248 THE MAJOR PLANETS. 



This apparent mystery has however been most clearly and beautifully ex- 

 plained by Biot, to whom the happy idea occurred that the rings could be 

 brought under the same laws of motion as moons. To make this explanation 

 clearly understood, let us first imagine a globe like the moon moving period- 

 ically round the planet like the earth. The manner in which the attraction of 

 gravitation combined with centrifugal force causes it to keep revolving round 

 the earth without falling down upon it by its gravity on the one hand, or 

 receding indefinitely from it by the centrifugal force on the other is well 

 understood. In virtue of the equality of these forces, the moon keeps con- 

 tinually at the same distance from the earth while it accompanies the earth 

 round the sun. Now it would be easy to suppose another moon revolving by 

 the same law of attraction at the same distance from the earth. It would re- 

 volve in the same time, and with the same velocity, as the first. We may ex- 

 tend the supposition with equal facility to three, four, or a hundred moons, at 

 the same distance. Nay, we may suppose as many moons placed at the same 

 distance round the earth as would complete the circle, so as to form a ring of 

 moons touching each other. They would still move in the mame manner and 

 with the same velocity as the single moon. Nor will the circumstances be 

 altered if this ring of moons be supposed to be beaten out into a thin flat ring 

 like those of Saturn. It is plain, then, that if the ring of Saturn revolve in its 

 own plane round the planet in the same time as that in *hich a single satellite 

 placed at the same distance would revolve, the stability of the ring with refer- 

 ence to the planet is explicable exactly upon the same principles as those by 

 which we explain the motion of a satellite. But Sir William Herschel, as has 

 been already stated, discovered the important fact that the rings do move round 

 their own centre and that of the planet in the same time that a satellite placed 

 at the same distance would do. Biot, therefore, has, with a happy adroitness, 

 adopted this as the key to the explanation of the stability of the ring. 



The following observations of Sir John Herschel on the rings indicated 

 another cause of their stability : 



Although the rings are, as we have said, very nearly concentric with the 

 body of Saturn, yet recent micrometical measurements of extreme delicacy have 

 demonstrated that the coincidence is not mathematically exact, but that the 

 centre of gravity of the rings oscillates round that of the body describing a 

 very minute orbit, probably under laws of much complexity. Trifling as this 

 remark may appear, it is of the utmost importance to the stability of the sys- 

 tem of the rings. Supposing them mathematically perfect in their circular 

 form, and exactly concentric with the planet, it is demonstrable that they would 

 form (in spite of their centrifugal force) a system in a state of unstable equilib- 

 rium, which the slightest external power would subvert not by causing a rup- 

 ture in the substance of the rings but by precipitating them, unbroken, on the 

 surface of the planet. For the attraction of such a ring or rings cm a point or 

 sphere eccentrically situate within them, is not the same in ill directions, but 

 tends to draw the point or sphere toward the nearest part of .e ring, or away 

 from the centre. Hence, supposing the body to become, from any cause, ever / 

 so litt.e eccentric to the ring, the tendency of their mutual gravity is, not to > 

 correc; but to increase this eccentricity, and to bring the nearest parts of them 

 together. Now, external powers, capable of producing such eccentricity, exist 

 in the attractions of the satellites ; and in order that the system may be stable, 

 and possess within itself a power of resisting the first inroads of such a ten- 

 dency, while yet nascent and feeble, and opposing them by an opposite or 

 maintaining power, it has been shown that it is sufficient to admit the rings to 

 be loaded in some part of their circumference, either by some minute inequality 

 of thickness, or by some portions being denser than others. Such a load 



