16 GEOLOGY, 



1. Assemblage from a Primitive State of Dispersion. 



Conditions affecting the initiation of a meteoritic swarm. — The start- 

 ing of a meteoritic swarm, theoretically, from dispersed meteorites 

 moving in diverse directions, as typical meteorites do, encounters a 

 grave obstacle in the high ratio of the kinetic energy of the meteorites to 

 their mutual gravitation. Since gravitation is the agency of assemblage, 

 and the kinetic energy of their motion is the dispersing agency, it is 

 obvious that if the potency of the latter is superior to that of the former, 

 the conditions for gathering meteorites into a swaim are adverse, at 

 least in general. The relations between the kinetic energy of meteorites 

 and their mutual gravitation therefore require consideration as a basal 

 feature. 



The velocities of meteorites. — While the velocities of meteorites 

 have been only approximately ascertained, between those that have 

 been directly measured and those that have been estimated from inci- 

 dental facts, they are sufficiently well known for the purposes of general 

 discussion. The meteorites which are seen in the evening strike the 

 earth from behind, and since the earth is moving in its orbit at an 

 average rate of 18.5 miles per second, these meteorites must overtake 

 it with a velocity sufficiently in excess of this to become incandescent 

 in passing into the atmosphere. From present data, the average 

 velocity of meteorites, when they encounter the earth's atmosphere, is 

 estimated to be between 20 and 30 miles per second. Occasionally 

 the velocity appears to reach 40 or 50 miles per second, and even more. 

 The velocities of meteorites seem to be of the same order as the veloci- 

 ties of the stars, whose average has recently been placed by Newcomb 

 at 23 miles per second. This of course is merely the average of those 

 that have been thus far measured, and will doubtless be changed by 

 additional measurements; but this velocity may be taken as roughly 

 representative of the velocities of stars and of meteorites in this part 

 of the sidereal system. 



Now to give to the meteoritic hypothesis the best possible condi- 

 tions, let all the matter of the sidereal system be supposed to be con- 

 verted into meteorites, and let these meteorites have directions of motion 

 and velocities identical with the parent bodies. It seems necessary 

 to assign them these motions to preserve the moment of momentum 

 of the system as the laws of mechanics require. We have thus a kind 



