38 



HISTORICAL GEOLOGY 



matter occupied the spaces between the arms. Such a distribution 

 of matter in a spiral shows that the form could not have been 

 maintained by gaseous pressure, as in the Laplacian hypothesis, 

 but rather by the movements of the separate particles or masses. 

 Since these particles are thought to have moved like miniature 



planets, they are called 

 planetesimals. Each 

 planet esimal is con- 

 sidered to have moved 

 in its own orbit around 

 the central mass. The 

 planetesimals did not 

 move along the arms 

 of the spiral, but 

 rather crossed them at 

 considerable angles 

 (Fig. 20). "When we 

 see a spiral we do not 

 see the paths which 

 the separate masses 

 have described, but 

 the positions which 

 they occupy at the 

 time. In the present 

 case (Fig. 20) if a 

 smooth curve is drawn 

 through the regions 

 where the matter is 

 densest, it will form a 

 sort of double spiral 

 as represented by the 

 full lines" (Moult on). 

 The dotted lines in 

 the figure represent orbits of some of the particles or knots. Due 

 largely to crossing of orbits, the knots increased in size by a gather- 

 ing in or accretion of the planetesimals. Meteors, which now strike 

 the earth, are thought to be planetesimals still gathering in, though 

 very slowly at present. The spiral orbits of the knots (planets^ 

 gradually gave way to the elliptical orbits due to a sort of wrapping 

 up process around the central attracting body (sun). 



Fig. 20 



Diagram to illustrate the formation of a spiral 

 nebula, *S, sun; S', passing star whose direc- 

 tion of motion is indicated by the arrows. 

 The numbered dotted lines show the paths 

 followed by masses pulled out from S by S'. 

 The straight dotted lines are paths which the 

 rupted masses would have followed had S' 

 remained stationary in the respective posi- 

 tions indicated. (Modified after Moulton 

 by W. J. M.) 



