o rush up hill.

After examining this for some time, Mary moved the cork float, with the
needle upon it, back into the middle of the basin, and then left it to
itself. It slowly moved around until it pointed to the garden gate, as
it had done before.

"Now what is the reason?" said Rollo; "that isn't north."

Mary looked upon it very attentively for a few minutes in silence, and
then said, suddenly,

"O, I see."

"What?" said Rollo.

She did not answer, but pointed down to the platform by the side of the
basin.

Rollo looked where she pointed, and saw the hammer lying there. He had
had it to play with a short time before, and, when he brought the basin
of water, he had laid it down by his side.

"What?" said Rollo.

"The hammer attracts the needle," replied Mary.

"The hammer?" said Rollo.

"Yes," replied Mary. "Don't you know that iron attracts the needle, and
it will not point to the north if there is any iron near to draw it
away?"

Rollo was just going to take the hammer up, but Mary stopped him,
saying,

"Wait a moment. Let me take it away slowly, and see the effect."

So Mary told Rollo to watch the needle, while she carefully drew the
hammer away.

Rollo did so. He and Mary both watched the needle. It was pointing
pretty nearly toward the hammer, and when Mary gently moved the hammer
away, the needle, released from the influence which the iron exerted
upon it, slowly moved back towards the direction of the barn, that is,
the direction of a north and south line, which is called the meridian.

"It's going back! it's going back!" said Rollo.

Mary said nothing, but watched it carefully. The needle swung beyond the
direction of the meridian a little way, and then came slowly back
again. So it continued vibrating from one side to the other, though to a
less and less distance every time. Finally, it came to a state of rest;
but it was not then, however, exactly in the meridian.

"What makes it swing so, back and forth?" said Rollo.

"I don't know exactly," said Mary. "I suppose the force that it moves
with, carries it a little beyond, and then it is drawn back again, and
that makes the oscillations."

"Oscillations?" said Rollo, inquiringly.

"Yes. They call this swinging back and forth, _oscillating_; and each
movement is an oscillation."

"Is that the name of it?" said Rollo.

"Yes," said Mary. "When you tie a little stone to a string, and hold the
upper end of the string still, and le