t the stone swing back and forth,
it makes oscillations."

"I mean to try it," said Rollo.

"Yes," said Mary; "and I will help you by and by, after I have studied
my philosophy lesson."

"Your philosophy lesson?" asked Rollo. "Have you got a philosophy lesson
to get?"

"Yes," said Mary, "in that great book."

So Rollo took up Mary's book, which she had laid down upon the platform
near Rollo's apparatus. He found that the title of it was "Intellectual
Philosophy."

"Intellectual Philosophy?" said Rollo; "and what sort of philosophy is
_intellectual_ philosophy?"

"It is the philosophy of the _mind_," replied Mary. "It explains to us
about the thoughts and feelings of our minds."

"Are there any _experiments_ in intellectual philosophy?" asked Rollo.

"Yes," said Mary, "we can try experiments in intellectual philosophy."

"What experiments?" said Rollo.

"Why, there is a question whether we always dream when we are asleep."

"_I_ do," said Rollo, "every night."

"Yes, but perhaps not all night long."

"Yes, I do," said Rollo. "I have good long dreams."

"But," replied Mary, "you may dream several hours in the night, so as to
remember good long dreams in the morning, and yet perhaps you might have
been, for some time, perfectly sound asleep, so as not to have any
dreams in your mind at all. Some persons think we dream all the time
when we are asleep, and others think we don't dream all the time. Now we
might contrive some experiments to decide the question."

"How?" said Rollo.

"Why, you and I might agree to wake each other up several times, from a
sound sleep, and then, if we were dreaming at that time, we should
probably remember it."

"Well," said Rollo, "let us try it."

"That would be an experiment in intellectual philosophy," said Mary.

Rollo determined to try the experiment; and then he took Mary's book,
and asked her where her lesson was that day. She found the place, and
Rollo read a little. He could not understand it very well, and so he
concluded that he would rather have Mary go and study her lesson, and
then come down and help him make the experiments of oscillation.

Mary accordingly took her book and went in, and left Rollo at his play.

OSCILLATIONS.

In about an hour, Mary came down into the yard in pursuit of Rollo, in
order to try the experiments which she had proposed.

When Rollo saw her coming, he left his play, and ran to meet her.

"Well, Mary," said he, "ha