516 BELL SYSTEM TECHNICAL JOURNAL 



explanation of tweeks. It does not appear probable that either 

 Barkhausen's or Eckersley's refraction theory properly explains the 

 tweek because of its lower limiting frequency of approximately 1600 

 c.p.s. It seems more than mere coincidence that this frequency is in 

 the range that the multiple reflection theory predicts. Any theory 

 adequately explaining the swishes or long whistlers should account 

 not only for long duration and apparently constant amplitude but for 

 upward as well as downward progression and freedom from diurnal 

 changes in tonal qualities. 



Acknowledgments 



The authors wish to acknowledge their indebtedness to Mr. A. M. 

 Curtis and Dr. W. S. Gorton for valuable advice, and to Messrs. J. 

 F. Wentz, A. B. Newell and E. W. Waters who through long hours 

 have worked patiently with us in procuring the data upon which this 

 article is based. 



