ASTRONOMY. 345 



a spheroidal shape and consisted, at the epoch when \re 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 r be the distance of any point from the 



center, the force is central and follows the law ar -\ — ^ wheje, 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 periodic 

 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 a in our formula for the force diminishes and the b increases, 

 but orbits which are circular still retain that form, notwithstanding 

 the i^rogressive 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 ot 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 process as that in 

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

 theory in 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 

 Professor Darwin himself. He has shown that the hypothesis that tidal 

 friction has had free play in the past leads to a remarkable quantitative 

 co-ordination of tte several demerits of the earth's rotation and of the 

 moon's orbital motion, and points to tiie genesis of the moon close to 

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

 satellite of Mars confirms in a remarkable waj' the truth of the by- 



