ON THE ORIGIN OF THE PLANETARY SYSTEM. 169 



attainable by terrestrial means scarcely exceed 2,000 

 degrees. In fact, the outer crusts of meteoric stones 

 generally show traces of incipient fusion ; and in cases 

 in which observers examined with sufficient prompti- 

 tude the stones which had fallen they found them hot 

 on the surface, while the interior of detached pieces 

 seemed to show the intense cold of cosmical space. 



To the individual observer who casually looks 

 towards the starry sky the meteorites appear as a rare 

 and exceptional phenomenon. If, however, they are 

 continuously observed, they are seen with tolerable 

 regularity, especially towards morning, when they 

 usually fall. But a single observer only views but a 

 small part of the atmosphere; and if they are calcu- 

 lated for the entire surface of the earth it results that 

 about seven and a half millions fall every day. In our 

 regions of space, they are somewhat sparse and distant 

 from each other. According to Alexander Herschel's 

 estimates, each stone is, on an average, at a distance of 

 450 miles from its neighbours. Eut the earth moves 

 through 18 miles every second, and has a diameter of 

 7,820 miles, and therefore sweeps through 876 millions 

 of cubic miles of space every second, and carries with 

 it whatever stones are contained therein. 



Many groups are irregularly distributed in space, 

 being probably those which have already undergone 

 .disturbances by planets. There are also denser swarms 



