CHAPTER XVI 



FIRE-BALLS. 



GREAT fire-balls are much more numerous than any one 

 would suppose who had not paid attention to the subject. 

 Nor need this be a matter for surprise if it be remembered 

 that when a fire-ball does arrive it is only by a favourable 

 combination of circumstances that any particular indivi- 

 dual is privileged to witness the exhibition. Let us 

 examine the conditions that are necessary. In the first 

 place, the observer who desires to look out for meteors 

 will naturally choose some station which commands the 

 most uninterrupted view of the sky on all sides. The 

 deck of a ship on the open sea, or, better still, the summit 

 of a lofty mountain, will represent ideal sites for the 

 purpose. But only a very small fraction of the entire 

 atmosphere of our globe is within the sphere of observa- 

 tion from one locality. If a fire-ball plunges into the air 

 it may perhaps be seen if in any part of its track it is 

 above the horizon. With sufficient precision we may 

 assert that the proportion of the total atmosphere which 

 is above the horizon at any one place is -j-J-^th of the whole, 

 and that anything which may occur in the remaining 499 



