214 SEE— RESULTS OF RECENT [April 23. 



ingly we may generally ignore the effect on the planet, and consider 

 only the effect on the satellite. It is thus clear how the satellite 

 drops down near these centers of attraction, and is finally captured 

 by one of them. 



(f) This is how all the satellites of the solar system were cap- 

 tured. At first they moved principally under the attraction of the 

 sun, and could pass from the sun's to the planet's domain, through 

 the neck of the hour-glass shaped space connecting the two spheres 

 of influence. When the neck is narrow, Darwin says that a particle 

 which passes from the sun's to the planet's control may revolve 

 about it hundreds of times before quitting the planet's sphere to 

 return again to the sun's control. And if resistance is meanwhile 

 encountered, so that the neck of the surface of zero velocity be- 

 comes closed, it is clear that the particle never will quit the sphere 

 of the planet's control, but will abide there permanently as a 

 satellite. 



(v) Thus it incontestably follows that the satellites of Jupiter, 

 Saturn and other planets formerly moved about the sun, and since 

 they were captured have had their orbits reduced in size and rounded 

 up under the secular action of the resisting medium formerly per- 

 vading our solar system. Satellites may cross over the line SJ before 

 coming completely under the planet's control, in zvhich case they 

 zmll move retrograde. In such cases the neck connecting the two 

 spaces is extremely narrow. But as the neck usually is not so 

 narrow as to produce crossing satellites, most of them naturally 

 move direct, in accordance with observation. This is the reason 

 also ivhy the planets have direct rotations on their axes. The planets 

 have in no case been inverted, as some have recently supposed, in 

 order to account for the retrograde motions of the outer satellites of 

 Jupiter and Saturn. 



(^) In the case of the terrestrial moon it is shown that the earth 

 simply captured one of the twenty-seven million such planets which 

 went to form the sun's immense mass. The moon came to us from 

 the depths of space, and never was a part of the earth, as has long 

 been supposed. Professor Sir G. H. Darwin's celebrated work of 

 1879 is shown to be based on chance coincidences, and not actual 



