36 GEOLOGY. 



But there are many binary, triple, multiple, and clustered systems of 

 suns which apparently divide the control of a common field, and this 

 control may reasonably be supposed to involve approaches of sufficient 

 nearness to one another to seriously perturb their outlying secondaries, 

 and introduce disturbances ultimately involving disruptive approaches. 

 The nebulous matter associated with some of these complex systems 

 perhaps implies something of this kind. 



Relations of meteorites to comets. — The hypothesis of disruption 

 by differential attraction goes one step farther in postulating that the 

 disrupted group of fragments may, in its earlier history, constitute a 

 comet, since it is the general belief of astronomers that the comet's 

 head is composed of a cluster of small bodies. The peculiar emanations 

 which arise from a comet may perhaps be referred as plausibly to 

 the occluded vapors and the radio-active substances of a shattered 

 asteroid, as to any other recognizable source. The recent discoveries 

 of the prevalence of radio-activity and allied phenomena render the 

 cometic emanations less strange and exceptional than they once seemed. 



The fragments of an asteroid, or other small body, disrupted in this 

 manner would be given a rotatory movement by the differential attrac- 

 tion that produced them, and hence the resulting cluster of fragments 

 should revolve about its center of gravity in a somewhat definite plane, 

 but at the same time in more or less irregular and inharmonious paths 

 as the result of the incidents of disruption, and this must render them 

 subject to mutual disturbance, and to frictional and glancing collisions. 

 If later conditions permitted a re-aggregation, a highly brecciated mass 

 would result. 



It is now accepted as highly probable that comets, particularly those 

 that have short orbits and frequently return to the vicinity of the sun, 

 are gradually dispersed by the latter's differential attraction. The 

 mutual gravity of the cometic fragments being very small, the differ- 

 ential gravity of the sun, in its own neighborhood, becomes superior 

 to it, and the members of the cometary cluster are drawn apart, and 

 thenceforth revolve about the sun in their own individual orbits, irre- 

 spective of the other members. In other words, the swarm of mete- 

 oritic fragments that constitutes the comet's head passes into the 

 planetesimal state by dispersion. In this we have an instance of that 

 tendency of a swarm to pass into a planetesimal condition, to which 

 allusion has heretofore been made. 



