211 



areas in lower latitudes may assist to induce aurorae in the 

 Polar zones. 



At Cape Royds, in the case of very faint or sluggish 

 aurorae, there did appear to be some relation to the existing 

 local meteorological conditions. For instance, in certain calm 

 weather, with north-easterly airs blowing, one walked out of 

 the hut to observe with the assurance that certain effects 

 would be visible, and it was usually so. However, much more 

 extended observations would be required to establish this 

 point. 



11. Colour. 



Normal aurorae of high latitudes are of a typical and 

 constant somewhat greenish-yellow tint. That is conditioned 

 by the predominance of the krypton line in its spectrum. 

 Intense displays are accompanied by crimson and greenish 

 tints. This journal contains mention of only about six cases 

 where such colours were observed. The absence of frequent 

 colouration effects indicates that the Cape Royds region. 

 though auroral manifestations are very prevalent, is not a 

 region of intense auroral storms. 



In the case of curtains, as the intensity of the lumin- 



*/ 



escence increases, a narrow selvage of crimson makes its 

 appearance along the lower border. When more intense, the 

 upper portions of the streamers may show a siskin or peacock 

 green. That portion of the curtain between these limiting 

 colours remains the normal aurora tint. 



12. Suggestions in Conclusion. 



Most of the characters of the aurora polaris are best 

 studied by analysis and scrutiny of the regular weak effects 

 which form the normal daily cycle in the Polar regions. 

 Superimposed, and operating at irregular intervals, are auroral 

 storm periods of intense displays. The circumstance of 

 increased intensity leads to progressive changes in the charac- 

 ter of the latter, and has the effect of concealing some of the 

 more fundamental features. 



Extra-terrestrial influences (e.g., sun-spots) may be 

 brought in to explain auroral storms, but the weaker effects 

 are a regular feature of the Polar zones. 



Recent experimenters — Birkland, and others — have given 

 good reason to believe that a definite flow of electricity takes 

 place along auroral arcs and curtains. The extremely small 

 resistance to the passage of electricity in the tenuous upper 

 atmosphere has been dwelt upon by C T. R. Wilson. In the 

 Polar regions this resistance would be expected to be less, 

 for on account of the attractive force exerted on free ions 



