ELECTEIC WAVES OVER THE SUEFACE OF THE EARTH. 131 



according to the law of spherical divergence. These facts suggest emphatically that 

 there is, especially at night, some other cause at work besides diffraction, and that it 

 may be necessary to take into account the possibilities involved in transmission 

 through a heterogeneous, and perhaps in parts conducting, atmosphere, as is 

 maintained particularly by EccLEs( 16 ) and ( 17 ). The view that ionization in the upper 

 and middle atmosphere may be a cause of variations of the medium, large enough to 

 be favourable to long-distance transmission, inasmuch as the waves may be partially 

 refracted downwards, and liable, on occasion, to be unfavourable, inasmuch as the 

 refracted waves may be subject to absorption, demands scrutiny, to be conducted by 

 means of careful quantitative experiments under varying conditions and controlled by 

 an adequate mathematical analysis. The adoption of this view at present seems to 

 me to be premature. 



It remains to acknowledge gratefully the valuable help which I have received from 

 Prof. J. S. TOWNSEND, F.R.S., in the discussion of the experimental evidence as to 

 the strength of signals transmitted to great distances. 



S 2 



