460 BELL SYSTEM TECHNICAL JOURNAL 



which he uses. It can be done, and it was done by Appleton and 

 Barnett in 1925. What they measured was the angle between the 

 directions of propagation of the sky wave and the "ground wave," 

 of which the wave-fronts are shown in Fig. 1 creeping along the 

 ground with a rapid attenuation. This ground wave, by the way, 

 is the only one by which radio transmission could be effected but for 

 the ionosphere; and it is seldom detectable beyond a few hundred 

 miles. 



Another scheme is much more complicated, and owing to its super- 

 session I may be excused for giving only the merest outline of it. 

 It is a clever way of putting to useful service the very great incon- 

 venience known as "fading." This term refers to a train of signals 

 which dies out and revives and keeps on fluctuating over and over 

 again, in a most irregular fashion. This sort of thing occurs in the 

 region where the sky wave and the ground wave both arrive and 

 overlap one another, and it has been traced to what in optics is called 

 the "interference" of the two. If conditions were absolutely stable, 

 then in taking a walk in the region of overlapping one would pass 

 through several maxima and minima of intensity. Since conditions 

 are never absolutely stable, the observer need not take the walk; 

 while he stands at any fixed point, the maxima and the minima float 

 past him while the ionosphere wavers in the sky and this is "fading." 

 But imagine the conditions relatively stable for a time, and the 

 observer standing still; and suppose that the engineer at the sending 

 aerial changes the wave-length by a small and known amount — then, 

 several maxima and minima will float past the observer, and by 

 counting them he can (though this is not at all obvious !) get a datum 

 which enables him to figure out the altitude of the reflecting layer 

 in the sky. This method also was invented by Appleton, and can 

 be found explained in the literature under the name "wave-length- 

 change method." 



The third of the methods has crowded out the others, and henceforth 

 will figure alone in these pages. It is the "echo-method," still some- 

 times called by the clumsy name of "group-retardation method." 

 Anticipated by Swann, it was realized by Breit and Tuve at the 

 Carnegie Institution of Washington. 



What is sent up to the sky is here a short sharp signal; if it could 

 be heard, it would be called a click. What comes back is the echo 

 of the signal. Passing over the receiving device, it produces a short 

 sharp kick on an oscillograph-record. Some of these are shown in 

 Fig. 2. The kicks marked Ei and Fi are due to signals echoed from 

 the layers E and F respectively. Those marked F^- - ■ F^ are due to 



