346 On the Propagation of Electromagnetic Impulses. [June 10, 



February to 1st March inclusive. On board the "Philadelphia," I did 

 not notice any apparent difference between the signals received in the 

 day and those received at night-time, until after the vessel had reached 

 a distance of 500 statute miles from Poldhu. At distances of over 

 700 miles, however, the signals transmitted during the day failed 

 entirely, while those sent at night remained quite strong up to 1551 

 miles, and were even clearly decipherable up to a distance of 2099 

 miles from Poldhu. 



It is interesting to note that at the time of the year at which these 

 experiments took place, daylight at Poldhu was rapidly increasing 

 between the hours of 6 and 7 A.M., and on the " Philadelphia," I 

 noticed that at distances of over 700 miles from the sending station, the 

 signals at 6 A.M. were quite clear and distinct, whereas by 7 A.M. they 

 had grown weak almost to total disappearance, their strength thus 

 apparently diminishing in proportion as daylight increased at Poldhu. 

 No such weakening of the signals was noticeable between the hours of 

 12 midnight and 1 A.M. 



With a view to further tests in this same connection, I carried out 

 other experiments between the station at Poldhu and a receiving 

 station (in all respects similar to the one on the "Philadelphia ") situated 

 at the North Haven, Poole, Dorset. The distance between the North 

 Haven and Poldhu is about 152 statute miles, of which 109 are over 

 sea and 43 over high land. It was found that the signals from Poldhu 

 could be perfectly well received at the North Haven during the night 

 when four vertical wires 12*1 metres high were used in connection with 

 the receiving instruments, whilst, all other conditions being the same, 

 during the day the height of the wires required to be 18 '5 metres 

 in order to receive the same signals with equal clearness. 



The cause of these observed differences in the effects obtained by 

 night as compared with those noticed by day may be due to the dis- 

 electrification of the transmitting elevated conductor, operated by the 

 influence of daylight. The electrical oscillations in the transmitting 

 elevated conductor may thus be prevented by the discharging influence 

 of light from acquiring so great an amplitude as they attain during 

 darkness. 



The diselectrification of negatively charged metallic bodies by light 

 has been noticed by many observers,* and as each alternate half-oscil- 

 lation in the transmitting elevated conductor must necessarily charge 

 it negatively, the dissipating effect of light on each alternate oscilla- 

 tion of the electrical wave in the transmitting wire may be sufficient to 

 cause a material decrease in the amplitude of the oscillations. 



* See papers by Messrs. Elster and Geitel in Wiedemann's ' Annalen,' pp. 38 — 40, 

 also p. 497; also remarks of Professor Eigbi in ' Comptes Rendus/ vol. 107 

 p. 559. 



