440 APPENDIX. 



arches were formed by short perpendicular rays passing from 

 one quarter to the other with a lateral motion, or by their 

 being met by similar rays from the opposite direction. The 

 arches, when formed, in general remained nearly stationary, 

 and save out coruscations, which streamed toward the zenith. 

 "When at rest the light was colourless, but when any move- 

 ment took place it exhibited prismatic colours, which increased 

 in strength as the motion became rapid. The coruscations 

 seldom reached our zenith, and more rarely passed to the 

 southward of it, but when that occurred the display was 

 always brilliant : on one occasion only they extended to the 

 southern horizon. 



We remarked, that when any material change was about to 

 occur one extremity of the arch became illuminated, and that 

 this light passed along the belt with a tremulous hesitating 

 movement toward the opposite end, exhibiting the colours of 

 the rainbow. An idea may be formed of this appearance 

 from the examination of the rays of some moluscous animals 

 in motion, such as the nereis, but more particularly the he- 

 roes. Captain Parry has compared its motion to the waving 

 of a ribbon. See Second Voyage, p. 144. As the light pro- 

 ceeded along the arch, coruscations emanated from it ; and as 

 the motion iDccame violent the curve was often deflected and 

 sometimes broken into segments, which were brightest at their 

 extremities, and in general highly coloured. When one ray 

 of the Aurora ci'ossed another, the point of intersection was 

 sometimes marked by a prismatic spot, very similar to that 

 which occurs in the intersections of coronae about the moon, 

 but far more brilliant ; and when the segments, which gene- 

 rally crooked toward the zenith, were much curved, colours 

 were perceptible in the bend. Generally speaking, after any 

 brilliant display, the sky became overcast with a dense haze, 

 or with light fleecy clouds. 



The Aurora has been frequently observed to rest upon a 

 dark nebulous substance, which some persons have supposed 

 to be merely an optical deception, occasioned by the lustre of 

 the arch; but this appearance never occurs above the arch, 

 which would be the case, I think, if these surmises were well 



