346 Mr Blackader's Account of' an Aurora Borealis 



be supplied by proximity, although defective in point of ra- 

 diance. But its position relatively to the star, and the relation 

 of its extremities to the earth, combined with its apparent length, 

 may possibly aiFord some criteria. Its nebulous, evanescent ac- 

 companiments, were too circumscribed to offer any certain data. 

 They may have been thin clouds, illuminated by the aurora, 

 which seems to be corroborated by the fact, that only the most 

 southerly and lowest portions were accompanied with them, and 

 this only under their lower extremities. Their forms seemed to 

 bear relation to the position and intensity of the lights, in the 

 absence of which they would have been invisible. 



At whatever altitude the aurora makes its ap}:)earance, it in 

 general gradually retires in the direction of north-west, until it 

 descends below the horizon, which is commonly accomplished 

 by 11 p. M. In proportion as the horizontal distance is increas- 

 ed, the longitudinal extent of the meteor is contracted. In the 

 Arctic Regions, when the aurora presents a continuous arch, it 

 is a small segment, m. this particular resembling the arch de- 

 scribed by the clouds of that region, which, in the winter at 

 least, is comparatively low, and the aurora also is perceptibly low 

 in proportion, — which circumstances combine in producing an 

 appearance of radiation, with divergent rays. 



The singular and very striking aurora noticed on a former 

 occasion, as occurring here, in March 1825, was not without 

 parallel. I have since learned that Professor Hallstrom of Abo, 

 bad observed similar black rays. The sketch 3, of Plate IV. 

 may assist to form an idea of it. And it may be permitted to 

 account for their appearance on possible principles. Thus the 

 stratum of clouds often formed immediately above that which, 

 on that occasion, was interposed as a dark screen betwixt the eye 

 and the lower portion of the northern regions of the heavens, is 

 vei'y fertile of detached turgid clouds, which seldom move with 

 rapidity. Over that screen of continuous clouds, the aurora 

 hght was brilliant, proceeding from a quarter depressed below 

 its margin, and illuminating its superior edge ; all below, with 

 the intervening landscape, being involved in pitchy darkness. 

 Suppose a few of these detached masses of cloud to be extended, 

 from right to left, at various distances behind the screen, it is 

 certain that the light beyond them would project their shadows 



