364 SCIENCE PROGRESS 



were not blowing against it. A similar effect is possible in 

 connexion with electromagnetic waves passing through air. 

 It has been shown by Dr. Eccles that, on certain assumptions 

 as to the nature of the ions, an electromagnetic wave travels 

 faster in ionised air than in non-ionised air. It has also been 

 proved experimentally by the writer that air containing con- 

 densed moisture, in the form of water spherules, has a slightly 

 higher dielectric constant than dry air. Hence the wave 

 velocity is slightly less in moist than in dry air. Also, there is 

 experimental evidence for the statement that ultra-violet light 

 can ionise air and separate from the molecules positive and 

 negative ions. Owing to the rapid absorption of the ultra-violet 

 light of the sun by the atmosphere, this kind of ionisation is 

 principally confined to layers of air at a considerable height, 

 in fact above the level of ordinary clouds. We have then the 

 necessary conditions for producing a refractive effect. The 

 electromagnetic waves radiated from an antenna are sent off 

 with greatest intensity in a horizontal direction but radiation 

 takes place to some extent in directions elevated above the 

 horizontal. When these upward-trending waves reach the 

 ionised layer of the atmosphere, owing to the greater velocity 

 of the upper part of the wave-front, a refractive effect takes 

 place which bends them down again. The effect may be com- 

 pared with an inverted mirage effect, the layers of ionised air 

 corresponding to the layers of hot air and the layers of non- 

 ionised air to the cold air. Again if the atmosphere from any 

 cause become ionised in patches, such non-homogeneous air 

 would behave to electromagnetic waves as water full of bubbles 

 behaves to light; it would become more or less opaque and 

 break up the wave-front passing through it. 



Before applying this theory in explanation of observed facts, 

 it will be well to turn attention for a moment to the fundamental 

 scientific question in connexion with long-distance radio- 

 telegraphy, viz. why is it possible to send electromagnetic waves 

 round the earth over long distances ? Suppose we consider an 

 analogy with light. If a luminous point of mathematical 

 dimensions were placed on the pole of a sphere one-quarter of 

 an inch in diameter, the radiant light would be diffracted to 

 a very small extent round the sphere but certainly not to such 

 a degree as to illuminate the sphere at its equator or even at 

 45° latitude. 



