At the end of the summer of 1927 I repeatedly heard signals from the Dutch short-wave transmitting station PCJJ at Eindhoven. At the same time as I heard these I also heard echoes. I heard the usual echo which goes round the Earth with an interval of about 1/7th of a second as well as a weaker echo about three seconds after the principal echo had gone. When the principal signal was especially strong, I suppose the amplitude for the last echo three seconds later, lay between 1/10 and 1/20 of the principal signal in strength. From where this echo comes I cannot say for the present, I can only confirm that I really heard it.
The pair, joined by another physicist Balthasar van der Pol researched the echos for some years, but failed to come up with an explanation. The reason for this is that the effect only occurs sporadically and that the time-delay of the echos varies dramatically. One would expect that, if these were echoes off some region of the atmosphere or something in space (e.g. the moon) the echoes would exhibit a predictable time delay (the time taken to travel to, and back from, the deflecting entity).
Long delayed echoes have been heard sporadically from the first observations in 1927 and up to our time.
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