472 



BELL SYSTEM TECHNICAL JOURNAL 



F-brarich sweeps smoothly and slowly upward from its commencement. 

 (The forking signifies that here are two similar curves lying side by 

 side as in the previous figures, but overlapping so much that over a 

 large part of their course they are not distinct.) Many observations 

 have concurred in showing that the crinkle is present only when the 



1700- 



1600- 



1500- 



1400- 



1300- 



^1200- 

 a. 



UJ 



t 1100- 



o 



d 1000- 



? 900- 



UJ 

 Q 

 3 800- 



< 700- 



3 600- 



500- 

 400- 

 300- 

 200- 









•t«» Ml<i 



0.5 



2 3 4 5 6 



FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 8 — Multiple echoes. (Carnegie Institution of Washington.) 



sun is high in the sky (within some 40° of the zenith) — therefore 

 absent by night and at the beginning and end of day, and indeed 

 absent all day in winter where the latitude is high. This is our first 

 example of the dependence of the ionosphere on sunlight, a very 

 important feature. 



