256 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 11 



in the daytime only, nor why they fade only at night. It seems more 

 nearly correct to consider the daylight transmission of waves as being 

 mainly along the earth as a guiding conductor, none of them being 

 able to reach the Heaviside surface because of the intervening 

 ionized stratosphere, while the waves at night reach the Heaviside 

 surface and travel or slide along it without appreciable absorption. 



If this be true, then the waves should reach enormous distances 

 at night, short waves traveling farther than long ones. Because 

 of the variable absorption which may be introduced by the irregulari- 

 ties of the Heaviside surface and the adjoining regions, the waves 

 may vary rapidly in intensity. Small irregularities would affect 

 short waves more than long waves; hence short waves would fade 

 most. These conclusions are in precise agreement with the facts. 



Within the distance from a transmitting station in which ground 

 absorption is negligible there is no fading and the night and day in- 

 tensities are equal, since the waves are not affected by conditions 

 in the upper atmosphere. If the theory given here be correct the 

 maximum intensity of signals received at night should be that given 

 by the transmission formula with the absorption factor equal to unity, 

 and this has been observed to be true. This should be taken as a 

 standard transmission distance. Certainly this is the only unique 

 value, the only value in which the nature of the location of the trans- 

 mitting and receiving stations does not enter. Thus the transmission 

 formula cannot predict the varying intensity of signals observed 

 at night, but it does give the limiting value of signal intensity. It 

 also shows why extraordinary ranges are more likely to be obtained 

 at night with short waves than long waves. 



This theory of night wave transmission is strikingly like the ex- 

 planation of the flight of the projectile from the German long-range 

 gun. In both cases it is now realized that there exists a region of 

 the upper atmosphere of surprisingly low opposition or resistance. 



G. Sagnac says that "zones of weak signals" observed between 

 moving ships are due to the earth's orbital motion dragging the sur- 

 rounding ether with it. It seems more probable, however, that such 

 phenomena are caused by regions of exceptionally great absorption 

 either of the wave transmitted along the ground or of the wave trans- 

 mitted along the Heaviside surface, depending upon the conditions. 



The relatively small fading on long wave-lengths is partly attri- 

 butable to the fact that the transmission is along the ground. 



