ASTRONOMY, 345 



a spheroidal shape and consisted, at the epoch when we began to watch 

 it, largely of separate meteorites. It is at first supposed that the sphe- 

 roidal aggregate consists of matter pretty nearly equally distributed, 

 and later a nucleus is formed. If rbe the distance of any point from the 



center, the force is central and follows the law ar + — -- where, in the 



beginning of the evolutionary process, b is very small, and later a be- 

 comes small. Initially, then, when the force is simply as the distance, 

 each meteorite moves in an ellipse about the center, and the i)eriodic 

 time of all is the same, whatever their eccentricity of orbit. In con- 

 sequence of collisions, a central nucleus is soon formed ; as this in- 

 creases, the o in our formnla for the force diminishes and the h increases, 

 but orbits which are circular still retain that form, uotwithstaudiug 

 the progressive change in the law of force. At the same time that the 

 nucleus is being formed, a series of flat and nearly circular rings arise 

 around it, those near to the nucleus attaining a definite shape sooner 

 than the remote ones. It is not adequately explained why the mat- 

 ter should be sifted, and should arrange itself in rings at definite in- 

 tervals around the nucleus, still less is any light thrown on the law 

 of Titius concerning the distances of the planets from the sun. Con- 

 sidering now the case of the first ring, M. Faye supposes that slight 

 differences ol angular velocity, mutual attraction, and collisions grad- 

 ually cause the aggregation of all the matter in the ring around some 

 center in its line. When the nucleus is small, the rings moved as a 

 rigid whole, and the linear velocity of the outer meteorites was greater 

 than that of the inner ones; therefore when the planetary aggregate is 

 formed, it will be found rotating with direct motion about an axis nearly 

 perpendicular to the plane of its orbit. As we proceed from the first 

 ring outwards, in each successive case the tendency to direct motion is 

 weaker, because the increase of the solar nucleus by absorption of me- 

 teorites has prevented so large an excess of linear velocity of the outer 

 meteorites over those of the inner ones as in the first case. By degrees, 

 therefore, we come to planets in which the meteorites move nearly ac- 

 cording to Kepler's laws, and here the resulting planet has a retrograde 

 rotation. Each planetary agglomeration in its turn forms a miniature 

 solar system, and generates satellites by the same ])rocess as that iu 

 which the planets were formed. After having thus sketched M. Faye's 

 theory iu its main outlines. Professor Darwin points out that no refer- 

 ence is made to the possible effect of tides in the evolution of the solar 

 system, a part of the subject which has been so ably worked out by 

 I'rofessor Darwin himself. He has shown t hat the hypothesis that tidal 

 friction has had free i)lay in the past leads toaremaikable quantitative 

 co-ordination of Hie several elements of the earth's rotation and of the 

 moon's orbital motion, and ])oints to the genesis of the moon close to 

 the present surfiice of the earth. The rapid orbital motion of the inner 

 satellite of Mars confirms in a remarkable way tlic^ truth of the hy- 



