Do you not remember the old fable of the oak tree and the pliant reed—how when the storm was about to arise the oak laughed at the poor reed, and said it would never be able to withstand the blasts? But matters did not so turn out. The mighty oak, which would not yield to the storm, was blown down, while the slender reed bent to the wind and suffered no injury. This gives us a hint as to the true constitution of Saturn’s ring; it is not a solid body, trying to resist by mere strength; it is rather to be explained as an excessively pliant structure. Indeed, I ought not to call it a structure at all; it is rather a multitude of small bodies not in the least attached together. I do not know what the size of these bodies may be. For anything we can tell, they may be no larger than the pebbles you find on a gravel walk.
Let us see how we could encircle our earth with rings like those which surround Saturn. I shall ask you to be provided with a sufficiently large number of pebbles, and you must also imagine that I have the means of ascending high up into space, halfway from here to the moon. Suppose I went up there and simply dropped the pebble, of course, it would tumble straight down to the earth again. If, however, I threw it out with proper speed and in the proper direction, I could start it off like a little moon, and it would go on round and round our earth in a circle. I mention a pebble, but really it is little matter what the size of the object may be—it may be as small as a grain of shot or as big as a cannon-ball. Now take another pebble. Cast it also in a somewhat similar path, taking care, however, that the planes of the two orbits shall be the same. Each of these little bodies shall pursue its journey without interference from the other. Then proceed in the same way with a third, a fourth, with thousands and millions and billions of pebbles, until at last the small bodies will become so numerous that they almost fill a large part of the plane with a continuous shoal. Each little object, guided entirely by the earth’s attraction, will pursue its path with undeviating regularity. Its neighbors will not interfere with it, nor will it interfere with them. Let us circumscribe the limits of our flat shoal of moonlets. We first take away all those that lie outside a certain large circle; then we shall clear away sufficient to make a vacant space between the outer ring and the inner ring, and thus the two conspicuous rings have been made; at the inside of the inner ring we shall take out numbers of pebbles here and there, so as to make this part much less dense than the outer portions, and thus produce a semi-transparent crape ring; then we shall clear away those that come too close to the planet, and form a neat inner boundary.
Could we then view our handiwork from the standpoint of another planet, what appearance would our earth present? The several pebbles, though individually so small, would yet, by their countless numbers, reflect the sun’s light so as to produce the appearance of a continuous sheet. Thus we should find a large bright outer ring surrounding the earth, separated by a dark interval from the inner ring, and at the margin of the inner ring the pebbles would be so much more sparsely distributed that we should be able to see through them to some extent. That beautiful system of rings which Saturn displays is undoubtedly of a similar character to the hypothetical system which I have endeavored to describe. No other explanation will account for the facts, especially for the semi-transparency of the crape ring. The separate bodies from which Saturn’s rings are constituted seem, however, so small that we are not able to see them individually. There are some other fine lines running round the rings beside the great division, and these can also be explained by the theory I have stated.
Saturn has other claims on our attention besides those of its rings. It has an elaborate retinue of satellites—no fewer, indeed, than nine; but some of them are very faint objects, and not by any means so interesting as the system by which Jupiter is attended. The ninth of these was discovered quite recently by Professor W. H. Pickering, of Harvard College Observatory. This little moon, for which the name “Phoebe” has been suggested, is further from the planet than any of the others. It is a minute object shining as a star of the 15th or 16th magnitude, and moves around the planet in a period of about sixteen months.
Saturn was the last and outermost of the planets with which the ancients were acquainted. Its path lay on the frontiers of the then known solar system, and the magnificence of the planet itself, with its attendant luminaries and its marvellous rings, rendered it worthy indeed of a position so dignified. These five planets—namely, Mercury, Venus, Mars, Jupiter, and Saturn—made up with the sun and the moon the seven “planets” of the ancients. They were supposed to complete the solar system, and, furthermore, the existence of other members was thought to be impossible. In modern times it has been discovered that there are yet two more planets. I do not now refer to those little bodies which run about in scores between Mars and Jupiter. I mean two grand first-class planets, far bigger than our earth. One of them is Uranus, which revolves far outside Saturn, and the other is Neptune, which is much further still, and whose mighty orbit includes the whole planetary system in its circuit. To complete its journey round the sun not less than 165 years is required.
WILLIAM HERSCHEL.
I have to begin the account of this discovery by telling you a little story. In the middle of the last century there lived at Hanover a teacher of music whose name was Isaac Herschel. He had a family of ten children, and he did the best for them that his scanty means would permit. Of his children William was the fourth, and he inherited his father’s talents for music, as did most of his brothers and sisters. He was a bright, clever boy at school, and he made such good progress in his music that by the time he was fourteen years old he was able to play in the military band of the Hanoverian Guards. War broke out between France and England, and as Hanover was then under the English crown, the French invaded it, and a battle was fought in which the poor Hanoverian Guards suffered very terribly. Young Herschel spent the night after the battle in a ditch, and he came to the conclusion that he did not like fighting, though he was only a member of the band, and he resolved to change his profession. That was not so easy to do just then, for even a bandsman cannot leave the service in war time at his own free will. William Herschel, however, showed all through his life that he was not the man to be baffled by difficulties. I do not know whether he asked for leave, but at all events he took it. He deserted, in fact, and his friends succeeded in sending him away to England.
He was nineteen years old when he commenced to look for a career over here, and certainly he found his prospects in the musical profession very discouraging. Herschel was, however, very industrious; and at last he succeeded in getting appointed as organist of the Octagon Chapel at Bath. He gradually became famous for his musical skill, and had numbers of pupils. He used also to conduct concerts and oratorios, and was well known in this way over the West of England. Busy as Herschel was with his profession, he still retained his love of reading and study. Every moment he could spare from his duties he devoted to his books. It was natural that a musician should specially desire to study the theory of music, and to understand it properly you should know Euclid and algebra, and, indeed, higher branches of mathematics as well. Herschel did not know these things at first; he had not the means of learning them when he was a boy, so he worked very hard after he became a man. And he studied with such success that he made fair progress in mathematics, and then it appeared to him that it would be interesting to learn something about astronomy. After he had begun to read about the stars, he thought he would like to see them, and so he borrowed a telescope. It was only a little instrument, but it delighted him so much that he said he must have one for himself. So he wrote to London to make inquiries.
Telescopes were much dearer in those days than they are now, and Herschel could not give the price that the opticians demanded. Here again his invincible determination came to his aid. What was there to prevent him from making a telescope? he asked himself; and forthwith he began the attempt. You will think it strange, perhaps, that a music-teacher who had no special training as a mechanic should at once commence so delicate and difficult a task; but it is not really so hard to make a telescope as might be imagined. The amateur cannot make such a pretty-looking instrument as he is able to buy at the shops—the tubes will not be so beautifully polished and the finish will be such as a trained workman would be ashamed of—but the essential part of a telescope is comparatively easy to make; at least, I should say of a reflecting telescope, which is the kind Herschel attempted to make, and succeeded in making. You must know that there are two kinds of telescopes. The commoner one with which you are more familiar is called the refracting telescope, and it has glass lenses. It was an instrument constructed on this principle that we spoke of in a former lecture ([p. 97]). The reflecting telescope depends for its power upon a bright mirror at the lower end, and when using this instrument you look at the reflection of the stars in the mirror. It was a reflector like this that Herschel began to construct, and he engaged in the task with enthusiasm. His sister Caroline had come to live with him, and she used to help him at his work. So much in earnest was he that he used to rush into his workshop directly he came home from a concert, and without taking off his best clothes he would plunge into the grinding and polishing of his mirrors. His sister tried to keep the house as tidy as possible, but Herschel put up a carpenter’s shop in the drawing-room, and turning-lathes in the best bedroom. At last he succeeded. He made a mirror of the right shape, and found that it exhibited the stars properly. It was not a looking-glass in the ordinary sense, with glass on one side and quicksilver on the other. The mirror that Herschel constructed was entirely of metal. It consisted of a mixture of two parts of copper with one of tin.
