1902.] affecting the Transmission of Electric Waves. 



265 



screening effect of the land varies with its nature, and is greater for 

 iron ores than for limestone alone, and that for this latter, it is greater 

 than for soft rocks. No effects which could be attributed to inter- 

 ference of waves, due to reflection from a hilly background, have been 

 recorded by me. 



% 7. 



DisCance in mibes. 



No signals. 



ah) 



Fig. 3. 



30 



(da) 



40 



50 



60 65 



(dh) 



50 



<6o 



' ^ • r^-^ T — a 1 



65 



65 



The next cause that I shall consider is that due to the varying con- 

 ditions of the atmosphere ; some of these conditions constitute a most 

 serious obstacle to the effective transmission of electric waves over 

 medium distances, and are, in consequence, a source of error likely to 

 be encountered, and which cannot be foretold nor allowed for in wire- 

 less telegraphy. 



As far as my experiences go, these effects are much less frequently 

 noticed in temperate than in sub-tropical regions. In the Mediter- 

 ranean Basin they seem to be particularly prevalent, and most per- 

 sistent in summer and autumn. 



Owing to their sudden advent and their equally sudden cessation, it 

 is most difficult to carry out systematic or pre-arranged experiments, ' 

 .and I must therefore confine my remarks to general observations as to 

 their effects in various parts of the Mediterranean Sea. 



