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."

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