320 



KNOWLEDGE & SCIENTIFIC NEWS. 



[January, 1906. 



The DisirvtegroLtion of 

 Saturn's Ring System. 



By W. J. Knight, LL.D. 



In his .statuiard work un " Saturn and its S^ystem," 

 Mr. K. A. I'roctor has so hicidly and convincinjfly dis- 

 cussed the disintegration of the rings of Saturn and 

 the consequent and recent formation of the dark ring 

 that subsequent research has done httle more than 

 ratify the conclusions at which he arrived. It is there- 

 fore' to be regretted that he has not extended his in- 

 vestigations to tlie phenomena of the outer edge of the 

 outer ring, where problems of great beauty present 

 tlicmselvos for our consideration. 



Mr. Proctor remarks that the exterior diameter of 

 the outer bright ring has not perceptibly increased, and 

 suggests a resisting medium as the reason why. I 

 woidd, yyith all respect, prefer to think of the accel- 

 erated satelloids, at the outer edge, being thrown off 

 bodily, like fragments from the rim of a rapidly re- 

 yohing grindstone. The subsequent fate of such a 

 satelloid is full of interest. The paUi it would describe 

 would be that due to the resultant of the three forces 

 acting on it at the moment it started on its separate 

 career, yiz., the force producing its orbital motion, in 

 conjunction with Saturn round the Sun ; the force in- 

 ducing rotation around its primary, and the force of 

 impact. Thispath would lie in a plane but little inclined 

 to- the plane of the rings, but might have any direction 

 in that plane, depending on its instantaneous path at 

 the moment when it left the ring. Dismissing from 

 c.ur consideration all but those trending sunwards, let 

 us follow these tiny planetoids as they trace out their 

 spiral orbits towards the sun. At first inyisible, owing 

 to their small size and great distance, the larger ones 

 become perceptible, when, in the absence of The Giant 

 I'lanet, they succeed in crossing the orbit of Jupiter 

 and are registered in our catalogues as The I^lanetoids, 

 Ceres, Pallas, Juno, Vesta, with hundreds of others of 

 kindred origin. All, however, are not so successful. 

 Attempting the passage near Jupiter itself, the in- 

 fluence of its mighty mass is sufficient to tear them 

 from their independent orbits and to compel them to 

 assume the suljordinate positions of satellites ol Jove. 

 Tile motions of these satellites is direct if they are ar- 

 rested whilst crossing in front of Jupiter, but retro- 

 grade if behind. Possibly the Great Red Spot indicates 

 the fate of one which actually collided with Jupiter 

 itself. -After passing the orbit of Jupiter, the track of 

 the plnnetoids was clear until they approached the 

 orbi; of !\Iars, which thev appear toi have done quite 

 recently, tw'o, Phobos and Deimos, assuming the role 

 of .Marsian Moons, and one. Eras, passing within that 

 orbit and holding out to terrestrial observers the possi- 

 ble acquisition of a second Moon bv the Earth itself. 



.'\s a confirmation of our theory, it should be ob- 

 served that the orbits of all the planetoids have but a 

 .small inclination tO' the plane of the rings, but we have 

 yet to assign the causes, both of the disintegration of 

 the rings and of the spiral form of the planetoidal 

 (irbits. We take these in order. Saturn, though pro- 

 bably the hottest planet of our system, is yet a cooling 

 body, and the rings, from their extreme thmness and 

 great extent of surface, are cooling much faster than 

 the planet. \\'hat wonder then, since the rigours of 

 .■m arctic winter can split up the crystalline rocks of 



Spitzbergen, though shielded by the earih's atmo- 

 sphere—what wonder that the fearful cold of inter- 

 stellar space should crumble up the rings of Saturn ? 

 The wonder would be if it did not. Here, then, we have 

 a simple but adequate explanation of this phenomenon. 

 Let us now investigate the form of the orbits of the 

 satelloids after assuming an independent existence. 

 We shall best dO' so by making our observations from 

 the sun, at a time when the plane of the rings passes 

 through it. A straight line joining the centres of sun 

 and planet would then cut the outer edge of the outer 

 ring in a point occupied by a satelloid, which we will 

 call "Quesita." Just prior to impact, the whole sat- 

 urnian system is moving eastward with a velocity just 

 sufficient to* counterbalance the attraction of the sun, 

 but, as Quesita is in inferior conjunction with Saturn, 

 it appears tO' lag behind the planet in its eastward 

 course, and if now it receives an impact from a follow- 

 ing satelloid, it is detached from the ring, is abandoned 

 by Saturn, and left, with diminished velocity, to pursue 

 its eastward path alone. But, as its velocity before 

 impact was only just enough to balance the attraction 

 of the sun, its velocity after impact is insufficient for 

 that purpose, and so Quesita is drawn sunwards. The 

 amount of this disturbance may be small at first, but, 

 as there is no force tO' increase the orbital velocity of 

 Quesita, and as the sun's attraction increases as the 

 inverse square of the distance, the planetoid is con- 

 tinually deflected from its instantaneous orbit and 

 forced to describe a spiral curve, at first differing little 

 from its original ellipse, but becoming more and more 

 inclined tO' it as the years roll on, until it will finally 

 terminate in the sun itself, just as the spiral orbits of 

 the satelloids of the Crape Ring are rapidly approach- 

 ing the body of the planet. As a further corroboration 

 of O'ur theory, it may be noted that recent observations 

 have shown the necessity of applying " corrections " 

 to the elements of Ceres, Pallas, Juno and Vesta. 

 These corrections being required, not because the 

 former observations were faulty, but because, being 

 applied to an elliptic orbit, they failed to give the 

 position of a body really describing a spiral curve. 

 Still, as these helicoids are not greatly different from 

 ellipses through several revolutions of the planetoid, 

 it may be still convenient to calculate their places by 

 means of the approximate ellipse and then apply cor- 

 rections from time to time. 



CORRESPONDENCE. 



Magnetism of the Sun. 



To THE EorriiRs of " Knowledge." 



Sir, — M. Salet, appointed by the Paris Bureau des Longitudes, 

 to observe the late eclipse, in Algeria, made observations on the 

 magnetic field in the neighbourhood of the sun by observing 

 the amount of deviation in the plane of polarisation of the 

 coronal light. The plane of the bands was found to be deviated 

 in the right-hand direction 2 S. This visual observation was 

 confirmed by photography which showed the plane of polari- 

 sation to be almost radial. 



The sun, therefore, has very little magnetism. 



Magnctisable material can be made a magnet by sur- 

 rounding it with a belt of electricity; but if completely sur- 

 rounded by a sphere of electricity there could be no poles 

 and no magnet. This is nearly the case with the sun, the 

 electrical activity of which is nearly equal over its whole 

 surface, and were it not for the slight increase of electricity 

 which pervades the sunspot bands, it would have no mag- 

 netic force at all. W. Badgley. 



Exmouth, November 24, 1905. 



