“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.”