Rollo's Experiments

“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.