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About one hundred years ago there was an unpopular professor in the Government Polytechnique School of Paris, still the great school of mathematics for the French public service, who loved to get his students into difficulties. One morning he addressed one of them the question:

"Monsieur, have you ever seen the moon?"

"No, sir," replied the student, suspecting a trap.

The professor was nonplussed. "Gentlemen," said he, "see Mr. ———, who professes never to have seen the moon!"

The class all smiled.

"I admit that I have heard it spoken of," said the student, "but I have never seen it."

I take it for granted that the reader has been more observant than the French student professed to be, and that he has not only seen the moon, but knows the phases through which it goes and is familiar with the fact that it describes a monthly course around the earth. I also suppose that he knows the moon to be a globe, although, to the naked eye, it seems like a flat disk. The globular form is, however, very evident when we look at it with a small telescope.

Various methods and systems of measurement all agree