But in dealing with Uranus we come to another of those startling problems of which astronomy is full. So far we have dealt with planets which are more or less upright, which rotate with a rotation like that of a top. Now take a top and lay it on one side on the table, with one of its poles pointing toward the great lamp we used for the sun and the other pointing away. That is the way Uranus gets round his path, on his side! He rotates the wrong way round compared with the planets we have already spoken of, but he revolves the same way round the sun that all the others do. It seems wonderful that even so much can be found out about a body so far from us, but we know more: we have discovered that Uranus is made of lighter material than the earth; his density is less. How can that be known? Well, you remember every body attracts every other body in proportion to the atoms it contains. If, therefore, there were any bodies near to Uranus, it could be calculated by his influence on them what was his own mass, which, as you remember, is the word we use to express what would be weight were it at the earth's surface; and far away as Uranus is, the bodies from which such calculations may be made have been discovered, for he has no less than four satellites, or moons. Considering now the peculiar position of the planet, we might expect to find these moons revolving in a very different way from others, and this is indeed the case. They turn round the planet at about its Equator--that is to say, if you hold the top representing Uranus as was suggested just now, these moons would go above and below the planet in passing round it. Only we must remember there is really no such thing as above and below absolutely. We who are on one side of the world point up to the sky and down to the earth, while the people on the other side of the earth, say at New Zealand, also point up to the sky and down to the earth, but their pointings are directly the opposite of ours. So when we speak of moons going above and below that is only because, for the moment, we are representing Uranus as a top we hold in our hands, and so we speak of above and below as they are to us. It was Herschel who discovered these satellites, as well as the planet, and for these great achievements he occupies one of the grandest places in the role of names of which England is proud. But he did much more than this: his improvements in the construction of telescopes, and his devotion to astronomy in many other ways, would have caused him to be remembered without anything else. Of Uranus's satellites one, the nearest, goes round in about two and a half days, and the one that is furthest away takes about thirteen and a half days, so both have a shorter period than our moon.