lull'] nn Rf(4if)t(tleiiraphy. 201 



The <,'encrally acc.o{)te(l hypothesis of the Cfiuse of this ahsori)tion 

 of electric waves in sunli<jht is founded on the belief that the al)Sorp- 

 tiori is due to the ionisation of the gaseous molecules of the air 

 affected Ijy the ultra-violet light, and, as the ultra-violet rays which 

 emanate from the sun are largely aljsorhed in the upper atmosphere 

 of the earth, it is probable that that portion of the earth's atmosphere 

 which is facing the sun will coutiiin njore ions or electrons than that 

 which is in darkness, and therefore, as Sir J. J. Thomson has shown,* 

 this illuminated or ionised air will absorb some of the energy of the 

 electric waves. 



The wave length of the oscillations employed has much to do 

 with this interesting phenomenon, long waves being subject to the 

 effect of daylight to a very much lesser degree than are short waves. 



Although certain physicists thought some years ago that the day- 

 light effect should be more marked on long waves than on short, the 

 reverse has been my experience ; indeed, in some transatlantic ex- 

 periments, in which waves about HOOO metres long were used, the 

 energy received by day at the distant receiving station was usually 

 greater than that obtained at night. 



Recent observation, however, reveals the interesting fact that 

 the effects vary greatly with the direction in which transmission is 

 taking place ; the results obtained when transmitting in a northerly 

 and southerly direction being often altogether different from those 

 observed in the easterly and westerly one. 



Research in regard to the changes in the strength of the received 

 radiations, which are employed for telegraphy across the Atlantic, 

 has been recently greatly facilitated by the use of sensitive galvano- 

 meters, by means of which the strength of the received signals can 

 be measured with a fair degree of accuracy. 



In regard to moderate power stations such as are employed on 

 ships, and which, in compliance with the International Convention, 

 use wave lengths of ^00 and 6()0 metres, the distance over which 

 connnunication can be effected during day time is generally about the 

 same, whatever the Ijearing of the ships to each other or to the land 

 stations — whilst at night interesting and apparently curious results 

 are obUiined. Ships, over 1000 miles away, off the south of Spain or 

 round the coast of Italy, can almost always communicate during the 

 liours of darkness with the Post Office stations situated on the coasts 

 of England and Ireland, whilst the same ships, when at a similar 

 distance on the Atlantic to the westward of these islands and on the 

 usual track between England and America, can hardly ever communi- 

 cate with these shore stations unless by means of specially powerful 

 instruments. 



It is also to be noticed that in order to reach ships in the 



Philosophical Magazine," Ser. 6, vol. iv., p. 253. 



