212 



by the Magnetic Poles the atmosphere should there be more 

 highly ionized. 



The bombardment of the daylight hemisphere of the 

 earth by the sun ionizes the atmosphere and increases its 

 potential. Even the heating effect of the sun contributes by 

 causing thermo-electric charges. Difference of potential may 

 further be produced by the fact that water yapour condenses 

 more readily upon electro-negative ions, entrapping them and 

 carrying them down to the earth, thus leaving the upper air 

 positively electrified. As cloud formation and rain precipita- 

 tion are greatest in evening hours, this tends to a definite daily 

 cycle. By such means an average difference of potential 

 tends to be established between the daylight and dark hemi- 

 spheres of the earth. 



The fact that bands and curtains on normal occasions at 

 Cape Royds usually followed a daily cycle, swinging round 

 always directed from the daylight to the dark hemispheres, 

 suggests that such are lines of current flowing to equalize the 

 potential in the daylight and dark hemispheres. 



In support of this suggestion may be mentioned that 

 should such a flowage of electricity take place in the higher 

 atmosphere the Polar regions are most favourable for it. 

 Attention is drawn to the consideration that arcs from regions 

 on the earth of prolonged sun bombardment to regions most 

 remote (in time) therefrom are much longer round the earth 

 in lower latitudes than are trans-Polar arcs. 



Topography and the distribution of land and sea have a 

 considerable influence on the permeability of the ether to 

 "wireless" waves. So the fact that auroral arcs are not 

 always mathematically simple and direct — indeed, that they 

 seem to avoid certain regions and are crowded into others — 

 is merely repeating experience in the transmission of "wire- 

 less" waves. 



The existence of overhead currents flowing in arcs across 

 the Polar regions from the daylight to the dark hemisphere 

 would account for the diurnal swing of the compass. 



DESCRIPTION OF PLATES. 



Plate XIX. 

 Locality Map of the Observing Station at Cape Royds. 



Plate XX. 



Diagram to illustrate time and place distribution at the Cape 

 Royds Station. The graphs illustrate auroral frequency in relation 

 to the hour of the day considered for the seventeen principal 

 directions in the heavens. Radius vectors plotted to relative* 

 frequency values of observed phenomena. 



