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BELL SYSTEM TECHNICAL JOURNAL 



entirely toward variations with time. While the presence of wave 

 interference has been detected, and the movement of this inter- 

 ference effect across the frequency band has been recorded, little 

 effort has been made to form a picture of such interference in its 

 space relation. A discussion of similar stable, daytime phenomena is 

 therefore not out of place, and particularly so in view of an evident 

 relation of the fickle nocturnal interference phenomena to the steady 

 states which follow the appearance of daylight. 



In a previously published map of field strength distribution in 

 New York City,* it was indicated that the congestion of high buildings 



Fig. 26 — Map showing location of radio obstruction on Manhattan Island as de- 

 termined by the intersection of lines between various transmitting points and their 



corresponding shadows 



just below Central Park cast a heavy shadow. More recently it has 

 been determined from observations on a portable transmitter, set 

 up at various points, that this building center is a consistent per- 

 former. The position of this obstruction is determined in Fig. 26 

 wherein only partial contours from maps for the indicated sites are 

 given to prevent confusion. The intersection of these lines from 

 transmitter to shadow, falls at approximately 38th Street in the vicinity 

 of Sixth Avenue. 



The dissipation of wave energy at such a point is probably the 

 composite effect of many adjacent structures. Fig. 27 gives an 

 elementary idea of how this can occur. The structures filling in 



* See footnote 1. 



