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would get 3 turns each. Then Saturn, Jupiter, Mars would finish up the day, and the Sun's turn would come in the first hour of the next day. In Fig. 43 let us show that the Sun follows Saturn by joining up Saturn to the Sun by a line, missing two planets (Jupiter and Mars). Then to find what planet will rule the next day we miss Venus and Mercury and join to the Moon: miss two and join to Mars, and so on. If you follow round the star in the direction of the arrows you will find the days of the week in the right order, Saturnday, Sunday, Moonday, Mardi, Mercredi, Jeudi, Vendredi. I have begun with Saturday to make the explanation simpler; but you can see that one may begin anywhere; and the Sun is so important that they began the week with him.

Of course, when Uranus was found they could not upset so well-established an order as the days of the week to make room for him in it; and a very good thing it is they did not try, because there would have been another upset half-a-century later when Neptune was found.

That was a discovery even more remarkable than the discovery of Uranus, because it was made by calculation. We have already said that when a planet has been watched long enough, we can calculate just where it will be in the future; we must take into account, not only the attraction of the Sun, but that of other planets as well, since they are all pulling at one another. Though this is hard and toilsome work it can be done, and was done for the planet Uranus when it had been watched long enough. But the curious thing was that Uranus did