ON WIRELESS ECHOES. 431 



-directly from a station a few kilometres away are quite unreadable, while the signals 

 from the same station which have made a journey round the earth are received with 

 perfect clarity- 



So far, in this discourse, I have dealt with echo-phenomena for which there are 

 now fairly widely-accepted explanations. I now turn to a group of phenomena the 

 origin of which must still be regarded as being something of a mystery. I refer to the 

 remarkable echoes which can sometimes be heard, not a millisecond, but several 

 seconds after the cessation of the original signal and which were discovered three years 

 ago by a Norwegian wireless amateur observer. Here I would like to observe that no 

 subject seems to have been more fortunate in the assistance rendered to it by amateur 

 experiments than that of wireless transmission. It is agreed by those best qualified 

 to form an opinion that it was the amateurs who discovered the extraordinary suitability 

 of very short waves for long-distance propagation. Amateur observers have rendered 

 extremely valuable service on special occasions when observations were needed, for 

 example, during the 1927 eclipse in this country. As we have seen to-night, it was a 

 Bristol wireless amateur who first obtained satisfactory data about television echoes, 

 while it was a Norwegian amateur, Engineer Hals, who first observed the echoes of 

 long delay. 



Engineer Hals, who lives at Bygdo, near Oslo, in Norway, had for some time 

 been making observations on wireless signals during auroral displays and sending in 

 periodic reports to Prof. Stormer, the well-known authority on aurora, who lives in 

 the same locality. One day in December 1927, Prof. Stormer met Engineer Hals 

 accidentally and they began to discuss wireless reception, and in the course of this 

 conversation Hals stated that he had been listening to the Dutch Short Wave Station 

 at Eindhoven working on about 30 metres, and had heard echoes three seconds after 

 the original signal, and also ventured the suggestion that the echoes were due to 

 waves which had been to the moon and back. 



Prof. Stormer, recognising that if the existence of these echoes could be established, 

 a remarkable discovery had been made, asked Engineer Hals to make a written report 

 of his observations. When this was received it was found to read, somewhat naively, 

 as follows : — 



' I hereby have the honour to advise you that at the end of the summer, 1927, 

 I repeatedly heard signals from the Dutch short-wave transmitter PCJJ (Eindhoven). 

 At the same time as I heard the telegraph signals I also heard echoes. I heard the 

 usual echo, which goes round the earth with an interval of one-seventh of a second, 

 as well as a weaker echo about three seconds after the principal signal had gone. 

 When the principal signal was especially strong, I assume that the amplitude for the 

 last echo, three seconds after, lay between one-tenth and one-twentieth of the principal 

 signal in strength. I will only herewith confirm that I really heard this echo.' 



In the meantime. Prof. Stormer had, through the courtesy of the Norwegian 

 telegraphic department, arranged for the Eindhoven station to send special test 

 signals to see if oscillographic records would provide what might be termed 

 documentary evidence of the existence of the echoes. Unfortunately, however, 

 although audible echoes were observed, it was not possible to recognise them on the 

 photographic film because of the many other atmospheric disturbances. 



Another series of observations was arranged in the autumn of 1928, the same 

 transmitter being used and attention being concentrated on aural observations to 

 see if the same echo could be identified by several observers. In these observations 

 Dr. van der Pol, of Eindhoven, also joined and was able to confirm the existence of 

 echoes heard as long as 30 seconds after the original signal. Further confirmatory 

 evidence of the reality of the phenomenon was obtained by Mr. K. A. L. Borrow and 

 myself, who heard echoes on February 19, 1929, at King's College, London. 



But almost as startling as the discovery of the echoes themselves was the theory 

 immediately put forward by Prof. Stormer to account for their long delay. The 

 question to be answered was : Where had the wireless waves been during the relatively 

 long period of 25 to 30 seconds ? The moon could be ruled out as a reflector, for a 

 journey there and back for wireless waves is a matter of about two seconds. Could 

 there conceivably be a reflector at ten or twenty times the moon's distance ? Now 

 Prof. Stormer had, as far back as 1903, been led, in his study of aurora, to consider 

 the existence of streams of electrified particles sliot out from the sun and impinging 

 on the earth's atmosphere. Such streams of electrons themselves constitute a current 

 and as such would be deflected by the earth's magnetic field. The calculation of the 



