1888.] Conditions of a Swarm of Meteorites, $c. 15 



the square of the quasi-hydrostatic pressure, and inversely as the mean 

 velocity of agitation. The temperature of the gases volatilised pro- 

 bably varies by some law of the same nature. 



The path of a meteorite is approximately straight, except when 

 abruptly deflected by a collision with another. This ceases to be true 

 at the outskirts of the swarm, where the collisions have become rare. 

 The meteorites here describe orbits under gravity which are approxi- 

 mately elliptic, parabolic, and hyperbolic. 



In this fringe to the swarm the distribution of density ceases to be 

 that of a gas under gravity ; and as we recede from the centre tlie 

 density at first decreases more rapidly, and afterwards less rapid 'y 

 than if the medium were a gas. 



Throughout all the stages of its history there is a sort of evapora- 

 tion by which the swarm very slowly loses in mass, but this loss is 

 more or less counterbalanced by condensation. In the early stages 

 the gain by condensation outbalances the loss by evaporation, they 

 then equilibrate, and finally the evaporation may be greater than 

 condensation. 



Throughout the swarm the meteorites are to some extent sorted 

 according to size ; as we recede from the centre the number of small 

 ones preponderates more and more, and thus the mean mass con- 

 tinually diminishes with increasing distance. The loss by evaporation 

 falls principally on the small meteorites. 



A meteor swarm is subject to gaseous viscosity, which is greater 

 the more widely diffused is the swarm. In consequence of this a 

 widely extended swarm, if in rotation, will revolve like a rigid body 

 without relative motion (other than agitation) of its parts. 



Later in the history the viscosity will probably not suffice to secure 

 uniformity of rotation, and the central portion will revolve more 

 rapidly than the outside. 



[The kinetic theory of meteorites may be held to present a fair 

 approximation to the truth in the earlier stages of the evolution of the 

 system. But later the majority of the meteors must have been 

 absorbed by the central sun and its attendant planets, and amongst 

 the meteors which remain free the relative motion of agitation must 

 have been largely diminished. These free meteorites the dust and 

 refuse of the system probably move in clouds, but with so little 

 remaining motion of agitation that (except perhaps near 'the peri- 

 helion of very eccentric orbits) it would scarcely be permissible to 

 treat the cloud as in any respect possessing the mechanical properties 

 of a gas.]* 



The value of this whole investigation will appear very different to 

 different minds. To some it will stand condemned as altogether too 

 speculative, others may think that it is better to risk error in the 

 * Added November 23, 1888. 



