Wireless Echoes of Long Delay

The paper discusses the various possible ways in which long delayed radio echoes may be propagated. It is proved that the echoes cannot be due to waves which are propagated in any of the following manners: (1) Along very long paths in the highly ionized part of the earth's atmosphere. (2) In any part of the earth's atmosphere so highly ionized that the group or signal velocity is very small. (3) In any part of space outside the earth's atmosphere so highly ionized that the group velocity is very small. (4) In the non-conductive layer of air between the earth's surface and the lower surface of a highly conductive layer in the higher atmosphere. In all of these four cases the attenuation will be altogether prohibitive for all echoes with more than 10 seconds retardation. In case (3) a pure electron atmosphere would not give any attenuation, but it is also proved that such an atmosphere having the necessary density will be so rapidly dispersed by the electrostatic forces that it cannot persist for any appreciable length of time. In the ionization bands which may cause these echoes the total space charge must consequently be very nearly equal to zero. And in this case the attenuation is great. The long delayed echoes must therefore be due to signal waves having travelled very long distances (about 3.