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."