ve you come to make the oscillations?"

"Yes," said Mary. "I have brought some thread for strings, and I want
you to get some pebble stones--some large, and some small ones."

Rollo went for the pebble stones, while Mary looked about for a suitable
place for making the experiments. In a corner of the yard there was a
bench under a tree, and the branches came down pretty low. Mary thought
that this would be a good place, for she could tie her strings to these
branches with the pebbles hanging down below; and she and Rollo could
watch the oscillations, while seated upon the bench.

Mary took her station here, and Rollo presently appeared, with the crown
of his cap half filled with pebble stones. Mary said they would do
finely. She poured them out upon the bench by her side, and Rollo put
his cap upon his head again.

"Now, Rollo," said she, "we will study the art of experimenting."

"No," said Rollo, "we are going to study oscillation."

"Yes," replied Mary; "the experiments are to be on oscillations; but
what I want principally to teach you, is, the proper way to make
experiments."

"Well," said Rollo.

Mary said no more, but she proceeded to tie a small pebble to the end of
one of the long threads which she had brought out with her. Then she
tied the other end of the thread to the branch of the tree, which was
over her head. The pebble then hung down before them, so that both Rollo
and herself could plainly see all its motions.

"The first thing," said Mary, "is to get a clear idea of the nature of
the oscillation, for we must know what we are experimenting about."

So saying, Mary carefully took hold of the suspended pebble stone, and
began to draw it off towards one side. She showed Rollo that, as it was
confined by its string above, it must move in a curved line when she
drew it away from its place, rising higher and higher the farther it was
drawn away. And when she had drawn it out to a considerable distance, to
one side, it was at a much higher level, than when it hung down freely
in its natural position.

"Now," said Mary, "you see that if I let it go, it will descend of
course as much as it can, for the earth draws it downwards."

"The earth draws it?" said Rollo.

"Yes," said Mary. "The reason why things fall is that they are
attracted, or drawn down, by the earth. Now the earth draws the pebble.
It _would_ go straight towards it, if it could; but the string confines
it, and so it can only go dow