ADVANCE OF ASTRONOMY. 221 



equatorial regions. This ring afterwards broke up, 

 but its parts condensed to form the furthest planet. 

 With further shrinkages and accelerations of the 

 parent nebular mass, the various planets were thrown 

 off in succession, themselves to repeat the process 

 in forming rings like Saturn's, or satellites like those 

 of Jupiter. 



One of the chief reasons which led Laplace to 

 think out a possible unity of origin for the solar 

 system, was that the planets and their satellites 

 revolve and rotate in the same direction as that in 

 which the sun rotates, — a coincidence of many (40 or 

 more) motions which almost suggests a common 

 origin. We now know that the satellites of Uranus 

 and l^eptune move in the opposite direction, and that 

 there are other exceptions, e.g., that the inner 

 Martian moon revolves in a shorter time than Mars, 

 to the uniformity which Laplace proposed; on the 

 other hand we know that there are many more in- 

 stances of uniformity of motion than he was aware 

 of. 



There are many other sets of facts which favour 

 the general idea of the nebular hypothesis. Thus 

 we have a rapidly increasing mass of information 

 in regard to the nebulae which Herschel was the 

 first to begin to study in earnest, some of which look 

 like the primeval nebula which Laplace postulated, 

 while others present appearances suggestive of 

 systems in process of being made. The great IJ^ebula 

 in Andromeda, as photographed by Roberts, 

 " suggests," as Huggins, says, " a stage in a succes- 

 sion of evolutional events not inconsistent with that 

 which the nebular hypothesis requires." 



That the same substances occur (as the spectro- 

 scope proves) in sun and planets is another fact which 



