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suppose that our tides are tending to turn our Earth over again, though the action is so slight and so slow that we cannot detect it by direct observation.

The mention of our own tides reminds us that we have shamefully neglected our own Moon. We have been visiting other planets' satellites and neglecting our own child all the time. Jules Verne and H. G. Wells were very different: they spent all their attention on the Moon. Let us look first at some pictures of the Moon's surface, which is very mountainous. Now-a-days we can get such pictures by photography very easily, but before we had this great help it took a long time to make an accurate picture. A little more than a century ago a great artist, John Russell, R.A., spent about twenty years making a careful drawing which is still preserved at the Radcliffe Observatory at Oxford; we can now get as good a picture by photography in a second or less. Perhaps some of the fine detail would not be as good, for the human eye still beats the photograph in drawing fine detail; but the more conspicuous features would be more faithfully in their exact places. A photograph is wonderfully accurate and faithful, as can be proved by taking two photographs with different telescopes—say one in Paris and one in Chicago—and comparing the results. A great English astronomer whom we have recently lost, Mr. Saunder, had the most careful measures made on two such pictures and showed that they agreed extraordinarily well. Not only that, but he mapped out the surface of the Moon so accurately that in some ways we know it better than that of our own Earth. He gave one very striking proof of the accuracy of