The attraction on the surface of the sun is also very great--so great that we can hardly picture it to ourselves. If life exists there at all--supposing it possible that any kind of life can be in such a fiery atmosphere--it must be life very different from any known in this world. A man who on earth weighs one hundred and sixty pounds, and walks lightly erect, would, on the sun, lie helplessly bound to the ground, crushed by his own overpowering weight. It is said that a cannon-ball, reposing on the sun, if lifted one inch and allowed to fall, would dash against the ground with a speed three times greater than that of our fastest express-trains. For weight on earth is merely caused by the amount of force with which the earth draws downward a body towards herself--a force greater or less according to the density of that body. So weight on the sun would be immensely increased by his immensely greater power of attraction. [Illustration: COMPARATIVE SIZE OF THE EARTH AND SUN.] It is an interesting question how far the sun’s attractive influence reaches effectually through space. The nearer a body is to the sun, the greater the attraction which he exercises over it. At the distance of the planet Mercury, a speed of twenty-nine miles each second is needful to overcome or balance it sufficiently for the planet to remain in his orbit. At the distance of the planet Neptune, about three miles each second is enough. If a planet were journeying at four times the distance of Neptune, the speed would need to be not over two miles each second, lest the planet should break loose and wander away. But even two miles a second is no mean speed--more than seven thousand miles an hour. If we come to speak of that which we on earth call rapid motion, we shall gain a clearer idea as to the extent of the sun’s power. Suppose a planet were traveling through space at the rate of one of our fastest express-trains--sixty miles an hour. It has been calculated that, unless the sun’s attraction were interfered with and overpowered by some nearer sun, the said planet, though placed at a distance ten or twelve times as great as that of the far-off star Alpha Centauri, would still be forced by the sun’s attraction to journey round him in a closed orbit. At such a speed it would not be free to wander off into the depths of space. CHAPTER XIV. MORE ABOUT THE MOON. From a globe all fire, all energy, all action, we come to a globe silent, voiceless, changeless, lifeless. So, at least, the moon seems to us. But it will not do to speak too confidently.