380 Propagation of Electromagnetic Waves round the Earth. 



strongly support the theory of ionic refraction put forward 

 by Eccles *, who suggested a possible increase of phase 

 velocity of long electromagnetic waves when propagated 

 through an ionized gas, which would cause the wave-front to 

 follow more or less the earth's curvature. An increase with 

 height in the concentration of heavy ions in the atmosphere, 

 together with an increase of their mobility, will cause the rays 

 to bend downwards in a somewhat analogous way as sound 

 waves, emitted at a certain elevation, undergo refraction. 



A silent zone so often observed in the acoustical analogy 

 has, so far as we are aware, not been found in the propa- 

 gation of electric waves, and it does not seem likely that it will 

 be found, for the energy distribution round an ordinary wire- 

 less transmitter, unless special arrangements are provided, is 

 such that by far the greatest amount of energy is emitted 

 horizontally, and therefore the part of the alternating field 

 at a certain distance from the transmitter due to the rays 

 emitted with small elevation, will generally be great in com- 

 parison with the part due to a ray leaving the transmitter at 

 a greater elevation, even if the latter were refracted down- 

 wards without any absorption whatever. 



Roughly speaking the ionic refraction will amount to the 

 same as if a layer were present in the higher atmosphere 

 which is more or less impermeable to long electric waves, 

 and this may account for the view, adopted by some writers, 

 of a reflexion of the waves against a layer of high conduc- 

 tivity. Whether this latter interpretation of the phenomena 

 is appreciably different from the adoption of a refraction 

 obviously depends on the vertical density gradient of the 

 ions in the upper atmosphere. It is, however, not likely 

 that this reflexion could occur without any losses, for in a 

 transition region energy would be absorbed, but to what 

 extent it is difficult to predict. 



(After this paper had been written Professor Watson com- 

 municated to the Royal Society a theoretical investigation of 

 the electric field on the assumption of a spherical concentric 

 shell of finite conductivity surrounding the earth. With the 

 assumption of a sharp inner boundary of this shell and for a 

 certain conductivity of the latter, values of the field could be 

 obtained of the order of magnitude of the observations. 



It is, however, impossible to state whether a sharply 

 defined inner boundary of this reflecting layer can be 

 expected to exist in the upper atmosphere.) 



Cavendish Laboratory, 

 Cambridge. 



* Proc. Roy. Sue. (ser. A) vol. lxxxvii. p. 79 (1912). 



