"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."