70 GEOLOGY. 



able that it would have been moderate throughout most of the period 

 of aggregation, and certainly so in the declining stages of infall. 



The bearing of the mode of accretion on the direction of planetary 

 rotation. — We have now reached a point where the hypothesis can be 

 tested. It happens to be a point where all hypotheses of this class 

 have been supposed to be fatally at fault. The crucial feature lies 

 in the direction of rotation which would result from the gathering-in of 

 matter in this way. At the same time, the bearing of the discussion 

 broadens, for this vital question of direction of rotation attaches to 

 all forms of aggregation of independent bodies moving in orbits about 

 the common center of a system. For example, if the evolution of 

 the solar system be supposed to start with a gaseous spheroid of the 

 Laplacian type, and to proceed in the manner postulated by Laplace 

 until the planetary rings were formed, and if then the velocities of 

 the molecules, resulting from mutual impact, carried them beyond the 

 gravitative control of the rings so that they were scattered and re- 

 volved independently around the central mass, the hypothesis of 

 their aggregation would be as much subject to the test of rotation as 

 the special hypothesis now under consideration. So, too, if, instead 

 of forming definite rings, the molecules were separated from the sup- 

 posed gaseous spheroid, one by one, as seems more probable than 

 separation by rings, their aggregation is equally open to the supposedly 

 fatal weakness. So indeed is the concentration of any kind of an 

 assemblage of discrete matter in which the individual molecules or 

 aggregates revolve independently. 



The lion in the way — and he appears to have turned back many a 

 pilgrim — is as follows: In a ring revolving as a unit, as the Laplacian 

 rings are supposed to have done, the outer part moves faster than the 

 inner part; and so, if a planetary ring parts at its weakest point and 

 gathers into a globe about the center of its cross-section, it will rotate 

 forward, as shown in Fig. 27. If, on the other hand, the particles of 

 the ring revolve independently, the inner ones must move faster than 

 the outer ones, and if they collect about the middle part, it has been 

 held that the* rotation must be retrograde, as illustrated in Fig. 28. 1 



By way of exception, to meet the singular cases of Uranus and 

 Neptune, it has been suggested that if the matter of the planetary 



1 For ampler statements of this difficulty, see Faye, " Sur l'Origine du Monde," 

 pp. 165, 270-281, 1896. Also Young's General Astronomy, pp. 518-20. 



