unbiassed.”
“Well,” said Rollo, “and what else is there that the oscillations may depend upon?”
“The length of the string,” replied Mary.
“O yes,” said Rollo, “it may depend upon that.”
“Let us see,” continued Mary. “There are three experiments we have already proposed; a large and a small pebble; a pebble and a cork; a long and a short string; and now there is one more,—a long and short arc.”
“How?” said Rollo.
“Why, if I draw up the weight, which forms the pendulum, pretty high, it will swing back and forth through a long arc. But if I move it only a little way, it will swing through only a short arc, and that may make a difference in the length of the vibrations.”
“Well,” said Rollo, “and now let us try.”
“First, let us see whether we have got all the apparatus we want. Here are strings and pebbles,—only we want a cork.”
“I’ll go and get one,” said Rollo.
So Rollo went off towards the house to get the cork. In a few minutes he came back, saying,
“I have got the cork. Now how shall we begin?”
“First,” replied Mary, “we will try what effect the weight of the pebble will have upon the oscillations.”
“Very well,” said Rollo.
“Now, in order to test that,” added Mary, “we must take two pebble stones, of different sizes, and hang them together, by strings of the same kind, and of the same length; and then we must set them a-going exactly together, and then watch the oscillations. You see that as they will be alike in every respect, excepting the size of the pebble stones, whatever difference there is in the mode of vibration will probably be caused by the difference in the size of the stones.”
“Is that the way they do it?” said Rollo.
“Yes,” replied Mary. “Whenever we want to know what effect any one circumstance produces, in such a case, we always arrange two experiments, making them very different in respect to the circumstance which we wish to examine, and as nearly alike as possible in all other respects.”
“I think that is a very good way,” said Rollo.
“Yes,” replied Mary, “I think it is an excellent way.”
While Mary was thus explaining her plan to Rollo, she was going on steadily with preparations, Rollo standing all the time by her side, looking on with great interest. Mary selected two pebbles. One was as big as a walnut, and the other about as big as an egg. She tied two of her threads to these stones, one to each, and then tied the other ends of these threads to a small branch of the tree which extended horizontally over their heads. They hung down about two feet. She took care so to adjust the strings, as to have the centres of the stones as nearly as possible on a level.
“The big one is twice as large, and so it will go twice as fast,” said Rollo.
“We shall see,” said Mary.
She then drew them both carefully out a little way on one side, and holding them there steadily a moment, she let them go. They immediately began to swing back and forth, together.
After a few oscillations, however, the large stone began to gain a little upon the other, and seemed to be moving faster. Presently it had gained half an oscillation, i. e. when the large one was moving forward, the small one would be coming back.
“The big one moves the fastest,” said Rollo.
“Not much,” said Mary.
“No,” said Rollo, “not much.”
“And I don’t think it is owing to the difference in the bigness of the stones.”
“What else can it be?” said Rollo. “They are exactly alike in all other respects.”
“Not exactly,” said Mary. “We have made them as nearly alike as we could, but not exactly. There may be a good many little differences that we do not observe. But if the size of the stone would cause any difference in the vibrations, I should think it would make a much greater difference, for one is twice as big as the other.”
“Let us try a very big stone,” said Rollo.
“Well,” said Mary.
So Rollo got a stone as large as an orange, which was as heavy a one as Mary thought the thread would hold; and Mary suspended that from the branch of the tree, and then swung it in company with the two others. They all went very nearly together at first, though there was evidently a slight difference, which, in a short time, separated the oscillations, so that the stones did not keep together; while yet they each swung back and forth, in nearly the same time. Rollo and Mary both concluded, from the result of this experiment, that the size of the vibrating body did not perceptibly affect the rapidity of the vibrations.
“Now,” said Mary, “we will try different lengths of string.”
So she began to look over Rollo’s pebbles, to find two as nearly as possible alike.
“The pebble stones must be of the same size, this time, for we want the two pendulums to be alike in all respects, except the length of the string, for that is the circumstance which we are now going to consider. We will have one string twice as long as the other.”
Mary found two pebbles very nearly equal in size, and similar in shape. She tied them to two strings, making one string twice as long as the other. She suspended them as before, and then, taking hold of one with one hand, and the other with the other, she drew them out to the same distance on one side, and let them go. The short one began at once to swing back and forth very quick, while the other followed quite slowly.
“That makes a difference,” said Rollo, clapping his hands.
“It goes twice as fast,” said Mary.
“More than twice as fast,” said Rollo, “I think.”
“Let us see,” said Mary.
They set them vibrating again; but they did not succeed in ascertaining whether the short one went more or less than twice as fast as the other. The two motions, so rapid and so near together, confused them. At length, Mary proposed that Rollo should count the vibrations of the long pendulum, while she counted those of the short one, and when she had got up to twenty, she said they would both stop, and then Rollo could tell how many he had got in the same time. But this plan, though apparently a very simple one, they found it somewhat difficult to put into practice. Mary’s pendulum puzzled Rollo’s counting, and Rollo, who could not count very well without at least whispering the numbers, puzzled Mary, and so pretty soon they gave it up.
Rollo then said that he meant to try a very short pendulum indeed, and he asked Mary to tie one up for him, not more than an inch in length. She, however, said that it would not be necessary to tie it to the branch; but, instead of that, she took hold of the string of one of the pebbles which was already hanging before them, about an inch above the pebble itself, and then set the pebble in motion; and they were both very much interested in observing how quick it vibrated to and fro.
Rollo then wanted to try a very long one, and proposed that he should climb up into the tree, and tie the end of the string to a high branch. But Mary was afraid that he would fall; and besides, she said that the pendulum would not swing clear of the branches below. She, however, immediately thought of the chamber window, and said that she would try it there. She accordingly went up into her chamber, taking a large pebble stone with her, and Rollo remained below to set the pendulum in motion, when it should be ready. Mary soon appeared at the window, and Rollo watched her while she tied her pebble to the end of a thread.
“Have you got your thread long enough?” said Rollo. “It will take a good long thread to reach away down here.”
“It is a whole spool of cotton,” said Mary. And, so saying, she held up in her hand the spool, to the thread of which she was tying her pebble stone.
When it was secured, she slowly let it down, until it reached Rollo’s hand, which was held up from below, ready to receive it. Mary then held the thread steady above, at a little distance out from the window, while Rollo took the stone along the side of the house, three or four feet from the place where it would naturally hang. He then let it go, and it swung back very slowly.
“O, how slow!” said Rollo.
“Yes,” said Mary, “it is very slow, indeed.”
“I wish you had gone up to the garret window,” said Rollo.
“O, this will do very well,” said Mary.
Rollo determined to see how many he could count while the stone made one oscillation to and fro. He counted sixteen.
Mary then said she was tired of experimenting, and so she should not come down again. She, however, asked Rollo to set the pendulum swinging, and that then she would draw the thread in, and he could see that it would go faster and faster, the farther she drew it up, for that would make the string grow shorter and shorter.
Rollo did so; and this was the end of the experiments on oscillations.
