428 EVENING DISCOURSE. 



therefore, arose as to whether there was a real difference between the height of the 

 reflecting region in England and America, or whether the discrepancy was due to 

 the different distances and different wave-lengths used. The question of different 

 distances was ruled out when the English experiments were repeated at short distances 

 comparable with that used in America, and echo-times of 2 miUiseconds were again 

 obtained, indicating again a height of 100 km. {i.e. total path 200 km.). This 

 particular difference gained added interest when it was remembered that, of the two 

 men who had suggested the existence of this layer as a way out of theoretical difficulties, 

 Heaviside was an Englishman and Kennelly an American Professor. Had nature 

 arranged a sufficient difference between the heights of the region in question in this 

 country and America for both their claims to be separately recognised ? 



The question was settled when experiments at short distances were carried out 

 here in England using a number of different wave-lengths. In these experiments 

 it was found that the long waves are reflected at lower heights than the short waves. 

 Also it was found that there was a discontinuity in the curve in which echo-time 

 Ti-as exhibited as a function of wave-length. Our theory as to the mechanism by 

 which the echo-waves are reflected is not quite clear, due largely to the fact that 

 we are uncertain as to the exact influence of the earth's magnetic field, but we are 

 fairly certain that, the higher the frequency, the more electricity is required to 

 reflect the waves. It thus looks as if there are two regions which reflect waves. 

 All frequencies below the critical value are reflected by the lower region, and all 

 frequencies above it are reflected by the upper region. 



You will by now have seen how the discrepancy between the American and English 

 results is to be explained. Using long waves and low frequencies, we had been getting 

 echoes from the lower region, while in the American experiments the shorter waves 

 had been penetrating the lower region and reaching the upper one. 



The critical frequency or wave-length, which varies with the time of the day, is 

 an interesting one, for echoes from both regions are often obtained simultaneously 

 with it. Recognition of this led us to be able to suggest an alternative explanation 

 of some of the American records. The American observers had on one occasion found 

 examples of three echoes, such as are shown in the upper half of Fig. 2. 



(A) 



EiG. 2. 



Now the echo-times appear to be in the ratio 1, 2 and 4, and they were therefore 

 interpreted as being due to multiple reflection from a single region. Now this 

 interpretation presented three difficulties, as follows : — 



(1) The echo-times were not exactly 1:2:4, but about 0.9 : 2 : 4. 



(2) Why was there no third order reflection ? 



(3) Why was the second echo stronger than the first, although it had made two 

 reflections at the same region ? 



These difficulties were found to disappear when the two-region hypothesis was 

 applied to them, and simultaneous reflection at heights of 100 km. and 220 km. 

 was assumed. The alternative methods of explaining these results are shown dia- 

 grammatically in the lower half of Fig; 2 (A and B). 



I ought, of course, to mention that these echoes, due to waves which are reflected 

 from the upper atmosphere, are the causes of the signal variations which we experience 



