228 A CENTURY'S PROGRESS IN ASTRONOMY. 



observation or calculation." Laplace noticed 

 that in the Solar System all the planets re- 

 volved round the Sun in the same direction, 

 from west to east, and that the satellites of 

 the planets obeyed the same law. He also 

 observed that the Sun, Moon, and planets 

 rotated on their axes in the same direction as 

 they revolved round the Sun ; also that the 

 planets moved round the Sun, and the satellites 

 round their primaries, in almost the same plane 

 as the Earth's orbit, the plane of the ecliptic. 

 It was evident that these remarkable congruities 

 were not the result of chance, and accordingly 

 Laplace expressed his belief that the Solar 

 System originated from a great nebula, which 

 in condensing detached various rings in the 

 process of rotation. These rings condensed into 

 the various planets and their satellites. 



Laplace's theory was powerfully supported by 

 Herschel's observations of the various nebulae 

 in the heavens. But, with the supposed resolu- 

 tion of the various nebulae after the erection of 

 the Rosse reflector in 1845, the evidence in 

 favour of the nebular theory seemed to be 

 greatly reduced. In 1864, however, the dis- 

 covery of the gaseous nebulae, by means of the 

 spectroscope, gave further support to the theory. 

 Powerful aid was lent to the nebular hypoth- 



