Notable Display of Northern Lights. 69 



pears, and I think the question of height might be sus- 

 ceptible of correction in some particulars. 



I have long been of the opinion that the silent auroral 

 discharge takes place between the higher reaches of the 

 atmosphere and lower strata, where it becomes dispersed, 

 and where air-currents of different electric potential play 

 a somewhat similar part to thunder clouds. It would 

 seem that the present observation tends to confirm that 

 view. If, however, the aurora never descends to cloud 

 level, — if 45 miles above the earth's surface is the 

 lowest limit, and 500 the upper, it would be difficult to 

 understand what air-currents could exist at that elevation, 

 to play the part I have in mind. We would have to fall 

 back on a superior atmosphere of cosmic dust, or some- 

 thing of that kind ; and while meteoric dust no doubt gives 

 tone to the spectroscopic light of the aurora polaris, yet we 

 appear as far from the mechanism of the electric display, 

 when assuming a medium of intrusive meteoric dust, as 

 when we confine our flights of speculative ingenuity to 

 the air. 



In the second place, the West to East motion of the 

 succession of arches, which was a distinct feature of the 

 latter part of this display, leads me to suppose that almost 

 every aurora has a certain amount of motion of the kind. 



In support of this idea I may point out that while the 

 rays or beams move east or west, up or down, and while 

 the arch swells south, or sinks back north or sways east 

 and west, — yet during a period of say half an hour, or 

 more, spots and patches, and more particularly crests of 

 recurrent brightness have a marked tendency to drift off 

 to the east, and disappear there. 



Likewise when the display is fuming and flickering up 

 overhead, cloud like fields have the tendency to be every 

 now and then re-illuminated. If these patches are watched 

 for, they also exhibit a slow drift in their position, always 

 to the east. Frequently as the display fades out, these 

 drifting overhead patches are replaced by thickening fields 



