164 Auroral and Sun-spot Frequencies Contrasted. 



As to the question of a possible lag : In the ease 

 of Scandinavia the groups of 3 years selected from con- 

 sideration of sun-spot frequency for use in Table V., 

 represented an earlier epoch than those selected from con- 

 sideration of auroral frequency for use in Table VI. in 

 14 cases, the same epoch in 2 cases, and a later epoch in 

 only 4 cases. This is, to say the least, not unfavourable to 

 the view that auroral frequency tends to lag behind sun-spot 

 frequency. 



In every case we have found the annual variation in 

 auroral frequency, monthly values denoting percentages of 

 the total number for the year, to be more uniform when sun- 

 spots are numerous than when they are few. The differences, 

 however, between the frequencies in the contrasted years are 

 in some instances not very conspicuous, and may be partly 

 due to chance. Also, supposing it to be a fact that the 

 annual variation in temperate latitudes becomes more accen- 

 tuated as sun-spots diminish, this may mean one of two 

 things. There may be, as sun-spots decrease, a greater 

 relative diminution in summer than in winter of the physical 

 phenomena which appeal to our eyes as aurora, or there may 

 only be a general diminution in the brightness of auroras 

 throughout the whole year. From what happens during 

 magnetic storms, it can hardly be questioned that the cause 

 — presumably electric discharges in the upper atmosphere — 

 to which auroral phenomena are due is often active when 

 aurora is invisible. It may even conceivably be in continuous 

 operation, though incapable of appealing to the eye, however 

 favourable the visual conditions, until a certain minimum 

 intensity is reached. 



Only exceptionally brilliant auroras have much chance 

 of being seen until the sun is far below the horizon, 

 so that a general reduction of intensity might well be 

 more prejudicial to visibility at midsummer than at other 

 seasons. 



In conclusion, I should like to draw attention to the 

 utility for inAestigations such as the present of trustworthy 

 auroral observations taken on a uniform plan, desirablv 

 throughout more than one sun-spot cycle, at a consi- 

 derable number of stations suitably distributed over the 

 earth. 



