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Rh chief thoughtfully; "for I have seen bits of hard substance come from the rocks of our fireplaces. But I fear you could not teach us to make these wonderful machines."

"I'm afraid I could n't," I replied, regretfully.

"There's one thing I want to ask you," the chief said eagerly; "and that is about the tides. Sometimes the water is high and then it is low. Do you know what makes the tides?"

Now that was a question I ought to have been able to answer. I knew it had something to do with the moon, and faint memories of the words perigee and apogee came into my mind. But so vague were my ideas that I could n't make it clear to myself, and so I thought it wise to tell the plain truth. I said I did n't know.

"At times the sun turns black," said the chief. "Why is that?"

"The moon gets in front of it," I answered, glad of an opportunity to make any kind of a reply.

"But the moon is n't black," he said.

"No, but it looks so," I said. "The moon has no light of her own. She looks bright only because the sun lights her."

"We know that," he said, "for the light on the edge of the moon is always toward the sun. But how often does the sun turn black?"

"I don't know," I was forced to confess.

"Why does n't it happen oftener?"

This was worse than a school examination. I made up my mind to end it.

"Chief," I said, "if I have not shown learning, at least I have learned my own ignorance. I am going to go back to my own time, if I can (and I think I can, for my wish was only to stay a while), and when I do get back there I'm going to know some of those things you asked me about. I'm going to know them all through. Then, if I can, I'm going to come back and teach you many things."