ON THE ORIGIN OF THE PLANETARY SYSTEM. 167 



heavy masses diffused in cosmical space is more dis- 

 tinctly revealed by the phenomena of asteroids and of 

 meteorites. We know now that these are bodies 

 which ranged about in cosmical space, before they came 

 within the region of our terrestrial atmosphere. In 

 the more strongly resisting medium which this atmos- 

 phere offers they are delayed in their motion, and at 

 the same time are heated by the corresponding friction. 

 Many of them may still find an escape from the terres- 

 trial atmosphere, and continue their path through 

 space with an altered and retarded motion. Others 

 fall to the earth ; the larger ones as meteorites, while 

 the smaller ones are probably resolved into dust by the 

 heat, and as such fall without being seen. According 

 to Alexander Herschel's estimate, we may figure shoot- 

 ing-stars as being on an average of the same size as 

 paving-stones. Their incandescence mostly occurs in 

 the higher and most attenuated regions of the atmos- 

 phere, eighteen miles and more above the surface of 

 the earth. As they move in space under the influence 

 of the same laws as the planets and comets, they 

 possess a planetary velocity of from eighteen to forty 

 miles in a second. By this, also, we observe that they 

 are in fact stelle cadente, falling stars, as they have 

 long been called by poets. 



This enormous velocity with which they enter our 

 atmosphere is undoubtedly the cause of their becom- 



