“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.”