HERTZIAN WAVE WIRELESS TELEGRAPHY. 159 



stations, signals having Ijeen received by him during the night between 

 Poldhu and Poole with an aerial the height of which was not suffi- 

 cient to receive them by day. It has been found, however, that the 

 effect simply amounts to this, that rather more power is required by 

 day than by night to send signals by Hertzian waves over long dis- 

 tances. 



Some interesting observations have also been made by Captain H. 

 B. Jackson, E.jST.,* on the influence of various states of the atmosphere 

 upon Hertzian wave telegraphy. These experiments were all made 

 between ships of the British Eoyal Navy, furnished with Hertzian wave 

 telegraphy apparatus on the Marconi system. Some of his observa- 

 tions concerned the effect of the interposition of land between two 

 ships. He found that the interposition of land containing iron ores 

 reduced the signaling distances, compared with the maximum distance 

 at open sea, to about 30 per cent, of the latter; whilst hard limestone 

 reduced it to nearly 60 per cent, and soft sandstone or shale to 70 per 

 cent. These results show that there is a considerable absorption effect 

 when waves of certain wave-length pass through or over hard rocks 

 containing iron ores. It would be interesting to know, however, 

 whether this reduction was in any degree proportional to the dryness 

 or moisture of the soil. Earth conductivity is far more dependent 

 upon the presence. or absence of moisture than upon the particular 

 nature of the material which composes it other than water. 



The observations of Captain Jackson, however, only confirm the 

 already well-known fact that Hertzian waves, as employed in the 

 Marconi system of wireless telegraphy, within a certain range of 

 wave-length, are considerably weakened by their passage through 

 land, over land or round land. In some cases he noticed that 

 quite sharp electric shadows were produced by rocky promontories 

 projecting into the line of transmission. His attention was also 

 directed (loc. cit.) to the more important matter of the effect of 

 atmospheric electrical conditions upon the transmission. The effect 

 of all lightning discharges, whether visible or invisible, is to make a 

 record on the telegraphic receiver. On the approach of an atmospheric 

 electrical disturbance towards the receiving station on a ship, the first 

 visible indications generally are the recording of dots at intervals from 

 a few minutes to a few seconds on the telegraphic tape. Captain Jack- 

 son states that the most frequent record is that of three dots, the first 

 being separated from the other two by a slight interval like the letters 

 E I on the Morse code, and this is the sign most frequently recorded 

 by distant lightning. But in addition to this, dashes are recorded and 



* See Proc. Roy. 80c., May 15, 1902. ' On Some Phenomena affecting the 

 Transmission of Electric Waves over the Surface of the Sea and Earth,' by 

 Capt. H. B. Jackson, R.N., F.R.S. 



