Part 30

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 let 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."