ANALYSIS OF THE IONOSPHERE 



475 



sees the curve lengthening out to the right as year follows year. 

 During this series of years the sun spots were growing more frequent: 

 the sun was in the ascending part of that eleven-year cycle of its fever, 

 of which the sun spots are one of the manifestations, while the form 

 of these curves is another. Do not, however, misread this statement 

 as meaning that the curve lengthens out, when and only when there 

 are sun spots on the solar disc ! there is no such correlation. 



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EASTERN STANDARD TIME 



Fig. 11 — Dependence of virtual altitude of the crowns of the principal layers on 

 season and hour of day. (Smith, Gilliland and Kirby.) 



Another way to study dependence on the sun is to pick out salient 

 features of the i}i\j) curves, and see how they vary. Such features 

 are the abscissae and the ordinates of the right-hand ends of the 

 branches and of the top of the F\ crinkle. The abscissae are the 

 penetration-frequencies for the layers £, F\ and F2; the ordinates are 

 the virtual heights (not the true heights) of what I have called the 

 crowns of these layers. All are shown, in their dependence on season 

 and hour of day, in Figs. 10 and 11. 



It may have struck the reader that for the last five paragraphs 

 there has been no allusion to the theory, the correlation of the i}i\j) 



