about 10 hours; so that this must be about the length of time in which Jupiter rotates. But now comes a curious thing, quite different from what we should expect. If the Martians are looking at the Earth, they see the markings on the surface go round to the other side and then reappear again in exactly 24 hours. Everything comes round in precisely the same time, England and Ireland and China and Australia, the mountains, and the seas, all keep their places on the Earth, just as they do on a globe of the Earth which I can turn round. If they did not keep their places we could not learn geography, which would be a terrible distress to us all. But when we look at Jupiter the spots do not keep their places: some of them come round faster than others, so that we cannot make a map which will do for more than a very little time. It is as though the Martians noticed the positions, not of England or China or Australia, but of the ships crossing our oceans. They are scarcely likely to do this unless they have very much more powerful telescopes than ours, because you have seen how very small even a whole country like England would appear to them, and a ship would be far too small. But we keep building ships bigger and bigger, and perhaps the Martians might see them at last; and if they watched carefully they would find that after 24 hours exactly the ship was not quite in the same place on the Earth's surface. Shall we, then, say that the spots we see on Jupiter are ships moving on its oceans? It seems more probable that they are cloud effects of some kind; all we know is that we are not looking at solidly arranged countries like our Earth; and if we want to know what we are
