June 4, 1921 delunger and whittemore; : r.\dio fading 253 



that fading was small, signals were good, and strays were weak when 

 it was raining at the receiving station. (Cases of little fading, 17; 

 medium fading, 18; severe fading, 9. Signals were weak 4, moderately- 

 loud 31, and very loud 7 times. Weak strays 24, medium strays 

 13, and loud strays 6 times.) 



Clouds at the receiving station are conducive to good signals. (Weak 

 signals 9, moderately loud 75, and very loud 17 times.) 



It is possible that clouds blanket the upper air from the distur- 

 bances of temperature and other effects on the earth's surface. 



DISCUSSION AND EXPLANATION 



The complexity of the phenomena suggests that the causes are 

 complex, so that no one explanation will fit all the observations. How- 

 ever, many of the more clearly established facts lend themselves 

 very well to the explanation of the transmission of radio waves pro- 

 posed below. 



Daytime transmission. — The waves that travel along the earth's sur- 

 face (roughly, the sliding waves), and not the waves in the upper at- 

 mosphere, are those which are utilized in the daytime. This is in- 

 dicated by the formula given in equation (1) in which the absorp- 

 tion coefficient, a, varies with the character of the surface over 

 which transmission takes place. It is a minimum for sea water, and 

 is greatly dependent on land characteristics. 



For short distances, short waves give the loudest signals, the ab- 

 sorption being negligible. For long distances, during the day time 

 when formula (1) holds true, long-wave signaUing is more efficient, 

 for the absorption factor predominates and limits the transmission 

 for short wave lengths. 



During the day the waves which get up into the atmosphere can 

 be considered as entirely absorbed by the upper ionized regions, known 

 as the stratosphere. Thus the intensity of the transmitted waves 

 depends on the ground conditions. This is borne out by the fact 

 that day transmission over sea varies very little with time. This 

 is to be expected, because the sea exhibits entire uniformity in its 

 effect on waves passing over it, thus tending to confirm the idea that 

 ground conditions determine wave intensity in the daytime. 



The idea that waves are retarded by ionization is a very old one. 

 Elihu Thomson and Fessenden, however, first locaHzed the ionization 

 on the ground, but it has since been assumed to have its seat at higher 

 and higher levels. The ionization of the air, which is not nearly large 



