ake a pendulum so that it would oscillate
faster or slower, just as we pleased."

"A pendulum?" asked Rollo.

"Yes," said Mary, "we call it a _pendulum_. Any heavy body hung in this
manner, so as to swing back and forth by its weight, is called a
_pendulum_. So that we are experimenting upon the oscillations of a
pendulum."

"Yes," said Rollo, "I understand."

"Now the question which we are going to examine," said Mary, "is, what
the rapidity of the vibrations depends upon."

"O, it depends upon the bigness of the pebble," said Rollo.

"How do you know?" said Mary.

"Why, of course, a bigger pebble will be heavier, and will fall quicker,
and that will make it vibrate faster."

"That is _reasoning_ about it," said Mary, "and what we want to do, now,
is to _experiment_. Now, in order to decide it by experiment, we must
try two pendulums, one with a small pebble, and the other with a large
one."

"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_