June 4, 1921 dellinger and whittemore: radio fading 257 



If fading were to be explained by reflection and interference be- 

 tween the direct wave and the reflected wave it would seem obvious 

 that fading will be greater in the case of continuous wave stations 

 than spark stations, but apparently this is not the case. It seems 

 probable that the reflection in the upper regions may more truly be 

 considered a quasi-refraction resulting from an increase in the velocity 

 of the waves as they enter the more highly ionized air. 



Strays. — Of the several kinds of strays, some have their origin 

 near the receiving station while others are waves sent out by electrical 

 disturbances in certain definite regions such as the western part of 

 the Gulf of Mexico or the central part of Africa. vSuch regions are 

 mostly tropical. The fact that strays are more intense at night than 

 in the daytime is explained by the greater ease of propagation of all 

 waves at night so that strays which are only local in their effects 

 in the daytime spread to a distance at night. 



The strays at night seem to be caused by occurrences at or above 

 the Heaviside surface, because there are times when the strays are 

 intense and yet signals are inaudible or very weak and fading violently. 

 This indicates a turbulent region of ionization over the area involved, 

 which gives rise to strays and which absorbs waves that come into 

 it. The aurora itself represents an extreme case. 



Direction of signals.— There is no correlation between fading and 

 direction changes except that both are greater at night. Thus fading 

 is greatest for short waves and direction changes are greatest for long 

 waves. This makes it appear that the explanations of the two may 

 not be closely related. The interference caused by waves from the 

 Heaviside surface may explain the direction changes observed at 

 night (rapid direction changes on long waves), as it is too remote 

 to cause interference with short waves. 



Kinsley has suggested that the direction changes may be the result 

 of reflection at the radioactive boundary between the troposphere 

 and the stratosphere. 



Effects of sunrise and sunset. — The change from the ionized sun- 

 light condition to the insulating condition of darkness introduces a 

 surface of discontinuity which acts as an obstacle to the waves, partly 

 reflecting them back. It is to be noted that the change takes place 

 in the stratosphere, above the clouds. This deflection of the waves 

 depends on a change in velocity of the waves, produced by the ioniza- 

 tion. Since this change in velocity is proportional to the square of 



