1889.] on Meteorites and the History of the Stellar Systems. 381 



properties to the whole in this supposed case, the nebiiLa would a fortiori 

 possess those properties when it had shrunk to smaller dimensions. 

 The swarm was supposed to be arranged in a perfect sjihere, and 

 what may be described as the layers of equal density of population 

 were taken to be concentric spheres, but the density of population 

 would necessarily be much greater towards the middle than towards 

 the outside. 



The whole crowd of stones would arrange itself automatically into 

 a steady condition, in which the population had no tendency to shift, 

 although, of course, the dance and collisions between the constituents 

 of the crowd would be incessant. When this steady condition was sub- 

 mitted to calculation, it was possible to discover the average velocity 

 of the stones, the average density of population, and the average 

 frequency of collision at each point of the swarm. 



It will naturally occur to the reader to inquire as to the source 

 of the great velocity of the stones ; it arose from gravitation, the 

 stones having fallen in from a great distance towards a centre of 

 aggregation. 



If somewhere in space there were an aggregation of meteorites, 

 and if a stone were released from a state of rest at a very great dis- 

 tance, it would fall towards the swarm under the influence of gravi- 

 tation. On reaching the swarm it would have acquired a certain 

 velocity, and would penetrate to some uncertain distance, until it 

 happened to strike another stone. Henceforth its path would be 

 zigzag, as it happened to strike, and it became incorporated as a 

 member or molecule of the swarm. 



The supposed visitant from outside space imported energy of 

 motion into the swarm, and besides increased the total mass of the 

 swarm. Thus, if it be imagined that the swarm is increased by the 

 addition of stone after stone, each being let fall from a distance, it 

 is clear that, in the course of accretion, the energy of agitation of the 

 meteorites continually increases. When stones have ceased to fall 

 in, the materials of the nebula were collected, and by means of incessant 

 collisions the swarm gradually attained the steady condition above 

 referred to. 



By reasoning of this kind it was possible to discover how fast the 

 stones were moving, but it is proper to add that an important correction 

 had to be applied to allow for the fact that at each collision between 

 two stones some speed is lost. In the process of settling down into 

 the steady condition, each stone loses, by imperfect elasticity, three- 

 tenths of the speed it would have if it were a fresh arrival from space. 



It makes no material difference in the result by whatever process 

 the stones were collected together, and the account given above of the 

 formation of a swarm was not intended as a contribution to its history, 

 but was only meant to explain the mechanical principles involved. 



By this line of argument it may be concluded that when the solar 

 swarm extended half as far again as the planet Neptune, the average 

 velocity of the stones was three miles a second. 



2 D 2 



