SCIENTIFIC PROBLEMS IN RADIOTELEGRAPHY 363 



moreover, irregular disturbances caused by local storms are 

 superposed upon these diurnal variations. Many of the stray 

 waves observed in England affect radiotelegraphic stations 

 hundreds of miles apart nearly simultaneously and therefore 

 cannot have their origin in England. 



We have next to notice the diurnal variation in the strength 

 of artificial or message-bearing waves sent over long distances. 

 The fact was discovered by Marconi in 1902, in one of his 

 voyages across the Atlantic, that whilst by day he could (using 

 a certain receiver) only receive signals from his Cornwall station 

 at a distance of about 700 miles, he could receive similar signals 

 at a distance of 2,100 miles by night. The first hypothesis 

 suggested was that this was due to the discharging power of 

 daylight upon the sending antenna. The daylight effect, how- 

 ever, is only a long-distance effect and no such reduction in 

 the strength of day signals is noticed over short distances. 

 Hence it cannot be an effect produced merely on the sending 

 antenna. It was then suggested that the atmosphere became 

 ionised by the sunlight and that this was the cause of the 

 absorption of the electric waves. We can calculate the absorp- 

 tion due to any amount of assumed conductivity in the air when 

 long electric waves are passing through it and it is not difficult 

 to prove that the atmospheric conductivity which has been 

 observed at sea level or a few hundred or even thousand feet 

 above it is not sufficient to account for the observed diminution 

 of range of long electric waves by day as compared with night, 

 if it be attributed to mere absorption of wave energy, in other 

 words, to want of transparency due to some degree of conduc- 

 tivity in the air. 



There is, however, another possible explanation. It is well 

 known that sound travels better with the wind than against it. 

 The late Sir George Stokes explained this as follows : When 

 the wind is blowing strongly, friction against the earth retards 

 it more at the surface than at higher levels. If then a plane 

 vertical sound wave-front be travelling against the wind, the 

 velocity of the wave will be more reduced at higher levels than 

 at the earth's surface. Hence the wave-front is no longer 

 vertical but slopes backward. The direction of propagation 

 of the wave being normal to the wave-front, the wave is tilted 

 upwards and the sound will pass above a distant point on the 

 terrestrial surface which it would otherwise reach if the wind 



