158 POPULAR SCIENCE MONTHLY. 



notice any apparent difference between the signals received in the day 

 and those received at night until after the vessel had reached a dis- 

 tance of 500 statute miles from Poldhu. At distances of over 700 

 miles, the signals transmitted during the day failed entirely, while 

 those sent at night remained quite strong up to 1,551 miles, and were 

 clearly decipherable up to a distance of 2,099 miles from Poldhu. 

 Mr. Marconi also noted that at distances of over 700 miles, the signals 

 at 6 A. M., in the week between February 23 and March 1, were quite 

 clear and distinct, whereas by 7 a. m. they had become weak almost to 

 total disappearance. This fact led him at first to conclude that the 

 cause of the weakening was due to the action of the daylight upon the 

 transmitting aerial, and that as the sun rose over Poldhu, so the wave 

 energy radiated, diminished, and he suggested as an explanation the 

 known fact of the dissipating action of light upon a negative charge. 



Although the facts seem to support this view, another explanation 

 may be suggested. It has been shown by Professor J. J. Thomson that 

 gaseous ions or electrons can absorb the energy of an electric wave, if 

 present in a space through which waves are being transmitted.* If it 

 be a fact, as suggested by Professor J. J. Thomson, that the sun is 

 projecting into space streams of electrons, and if these are continually 

 falling in a shower upon the earth, in accordance with the fascinating 

 hypothesis of Professor Arrhenius, then that portion of the earth's 

 atmosphere which is facing the sun will have present in it more elec- 

 trons or gaseous ions than that portion which is turned towards the 

 dark space, and it will therefore be less transparent to long Hertzian 

 waves, f In other words, clear sunlit air, though extremely trans- 

 parent to light waves, acts as if it were a slightly turbid medium for 

 long Hertzian waves. The dividing line between that portion of the 

 earth's atmosphere which is impregnated with gaseous ions or electrons 

 is not sharply delimited from the part not so illuminated, and there 

 may be therefore a considerable penetration of these ions into the 

 regions which I may call the twilight areas. Accordingly, as the earth 

 rotates, a district in which Hertzian waves are being propagated is 

 brought, towards the time of sunrise, into a position in which the 

 atmosphere begins to be ionized, although far from as freely as is the 

 case during the hours of bright sunshine. 



Mr. Marconi states that he has found a similar effect between inland 



* See Phil. Mag., Vol. IV., p. 253. Series 6, August, 1902. J. J. Thomson, 

 ' On Some Consequences of the Emission of Negatively Electrified Corpuscles 

 by Hot Bodies.' 



t The opinion that ionization of the air by sunlight is a cause of obstruc- 

 tion to Hertzian waves propagated over long distances has also been expressed 

 by Mr. J. E. Taylor. See Proe. Roy. 80c. , Vol. LXXI., p. 225, 1903. ' Char- 

 acteristics of Earth Current Disturbances and their Origin.' 



