FUNDAMENTAI, PROBLEMS OF GEOLOGY. 207 



ing cluster of fragments should revolve about their common 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 these doubtless render them subject to 

 mutual disturbance and frictional and glancing collisions. 



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 

 differential 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, irrespective of the other members. In other words, the 

 cluster of fragments that is supposed to constitute the comet's head 

 passes into the planeteshnal state by dispersion. In this we seem to 

 have an actual instance of that tendency of a swarm to pass into a 

 planetesimal condition to which allusion has heretofore been made. 



These planetesimals constitute one variety of meteoroidal bodies 

 in the broader sense of the term meteoroidal, and it is to these that 

 the brilliant August and September meteoric showers are assigned. 

 It has not been quite demonstrated that they are identical with the 

 iron and stony meteorites above described, for they do not generally 

 reach the earth, and it is not positively known that they have done 

 so in any case, but their essential identit}* is extremely probable. 

 In the fact that they have come to have individual orbits about the 

 sun, and that these orbits are parallel to one another, and that their 

 velocities are of the same order, they do not represent the typical 

 meteoritic condition as heretofore defined. They illustrate rather 

 the planetesimal mode of organization. 



The foregoing hypothesis of the origin of meteorites makes them 

 but an incidental result of stellar and planetary action. If this be 

 correct, their genesis is wholly a secondary matter, and furnishes no 

 ground for regarding meteorites as the parent material of great 

 nebulae or of stellar systems. The quantity of matter dispersed in 

 this way is, by the terms of the hypothesis, limited to an extremely 

 small part of the total mass of the systems from which it is derived. 

 This scattered matter is presumed to be picked up individually by all 

 the larger bodies, as is being done daily by the earth, and the main- 

 tenance of the supply only requires the disruption of small bodies to 

 an extent equal to the trivial masses gathered in by the existing suns 

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