XLVIII. Rest and Motion. Parallelogram of Forces. Law of Fall- XLIX. Kepler's Second Law proved. Centrifugal Tendency. L.-Attracting and Attracted Bodies considered separately. INTRODUCTION. GENERAL NOTIONS. THE USES OF ASTRONOMY. I. AT night, if the sky be cloudless, we see it spangled with so many stars, that it seems impossible to count them; and we see the same sight whether we are in England, or in any other part of the world. The earth on which we live is, in fact, surrounded by stars on all sides; and this was so evident to even the first men who studied the stars that they pictured the earth standing in the centre of a hollow crystal sphere, in which the stars were fixed like golden nails. 2. In the daytime the scene is changed: in place of thousands of stars, our eyes behold a glorious orb whose rays light up and warm the earth, and this body we call the sun. So bright are his beams that, in his presence, all the "lesser lights," the stars, are extinguished. But if we doubt their being still there we have only to take a candle from a dark room into the sunshine to understand how their feeble light, like that of a candle, is “put out" by the greater light of the sun. 3. There are, however, other bodies which attract our attention. The moon shines at night now as a crescent B and now as a full moon, sometimes rendering the stars invisible in the same way as the sun does, though in a less degree, and showing us by its changes that there is some difference between it and the sun for while the sun always appears round, because we receive light from all parts of its surface turned towards us, the shape of the bright or lit-up portion of the moon varies from night to night, that part only being visible which is turned towards the sun. 4. Again, if we examine the heavens more closely still, we may see, after a few nights' watching, one, or perhaps two, of the brighter "stars" change their position with regard to the stars lying near them, or with regard to the sun if we watch that body closely at sunrise and sunset. These are the planets; the ancients called them "wandering stars." 5. But the planets are not the only bodies which move across the face of the sky. Sometimes a comet may by its sudden appearance and strange form awaken our interest and make us acquainted with another class of objects unlike any of those which we have previously mentioned. 6. Such are the celestial bodies ordinarily visible to us. Far away, and comparatively so dim that the naked eye can make little out of them, lie the nebulæ, so called because in the telescope they often put on strange cloud-like forms ; they differ as much from stars in their appearance as comets do from planets. 7. There are other bodies, to which we shall refer by and by. But we will, in this place, content ourselves with stating generally what Astronomy teaches us concerning star and sun, moon and planet, comet and nebula. 8. To begin then with the stars. So far from being fixed, and being stuck as it were in a hollow glass globe, which state of things would cause all to be at pretty nearly equal distances from us, they are all in rapid motion, and their distances vary enormously; although all of them are so very far away that they appear to us to be at rest, as a ship does when sailing along at a great distance |