ORIGIN OF THE WORLD 



tesimals" because they are supposed to be revolving 

 in independent orbits, like miniature planets. Hence 

 the name "planetesimal theory." 



It is obvious at a glance that the larger nuclei 

 bigger fragments of world stuff make up the struc- 

 ture of the spiral arms. It should be explained that 

 matter is not streaming along these arms as might 

 be supposed, but that the entire structure is revolving 

 as if it were a solid body. The larger nuclei, however, 

 necessarily exert a gravitational influence over the 

 smaller planetesimals in their neighborhood; hence 

 an incessant shower of smaller particles will fall 

 against each larger nucleus and this augments its size 

 and its gravitational power. 



As time goes on, each of these growing nuclei will 

 (through gravitation) suck in the matter from the 

 space about it, as a vacuum cleaner sucks in dust, un- 

 til ultimately each larger body will be revolving in a 

 clear space. 



Thus the myriads of planetesimals will have been 

 aggregated into a small number of planets; and the 

 spiral nebula will have been developed into a planet- 

 ary system. The original central nucleus of the 

 nebula, having drawn to itself the cloud of minor 

 planetesimals in its neighborhood, becomes a de- 

 tached central sun. 



According to this theory, then, our earth, in com- 

 mon with its sister planets, was never a gaseous ring, 

 nor yet a liquid globe ; but was built up about a more 

 or less solid nucleus by a perpetual meteoric bom- 

 bardment. 



Larger planets of our system may have gathered 

 2 9 



