124 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
is the detrimental effect produced by daylight on the propagation of 
electric waves over great distances. 
The generally accepted hypothesis of the cause of this absorption 
of electric waves in sunlight is founded on the belief that the absorp- 
tion is due to the ionization of the gaseous molecules of the air 
affected by the ultra violet light, and as the ultra violet rays which 
emanate from the sun are largely absorbed in the upper atmosphere 
of the earth, it is probable that that portion of the earth’s atmosphere 
which is facing the sun will contain more ions or electrons than that 
which is in darkness, and therefore, as Sir J. J. Thomson has shown,! 
this illuminated or ionized 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 experi- 
ments, in which waves about 8,000 meters 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 south- 
erly 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 galvanom- 
eters, 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 300 and 600 meters, the distance over which communica- 
tion can be effected during daytime is generally about the same, 
whatever the bearing of the ships to each other or to the land stations 
—whilst at night interesting and apparently curious results are 
obtained. Ships over 1,000 miles away, off the south of Spain or 
round the coast of Italy, can almost always communicate during the 
hours 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- 
1 Philosophical Magazine, ser. 6, vol. 4, p. 253. 
