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.”