488 BELL SYSTEM TECHNICAL JOURNAL 



radio very far over the earth. The reader may think that this is not 

 very important: our ancestors lived without radio, why should we 

 worry about lacking it? Well, it is probably quite true that if the 

 ionosphere were not overhead, we should not be worrying about the 

 lack of radio. We should in fact probably not be worrying about 

 anything at all, for we should not be here to worry. The ultra-violet 

 light of the sun, pouring down upon the surface of the globe unhindered, 

 would work changes so severe on organisms as we know them that life 

 would have to be very different, and perhaps impossible. This lethal 

 light is like an enemy, which in attacking a city spends itself in throwing 

 up a barrier against itself; and the barrier not only keeps the enemy 

 out, but is serviceable otherwise to the dwellers in the city. 



