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BELL SYSTEM TECHNICAL JOURNAL 



organized spacing of the minima is present in all of these night-time 

 band fading records. As has already been suggested such evidence 

 has an important significance, but before going into this phase of 

 the subject again let us examine a little more in detail the structure 

 of these band fading records. 



The steps in any one picture of Fig. 17 are, as we have said, snap- 

 shots of the wave amplitude for successively different radio frequen- 

 cies taken about a quarter of a second apart. The fact that the 

 fifteen snap-shots used to build up a single picture are not taken 

 simultaneously causes a skewing of the outlines when movement 

 of the depressions as shown in Fig. 17 occurs. If, for example, we 



Fig. 18 — Three dimensional diagram, showing the method of interpreting band fading 



records 



were to take fifteen separate and successive snap-shots of a mountain 

 through fifteen long vertical slits side by side it would be possible to 

 combine the narrow sections so as to form a true picture of the peak. 

 Now, if by some prodigious act of nature the mountain were shifted 

 suddenly to one side and back again during the time we were taking 

 the fifteen successive snap-shots through the vertical slits, the com- 

 bination of them would form a profile quite different from that ob- 

 tained when it was stationary. Or if it were simply moved steadily 

 across the field of vision during the time the snap-shots were being 

 taken one slope would be made to appear precipitous while the other 

 would be leveled to a gentle grade in the finally built-up picture. 



