Suppose that you stood on Saturn at his equator, and looked up at the mighty ring which would stretch edgewise across your sky. It would rise up from the horizon on one side, and, passing over your head, would slope down to the horizon on the other. You would, in fact, be under an arch of which the span was about 100,000 miles. Owing to the attraction of Saturn, every part of that structure would be pulled forcibly towards his surface, and thus the materials of the arch, if it were a solid body, would be compressed with terrific force.
It does not really signify that the arch I am now speaking of is half of a ring the other half of which is below the globe of the planet. That is only a difference with respect to the support of two ends of the arch, and does not affect the question as to the pressure upon its materials; nor does the fact that the ring is revolving remove the difficulty, though it undoubtedly lessens it. We know no solid substance which could endure the pressure. Even the toughest steel that ever was made would bend up like dough under such conditions. We cannot, therefore, account for Saturn’s ring by supposing it to be a solid, for no solid would be strong enough.
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.