ON WIRELESS ECHOES. 427 



parts of the journey, is a little less than this, but we may take this as our value for 

 the time being. Moreover, as in the first part of our discussion, we shall be dealing 

 with times of transit that are quite small, it is useful for us to remember that the 

 velocity of the waves is such that in one-millisecond, or one thousandth of a second, 

 the waves have travelled 300 kilometres or 186 miles. 



The first experiments on the timing of the journeys of wireless waves were carried 

 out almost simultaneously in this country and in America, though the methods of 

 timing in the two cases were quite different. The English experiments were carried 

 out using an 80-mile base. That is to say, there were 80 miles between sending and 

 receiving stations. In this case it was found that at night, but not usually in the 

 daytime, for every signal sent out from the sending station two or more were observed 

 at the receiving station, the second coming just over 1/3 millisecond after the first, 

 and the third about one millisecond after the first, indicating paths about 100 km. 

 and 300 km. greater than that traversed by the first signal. 



It was natural for it to be assumed that the first signal had travelled straight 

 along the ground from sender to receiving station, that is, a distance of 130 km., 

 while the second had travelled 100 km. further, and the third 300 km. further. 



Now the unexpected result that long waves were able to travel round a spherical 

 earth had led Kennelly and Heaviside to suggest that there is in the upper atmosphere 

 a layer of electricity which is sufficiently conducting to be a reflector of wireless waves. 

 It was therefore natural to assume that the echo-signals which were observed were 

 due to waves reflected from the layer. This is made clearer in Fig. 1. The 



B 



. C 



Fig. 1. 



first signal received is the one which travels by the path (1) along the ground, the 

 second is that which has been reflected once at this layer and marked (2), and the third 

 is one which has been doubly reflected and marked (3). Since the path length of the 

 first is known, that of the second and third can be calculated, and it is found that 

 they agree with a layer height of about 100 km. This would indicate that the angle 

 between the direction of the downcoming waves constituting the first echo and the 

 vertical at the receiver was about 30°. In another series of experiments in which 

 this angle could be measured by an entirely different method it was found to have 

 an average value of about 30°, thus confirming the results of the first series. 



The British method of timing echo-signals, though very accurate, is a little 

 complicated, so I will not burden you with details of it. In the American experi- 

 ments a more direct method of timing was adopted. The two stations, sending and 

 receiving, were closer together, being situated only six nules apart. Very short signal 

 impulses were sent out by the sending station and recorded at the receiving station 

 by means of a high-speed galvanometer. Evidence of echo-signals were obtained, 

 and from the speed of travel of the photographic film used the echo-time could be 

 estimated. 



In the earlier series of the American experiments the echo-delay was of the order 

 of 1^ mUHseconds, indicating a path-length difference of the order of 450 km. Now 

 as the stations were close together, we must assume that the waves causing the echo 

 had gone almost straight up and down, so that the height of the reflecting surface 

 was about 220 km. Now there is a big difference between 100 km. and 220 km., 

 though it must be remembered that in English experiments 400-metre waves were 

 used, and in the American experiments 75-metre waves were used. The question. 



