Page:Popular Science Monthly Volume 51.djvu/372

360 they are not liquid, for no liquid could be suspended in such a manner without being precipitated eventually upon the surface of the planet. Nor are they solid; for it has been demonstrated that no solid could hold together under such strains, such tremendous forces, as the attraction of the monster planet would subject it to; it would soon be broken up entirely.

The only supposition remaining is that it is composed of myriads of solid particles—a ring of dust and fragments of rock and stone. In this case we may imagine it as being an immense swarm of tiny moons or satellites, each revolving in its own particular path around the planet, and the aggregation presenting to us at this distance the appearance of a solid mass. Of course, the word "tiny" must be taken in an astronomical sense, which would not preclude one of these "dust" particles or fragments from being as large as a house, or even a mountain.

That the ring is composed of solid matter of some kind is proved by the fact that it reflects the sunlight which it receives, apparently unchanged in quality, and deprives of sunlight those portions of the planet on which its shadow falls. But here comes the question, If we know the ring is composed of solid matter, how do we know that it is in the form of dust and fragments? This question was long a stumbling-block, but, as Prof. George Darwin points out, the investigations of M. Roche, a French mathematician, seem to have solved the difficulty.

Briefly, the reasoning is as follows: We know that our moon always keeps the same face toward the earth, but perhaps it is not so generally known that the cause of this is in the moon's own shape, which is that of an egg with the longer diameter pointing toward the earth. Not that this egg shape is so very pronounced, but it is sufficient to keep the moon from rotating as the earth does, and to keep its longer diameter pointed toward the seat of that force which holds our satellite in its path.

The cause of this egg shape is simply in what is termed the "tide-generating force." The moon's effect upon the earth due to this force is rendered noticeable and well known in our tides. The earth also exerts the same force upon the moon, only, as the former is eighty times more massive, the effect is correspondingly greater, and the moon's globe has suffered under the strain has—been pulled out of shape, so to speak.

Now this force of course increases as its source is approached, and were the moon brought nearer and nearer the earth, a point might finally be reached where the solid materials of which she is composed could no longer hold together, and her globe would be torn to pieces by the tremendous forces to which she would be subjected. To determine this point was the problem which M. Roche solved, and his conclusions led him to place it at a