26 



HARD WICKE ' S S CIE NCE- G O SSI P. 



succession, from hoop-shaped to quoit-shaped, each 

 new ring formed being thicker than its predecessor. 

 The different systems of satellites offer a similar 

 contrast; thus in the! Jovian system there was a 

 regular increase in the thickness of the genetic rings 

 from the outermost to the innermost moon. The 

 thickness of the genetic rings appears also to have 

 determined the rate of rotation of each planet on its 

 own axis ; for, other things being equal, a genetic ring 

 which is broadest in the direction of its plane will 

 produce a mass rotating faster than one that is 

 broadest at right angles to its plane. Accordingly 

 we find that Jupiter and Saturn — which originated 

 from relatively quoit-shaped rings, notwithstanding 

 their enormous size — each rotate in about ten hours ; 

 while all the inner planets, originating from relatively 

 hoop-shaped rings, rotate with much less rapidity. 

 In considering the size of the planets it must not be 

 forgotten that the solar mass was continually growing 

 smaller and denser. This would further account for 

 the great difference between the sizes of the outer and 

 inner planets. 



The gravitative force of Jupiter is more than twice 

 that of all the other planets put together, so that the 

 thinnest and weakest of all the rings which started 

 comparatively close at hand, had very little chance 

 of forming into one planet. The gravitative force of 

 Jupiter is more than sufficient to have dragged the 

 pieces of the ring as it broke up so far out of their 

 normal path, that all hope of ultimate concentration 

 was hopelessly gone ; and accordingly, instead of one 

 planet, this ring formed the Asteroids — a collection of 

 planets. 



The attraction of Jupiter, however, does not account 

 for the great inclination of some of the asteroids to 

 the elliptic, and this circumstance, as I have already 

 said, at present remains inexplicable. A simple 

 mechanical consideration will show that the detach- 

 ment of a moon-forming ring from a contracting 

 planet depends on the excess of centrifugal force over 

 gravity at its equator, and it is therefore evident that 

 rings will be detached in greatest numbers from those 

 planets in which the centrifugal force bears the 

 greatest ratio to gravitation. Such planets ought to 

 have the greatest number of moons ; and such is 

 the case. Of the four inner planets, which rotate 

 . comparatively slowly, and in which the centrifugal 

 -.force is therefore small, only the earth and probably 

 Mars are known to have a satellite.* But Jupiter, 

 whose centrifugal force is twenty times greater than 

 that of any of the inner planets, has four satellites. 

 Uranus, with still greater centrifugal force, has at 

 least four, and probably more. And finally, Saturn, 



* Though it is now generally acknowledged that Mars has 

 ,t\vo very small moons, the writer considers that at present 

 sufficient is not known of them to enable him specially to 

 deal with them. The smaller and inner one appears to be 

 just outside "Roche's limit," and the period of its revolution 

 is less than one-third of the period of the rotation of its 

 primary. 



in which the centrifugal force is one-sixth of its 

 gravity, being nearly fifty times greater than in the 

 earth, has at least eight moons, besides three partly 

 broken rings. Mr. Spencer declares that this 

 emphatic agreement of observation with deduction is 

 an unanswerable argument in favour of the nebular 

 theory. He says, here where the dynamic relations 

 involved are so simple that we have no difficulty 

 in tracing them, the significance of the result is 

 unmistakeable. 



The Saturnian system is altogether exeptional and 

 very fascinating, well worthy of a great deal more 

 attention than there is time to give it in this article. 

 Saturn, the least dense and second largest planet, 

 weighs ninety-one times as much as the earth, but 

 being light as cork, it is six hundred and ninety 

 times the volume of the earth, and is nine times as 

 great in circumference. It is encircled by two 

 bright, quoit-shaped rings, together about 58,000 

 miles in width, and one dark inner ring under 

 18,000 miles in width. The thickness of the rings 

 is uncertain, but probably from 200 to 300 miles. 

 The rings probably consist of brickbats, or meteor- 

 ites. This conclusion is the result of a series of 

 calculations and observations too interesting to be 

 passed over in silence. Every satellite is subject 

 both to centrifugal and centripetal forces which tend 

 to elongate the satellite to an egg-shape. This 

 effect is well known in the case of our moon, which is 

 drawn out and now rotates only once during its 

 entire revolution round the earth, so that the same 

 side of the moon's surface is always turned towards 

 the earth. These opposing forces would drag a 

 planet to pieces as soon as they became greater than 

 the internal force of gravity which the satellite 

 possesses and which keeps its particles together. 

 Roche determined in 1848 that the greatest possible 

 elongation of which a fluid body is capable without 

 being broken up, is when the ratio between the 

 longest and shortest diameter is 1000 to 469 ; also 

 that the centre of such a satellite must be at least a 

 distance from the planet's centre of 2 '44 of the 

 planet's radius. This distance is termed "Roche's 

 limit." Now, the distance of the outside of Saturn's 

 ring system from the planet is 2*38 times the mean 

 radius, so that the rings are entirely within Roche's 

 limit ; hence Roche concluded that the ring is broken 

 and consists of fragments, and that no planet can be 

 formed in that position. In 1S51 Bond concluded, 

 from observation, that the ring consisted of frag- 

 ments, and in 1857 Clerk Maxwell came to the same 

 conclusion by calculation, and constructed a model, 

 which is in the laboratory at Cambridge, to exhibit 

 the movements of the pieces of the rings (which he 

 described as a mass of brickbats), now crowding 

 together in one place, now in another, making 

 almost waves as the centre of the crowded parts 

 moves along in the direction of rotation. Maxwell 

 also showed that a spreading of the rings both 



