How can we possibly tell what the stars are made of? If we think of the vast oceans of space lying between them and us, and realize that we can never cross those oceans, for in them there is no air, it would seem to be a hopeless task to find out anything about the stars at all. But even though we cannot traverse space ourselves, there is a messenger that can, a messenger that needs no air to sustain him, that moves more swiftly than our feeble minds can comprehend, and this messenger brings us tidings of the stars--his name is Light. Light tells us many marvellous things, and not the least marvellous is the news he gives us of the workings of another force, the force of gravitation. In some ways gravitation is perhaps more wonderful than light, for though light speeds across airless space, it is stopped at once by any opaque substance--that is to say, any substance not transparent, as you know very well by your own shadows, which are caused by your bodies stopping the light of the sun. Light striking on one side of the earth does not penetrate through to the other, whereas gravitation does. You remember, of course, what the force of gravitation is, for we read about that very early in this book. It is a mysterious attraction existing between all matter. Every atom pulls every other atom towards itself, more or less strongly according to distance. Now, solid matter itself makes no difference to the force of gravitation, which acts through it as though it were not there. The sun is pulling the earth toward itself, and it pulls the atoms on the far side of the earth just as strongly as it would if there were nothing lying between it and them. Therefore, unlike light, gravitation takes no heed of obstacles in the way, but acts in spite of them. The gravitation of the earth holds you down just the same, though you are on the upper floor of a house, with many layers of wood and plaster between you and it. It cannot pull you down, for the floor holds you up, but it is gravitation that keeps your feet on the ground all the same. A clever man made up a story about some one who invented a kind of stuff which stopped the force of gravitation going through it, just as a solid body stops light; when this stuff was made, of course, it went right away off into space, carrying with it anyone who stood on it, as there was nothing to hold it to the earth! That was only a story, and it is not likely anyone could invent such stuff, but it serves to make clear the working of gravitation. These two tireless forces, light and gravitation, run throughout the whole universe, and carry messages of tremendous importance for those who have minds to grasp them. Without light we could know nothing of these distant worlds, and without understanding the laws of gravity we should not be able to interpret much that light tells us.