230 A CENTURY'S PROGRESS IN ASTRONOMY. 



were urged by various investigators. Among 

 these was the retrograde motions of the satellites 

 of Uranus and Neptune, and the extremly rapid 

 revolution of the inner satellite of Mars. Other 

 objections were urged by Babinet, Kirkwood, and 

 others, and at length a sweeping reform of the 

 nebular theory was proposed by Faye in 1884, 

 in his work, ' Sur 1'Origine du Monde/ Faye put 

 forward the idea that all the planets interior 

 to the orbit of Uranus were formed inside the 

 solar nebula, while Uranus and Neptune came 

 into existence after the development of the Sun 

 was far advanced. But the objections to Faye's 

 theory are formidable, and the hypothesis has 

 not been accepted. 



A popular exposition of the nebular theory 

 was given in 1901 in Ball's work on 'The 

 Earth's Beginning/ He exhaustively discusses 

 the whole question, and explains the retrograde 

 motion of the satellites of Uranus and Neptune 

 as due to the fact that the planes of the orbits 

 of the satellites will eventually be brought to 

 coincide with the ecliptic. These motions, says 

 Ball, do not disprove the nebular theory. " They 

 rather illustrate the fact that the great evolu- 

 tion which has wrought the Solar System into 

 its present form has not finished its work : it 

 is still in progress." 



