156 AN2sUAL REPORT SMITHSONIAN INSTITUTION, 1936 



and the English. He foretold a British victory, because the noise 

 of battle became gradually fainter as the victors pursued the Dutch. 

 The fact that sounds are bent back again to the earth necessitates 

 a warmer layer above tlie cold. It seems that with increasing altitude 

 the temperature may gradually decrease down to many degrees below 

 zero Fahrenheit, but at a height of 30 miles there is an increase up 

 to 80° F., and the heat to maintain this may be connected with the 

 formation of ozone from oxygen by the sun's ultraviolet light. 



RADIO WAVES 



The most important method of throwing light on the nature of 

 the upper regions of the air is by projecting radio (or wireless) 

 waves directly upward, for it is found that with suitable frequencies 

 they will be reflected back to the earth. It will be recalled how 

 puzzling it was, in the early days of wireless, to account for the fact 

 that the electromagnetic waves, expected to move in a straight line 

 like light, could travel from Ireland to Newfoundland. Today wire- 

 less waves, carrying speech, music, or Morse, can be sent completely 

 round the world, so that a man can speak to himself and hear it a 

 fraction of a second later, using waves which have circumnavigated 

 the globe, changing local time in the most remarkable way as they 

 traveled. During a part of the journey it must have been yesterday, 

 or tomorrow though on return it was the same day and perhaps 

 about a seventh of a second since they started. It was surmised both 

 by Kennelly and by Heaviside, independently, that the possibility of 

 successful long-range wireless signals depended upon reflection or re- 

 fraction by an electrified or ionized region at a considerable height 

 above the earth. The proof of the existence of such a conducting 

 region was given by Appleton, who also showed that there is another 

 higher region also capable of reflecting radio waves back to the 

 earth. 



The lower or E region is at about 100 kilometers, or 60 miles from 

 the earth, and it is also called the Kennelly-Heaviside region. The 

 upper or F region is two or three times as high, and bears the name 

 of Appleton. It is possible to send a brief signal of suitable fre- 

 quency which will be reflected back from both the E and F regions, 

 so that both signals may be recorded on a suitable photographic plate 

 (pi. 2) by means of a cathode ray oscillograph. It is possible to 

 measure the very short period of time between the initial and return 

 signals, and, as the velocity of such waves is about 186,000 miles a 

 second, it is easy to deduce the height of the reflecting region. For 

 example, if the interval is one-thousandth of a second, the reflecting 

 layer would be about 93 miles above the earth. Experiments carried 

 out by Henderson and others, during a total eclipse of the sun in 



