HERTZIAN WAVE WIRELESS TELEGRAPHY. 157 



inch. In the case of Hertzian wave telegraphy, we are, however, dealing 

 with ether waves many hundreds of feet in length, and the waves sent 

 out from Poldhu have a wave-length of a thousand feet or more, say one 

 fifth to one quarter of a mile. The distance therefore between Poldhu 

 and Cape Cod is only at most about twelve thousand wave-lengths, and 

 stands in the same relation to the length of the Hertzian wave used 

 as does a body the diameter of a pea to the wave-length of yellow light. 

 There is unquestionably a large amount of diffraction or bending of 

 the electric wave round the earth, and proportionately speaking it is 

 larger than in the case of light waves incident on objects of the same 

 relative size. 



Quite recently Mr. H. M. Macdonald (see Proc. Roy. Soc. Lond., 

 Vol. LXXI., p. 251) has submitted the problem to calculation, and 

 has shown that the power required to send given electric waves 3,000 

 miles along a meridian of the earth is greater than would be required 

 to send them over the same distance if the sea surface were flat in the 

 ratio of 10 to 3. Hence the rotundity of the earth does introduce a 

 very important reduction factor, although it does not inhibit the 

 transmission. Mr. Macdonald 's mathematical argument has, however, 

 been criticized by Lord Eayleigh and by M. H. Poincare (see Proc. 

 Roy. Soc, Vol. 72, p. 40, 1903). 



The accomplishment of very long distances by Hertzian wave teleg- 

 raphy is, however, not merely a question of power, it is also a question 

 of wave-length. Having regard, however, to the possibility that the 

 propagation which takes place in Hertzian wave telegraphy is not 

 that simply of a free wave in space, but the transmission of a semi- 

 loop of electric strain with its feet tethered to the earth, it is quite 

 possible that if it were worth while to make the attempt, an ether dis- 

 turbance could be made in England sufficiently powerful to be felt in 

 New Zealand. 



Leaving, however, these hypothetical questions and matters of pure 

 conjecture, we may consider some of the facts which have resulted 

 from Mr. Marconi's long distance experiments. One of the most in- 

 teresting of these is the effect of daylight upon the wave propagation. 

 In one of his voyages across the Atlantic, when receiving signals from 

 Poldhu on board the S. S. Philadelphia, he noticed that the signals 

 were received by night when they could not be detected by day.* In 

 these experiments Mr. Marconi instructed his assistants at Poldhu to 

 send signals at a certain rate from 12 to 1 a. m., from 6 to 7 a. m., from 

 12 to 1 P. M. and from 6 to 7 p. m., Greenwich mean time, every day 

 for a week. He has stated that on board the Philadelphia he did not 



-» 



See Proc. Roy. Soc, June 12, 1902. ' A note on the EflFect of Daylight 

 upon the Propagation of Electromagnetic Impulses Over Long Distances,' by 

 G. Marconi. 



