14 GRAVITATION. [CHAP. only body pulled, and the only body that seems to move. We stand firmly on the earth because it pulls us, and quite unconsciously our bodies pull it; and when we jump up we exert ourselves against this mutual pull, which soon brings us back to the surface. What we call the weight of a body, such as a log of wood, is really the pull of the earth and the log towards each other, which pull we feel when we try to raise it and find it to be heavy. This is felt everywhere on the surface of the earth, and is very nearly the same (but not exactly the same) on every part of it.1 The power of pulling, which is exerted by everything that has weight, is called gravitation. And it is not only things on, or near, the surface of the earth that thus gravitate or are drawn towards it. The moon, although at so great a distance that it looks like a moderate sized ball, whereas it is in reality 2,153 miles across or the size of our earth, is attracted by the earth, and attracts it while it moves round it. Indeed it is because it is pulled towards the earth, that it remains always at nearly the same distance, and does not fly away from it into distant space. A simple experiment that everyone can try for himself will help to make this clear. Take a stone or a lump of clay, tie it up in a piece of cloth, and fasten it to the end of a string, two or three yards long: then, holding the other end of the string, stand in a clear space and whirl the stone rapidly round you, turning round yourself at the time. You will then feel that the string is pulled by the moving stone, and if it is thin and weak, it will break, and the stone will fly off. So the moon would fly off from the earth if it were not pulled towards it. There is, it is true, no visible link or bond between the two, like the string in our experiment, but neither is there any visible connection between the earth and a ball that is falling towards it, and yet evidently they attract each other. In the same way, the sun, which is much further off from us than the moon, and nearly one 1 In India it is a little less than in England, and in England less than in Greenland; while on the top of the Himálaya it is less than on the plains of Bengal. I.] THE SUN. 15 and a quarter million of times larger than the earth, attracts both the earth and the moon; and with such force, that if they were not both in rapid motion round the sun, they would fall into it and be first melted, and then in great part turned into vapour by his heat, which far exceeds that of the hottest fire we can make. And this leads us to say something about the sun, that glorious luminary, whose warmth and light are so grateful to all living things, and without which indeed no living thing on the earth's surface could continue to live for the space of two days, so needful are they to life itself. What is the sun? A few years ago we should have been unable to answer this question nearly so well as we can now. Its distance is so immense that if we could leave the earth and travel towards it, at the rate of a railway train at full speed, say thirty miles in an hour, it would take us 338 years to reach it; it is so large that if we were on its surface and were to travel round it at the same rate of speed, we should be more than nine years completing the journey; whereas, if we were to make a similar journey round our earth, we should come back to our starting point in about one month. And as to its heat, we all know that if we make a good sized bonfire, and stand ten or twelve yards off, we no longer feel the warmth given out by the flame; whereas, notwithstanding his enormous distance, the heat of the sun is sometimes so powerful that we can hardly bear it. By a method which was discovered only a few years ago, the light of the sun has been made to reveal to us that that luminary consists of materials for the most part like those of our earth. Iron, for instance, is one of them, but it is not solid nor even melted iron, at all events on the outside of the sun, but is rather vapour, that is, it is in much the same condition as the air we breathe, and it is kept in this condition owing to the intensity of the heat. Besides iron, there are many other substances, some of them metals, and ali like it in a state of vapour. There are liquid matters beneath these, but we cannot tell very well what they are. We have already seen in the introductory chapter, that the 16 THE EARTH'S MOTION. [CHAP. heat that reaches us from the sun, evaporates water from the surface of the sea. We shall see afterwards that the winds which bring us rain are set in motion entirely by the sun's heat. Plants will not grow unless they receive both heat and light, and we ourselves should be able to do very little without the light of the day, and we should soon perish of cold if deprived of the sun's genial rays. We must now turn our attention to one very important variation in the effect of the sun upon our earth, viz., the alternation of day and night. When we see the sun rise in the morning, and after ascending till almost over our heads, descend and set in the west, our first impression is that the sun really moves past us, and that we ourselves are at rest. But we have learned in our discussion of the shape of our earth to be cautious in trusting to first impressions, and that to arrive at the truth, we must test our first conclusions by further observation, and then compare together our several observations and impressions, correcting one by the other. The present is a case in which it becomes necessary to do so. The question which we have here to ask ourselves is this:—can we ourselves be in motion without being sensible of it? and, secondly, if this be so, and if we are moving past an object really stationary, will the appearance be the same as if the object were moving, and we ourselves at rest? A little experience will show that both these questions must be answered in the affirmative. If a person in a boat at anchor in the middle of a very wide river sees another boat drifting past him with the current, and if he does not actually know from previous observation that his own boat is anchored, he is unable to say whether his own or the other boat is in motion. All he can be sure of is, that one is moving past the other, but he cannot tell without looking at the rope which he knows to be attached to the anchor, whether his own boat is stationary and the other moving, or the former moving and the latter stationary, or finally whether both are not moving in opposite directions. So again, in a railway station at which two trains are standing, the observer being 1.] DAY AND NIGHT. 17 in one of them. If either train begins to move off gently, it is impossible to tell by merely looking at the passing carriages, whether they or the observer's own carriage is moving; until he either feels the rattling motion of the carriage, or looks at the station shed which he knows to be at rest. But the earth does not shake or rattle in moving, and the only mode of judging whether the sun or the earth is the moving body is to look at the stars. And they, too, will tell us nothing, unless we know or believe that like the station shed of the railway they do not move. What we then see is, that if it is the sun that moves, the stars must move too, and at nearly the same rate, for all seem to go round in the same way; and therefore we can only conclude that either the earth is turning round from west to east, and presenting every side in succession to the sun and to each particular star, or else that the earth is stationary, and all the heavens moving round it from east to west. So we have only arrived at this result: that we require a great deal more observation to decide the question. This astronomers have made for us with great care, and the result is, that it is proved that the earth turns round like a spinning top, and the sun, relatively to us, is almost stationary.' Every side of the earth is in turn presented to the sun and is illuminated by it, producing day, while it is night on that side which is turned away from the sun. The ball of white clay that we used a short time ago to illustrate the appearance of the moon may now be used to represent our earth. Thrust a wire or thin stick through the middle of it, make an ink mark on the ball a little above the middle to represent India, and then holding the stick a little inclined, turn 1 There is a little instrument called a Gyroscope, resembling a boy's top, by which it may be proved to the eye that the earth moves, and not the sun or the stars. It is made to spin very rapidly on a kind of moveable support, and it is a known property of this instrument that the axis of the spinning top keeps pointing in the same direction, however the support may be moved. Now, if it be set spinning pointing to a particular star, it will remain pointing to that star, although the star may seem to have moved some distance across the sky. The star then has not moved, but the earth has. B. G. с 18 THE SOLAR SYSTEM. [CHAP. it round slowly before the lamp. The marked spot will be lighted up while it passes in front of the lamp, representing day, and will be in darkness, while passing on the opposite side, representing night. The earth revolves once in 24 hours, or in one day and one night, around a line that passes through its centre. This line is termed its axis. In the clay ball or in a model globe, the axis is represented by the wire or stick thrust through it, and the two points at which it protrudes from the ball are called the poles. But in our earth this axis is merely an imaginary line, the ends of which are called the earth's poles; in free space, the earth requires no support; for there is nothing but gravitation, and the motion it already has, to make it go one way rather than another; and these, conjointly, make it travel round the sun, in much the same way as the moon travels round the earth. The earth travels round the sun once in a year, while the moon travels round the earth once in a lunar month. If either of them left a train of light behind it to show where it had passed, and some one could see this from one of the stars, it would look like a ring; that of the earth being very much larger than that of the moon. The sun would be seen in the middle of the ring traced by the earth, and the earth would be in the middle of that traced by the moon. Moreover, the moon and its ring would be seen to move round the sun together with the earth, the moon being always at nearly but not quite the same distance from the earth, and the earth at nearly but not quite the same distance from the sun. Other balls like our earth, one of them thirteen hundred times larger, and having four moons, and another with a peculiar ring round it, and accompanied by eight moons, would be seen at different distances from the sun, all moving round it in the same direction as the earth. As we see them from the earth, these bodies look like stars, except that they move among the real stars; but they are very different from the stars and are called planets. The brightest of them all can be seen at one time of year in the west for |