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.”