442 THE PRINCIPLES OF SCIENCE. 



in the production and dissipation of heat, the energy of 

 which must be drawn, partially at least, from that of the 

 moving bodies. This effect is probably identical, as I 

 have suggested (vol. ii. p. 213), with the very evident loss 

 of energy of comets attributed to a so-called resisting 

 medium. But whatever be the theoretical explanation of 

 these phenomena, it is almost certain that there exists a 

 tendency to the dissipation of the energy of the planetary 

 system, which will in the indefinite course of time result 

 in the fall of the planets into the sun. 



It is hardly probable, however, that the planetary 

 system wiU be left undisturbed throughout the enormous 

 period of time required for the dissipation of its energy 

 in this way. Conflict with other bodies is so far from 

 being improbable, that it becomes approximately certain 

 when we take very long intervals of time into account. 

 As regards cometary conflicts, I am by no means satisfied 

 with the negative conclusions drawn from the remarkable 

 display on the evening of the 2/th of November, 1872. 

 We may often have passed through the tails of comets, 

 which are probably electrical manifestations no more 

 substantial than the aurora borealis. Every remarkable 

 shower of shooting stars may also be considered as pro- 

 ceeding from a cometary body, so that we may be said to 

 have passed through the thinner parts of various comets. 

 But the earth has probably never passed, in times of which 

 we have any record, through the nucleus of a comet, which 

 consists perhaps of a dense swarm of small meteorites. We 

 can only speculate upon the effects which might be pro- 

 duced by such a conflict, but it would probably be a much 

 more serious event than any yet registered in history. 

 The probability of its occurrence, too, can hardly be 

 assigned ; for though the probability of conflict with any 

 one cometary nucleus is almost infinitesimal, yet the 

 number of comets is immensely great (vol. ii. p. 1 1). 



