^' 



Prof. Peirce on the Constitution of Saturn's Ring. 



107 



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d 



hibit Ihe possibility of this strange phenomenon, but they even 

 show the precise mode of action and how it mast be the case of 

 nature. If the ring had been originally one, it would soon have 

 subdivided itself at definite points, which can be exactly compu- 

 ted, into portions of a determinate width. The disturbing causes 

 must, however, drive these separate rings, sooner or later, against 

 each other. There must then follow an interchange and crossing 

 of currents, a mutual retardation, a momentary state of equiUb- 

 rmm as one ring and then another was breaking up, when the 

 same process would be repeated in endless succession. 



3. But even a fluid ring would not be permanently retained 

 by the direct action of the primary. For whatever may be its 

 figure, the velocity of its current must be slower at the points 

 which are more remote from the planet; so that there must be 

 an accumulation of fluid at these points. An exact analysis 

 shows that the accumulation precisely balances the greater dis- 

 tance, and hence the ring is attracted equally in every direction. 

 The regulating action upon the motion of the center of gravity 

 is, therefore, cancelled; and it must continue to move uniformly 

 m any direction in which it may have been started by a foreign 

 influence. It must move on until it is destroyed by striking the 

 surface of the planet. How has Saturn's ring escaped this cat- 

 astrophe? Simply, because the disturbing forces have counter- 

 acted their own effects- The satellites are constantly disturbing 

 the ring; but, in the. very act of perturbation, they are sustain- 

 ii^g it in its place. Their sustaining action is'not negative, but 

 positive ; and without satellites there can be no ring. The the- 

 ory (rf this curiously sustaining power may be variously illus- 

 trated: In the first place, each particle of the ring may be re- 

 garded as a satellite, which the other satellites disturb in the 

 usual way, 



in the least, and the disturbance of the eccentricity can 



reach certain definite limits, after attaining which it must dimin- 

 ish, ^ " - ' - 



Thus the mean distance from Saturn is not varied 



only 



Secondly, in consequence of the attraction of its satellites, 

 Saturn describes an orbit about the common center of gravity of 

 the system ; each particle of the planet tends to move in this 

 same orbit. The center of gravity of the ring must, likewise, 

 tend to describe nearly this same orbit, and its orbit would be 

 precisely the same if the attraction of the ring for the satellites 

 '«^ere the same as if its mass were accumulated at its center of 

 gravity. But the deviation may be safely neglected and referred 

 to the class of periodical perturbations! 



4. It follows, then, that no planet can have a ring, unless it is 

 surrounded by a sufficient number oi properly arranged satellites. 

 paturn seems to be the only planet, which is in this category ; and 

 Jt IS the only one, therefore, which could sustain a ring. Our 

 «un, also, does not appear to have its satellites properly disposed 



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