466 /*/!/-/- SYSTEM TECHNICAL JOrRXAL 



condition of iIil' iransatlaiuic transmission paili with ros|)fct to liay- 

 light and darkness is indicated 1)>' the bands beneath the curves. 

 The black portion indicates the time (hiring which the transathintic 

 path is entirely in darkness, the shaded portions the time during 

 which it is only partially in darkness, and the unshaded portions 

 the time during which da\light pervades the entire path. 

 The diurnal variation may lie traced through as follows: 



1. Relatively constant ('ulil >lrcni;i!) |)rt-\ails during the daylight 

 period. 



2. A decided drop in transmission accompanies the occurrence of 

 sunset in the transmission path between the two terminals. 



3. The advent of night-time con(liti<jns causes a rapid rise in field 

 strength to high values which are mainlained until dajlight approaches. 



4. The encroachment of daylight upon the eastern terminal causes 

 a rapid drop in signal strength. This drop sometimes extends into a 

 morning dip similar to, but smaller than, the evening dip. After 

 this, relatively- steady daylight field strengths again obtain. 



Three or four cur\'es similar to l-'ig. o are obtained each month. 

 B>' taking the a\erage of such curves for the month of September, 1923, 

 the lower cur\e on Fig. (> is obtained. The upper curves are for 

 similar averages of measurements made on the lower frequencies. 

 These curves show clearly that the range of the diurnal fluctuation 

 is less for the lower frequencies. This is because of the lesser day- 

 light absorption. 



The mechanism by wliiih llu- trans.iilaniir transmission path is 

 subjected to these daily and seasonal controls on the part of the sun, 

 would be more evident were we enabled to obser\-e the earth from a 

 fixed point in space. We should then be able to see the North Atlantic 

 area plunged alternately into daylight and darkness as the earth 

 rotates upon its axis, and to visualize the seasonal variation of this 

 exposure to sunlight as the earth revolves about the sun. Photographs 

 of a model of the earth showing these conditions have been made, and 

 are shown in Fig. 7. 'I'lie first cc)ndilion is that for January, in which 

 the entire path is in da\light. The curve of diurnal variation is 

 shown in the picture and that part which corresponds to the daylight 

 condition is indicated b\- the arrow. In the next position the earth 

 has rotated so that the London terminal is in darkness while the United 

 States terminal is still in da%light. This corresponds to the evening 

 dip, the period of poorest transmission. With the further rotation of 

 the earth into full nighl-time conditions for the entire |)alh, the re- 

 ceived signal rist's to the high niglu-timi' \alues. These high \alues 

 contiiuic until llu- p.iih appro.n Ins the d.i\liglil hemisphere as indi- 



