ch. v.] PLANETARY EVOLUTION. 375 



rings would prove a formidable nuisance. Mr. Proctor has 

 shown that, in latitudes corresponding to that of New York 

 and Naples, they cause total eclipses of the sun, which last 

 seven terrestrial years at a time. But the problem which 

 natural theology thus fails to solve, is completely solved by 

 a very simple mechanical consideration. Since the detach- 

 ment of a moon-forming ring from a contracting planet 

 depends on the excess of centrifugal force over gravity at its 

 equator, it is evident that rings will be detached in greatest 

 numbers from those planets in which the centrifugal force 

 bears the highest ratio to gravitation. Such planets will have 

 the greatest number of moons. And such, in fact, is the case. 

 Of the four inner planets, which rotate slowly, and in which 

 the centrifugal force is therefore small, only the earth is 

 known to have a satellite. 1 But Jupiter, whose centrifugal 

 force is twenty times greater than that of any of the inner 

 planets, has four satellites. Uranus, with still greater cen- 

 trifugal force, has at least four, and probably six or eight 

 moons. And finally Saturn, in which the centrifugal force is 

 one-sixth of gravity, being nearly fifty times greater than on 

 the earth, has at least eight moons, besides his three unbroken 

 (or partly-broken) rings. Mr. Spencer may well declare that 

 this emphatic agreement of observation with deduction is an 

 unanswerable argument in favour of the nebular theory. 

 Here, where the dynamic relations involved are so simple 

 that we have no difficulty in tracing them, the significance of 

 the result is unmistakeable. Where we are enabled thus 

 directly to put the question to Nature, there is no ambiguity 

 in her answer. 



In the quoit-shaped rings which girdle Saturn, we have 

 a curious vestige — upon the significance of which Kant 

 strongly insisted — of the ancient history of our planetary 



1 It is not improbable that Venus may have a satellite also. Several astro- 

 nomers have declared that they have seen such a. satellite ; but as their testi- 

 mony seems difficult to reconcile with that of other astronomers, equally 

 competent as observers, the question must remain an open one for the present. 



