Curiosities of Science. 165 



SPLENDOUR OF THE AURORA BOREALIS. 



Humboldt thus beautifully describes this phenomenon : 

 The intensity of this light is at times so great, that Lowenorn (on 

 June 29, 1786) recognised its coruscation in bright sunshine. Moticn 

 renders the phenomenon more visible. Round the point in the vault of 

 heaven which corresponds to the direction of the inclination of the 

 needle the beams unite together to form the so-called corona, the 

 crown of the Northern Light, which encircles the summit of the heavenly 

 canopy with a milder radiance and unflickex*ing emanations of light. 

 It is only iu rare instances that a perfect crown or circle is formed ; but 

 on its completion, the phenomenon has invariably reached its maximum, 

 and the radiations become less frequent, shorter, and more colourless. 

 The crown, and the luminous arches break up; and the whole vault of 

 heaven becomes covered with irregularly scattered, broad, faint, almost 

 ashy-gray, luminous, immovable patches, which in their turn disap- 

 pear, leaving nothing but a trace of a dark smoke-like segment on the 

 horizon. There often remains nothing of the whole spectacle but a white 

 delicate cloud with feathery edges, or divided at equal distances into 

 small roundish groups like cirro-cumuli Cosmos, vol. i. 



Among many theories of this phenomenon is that of Lieu- 

 tenant Hooper, R.N., who has stated to the British Association 

 that he believes " the Aurora Borealis to be no more nor less 

 than the moisture in some shape (whether dew or vapour, liquid 

 or frozen), illuminated by the heavenly bodies, either directly, or 

 reflecting their rays from the frozen masses around the Pole, 

 or even from the immediately proximate snow-clad earth." 



VARIETIES OF LIGHTNING. 



According to Arago's investigations, the evolution of Light- 

 ning is of three kinds : zigzag, and sharply defined at the 

 edges; in sheets of light, illuminating a whole cloud, which 

 seems to open and reveal the light within it ; and in the form 

 of fire-balls. The duration of the first two kinds scarcely con- 

 tinues the thousandth part of a second ; but the globular light- 

 ning moves much more slowly, remaining visible for several 

 seconds. 



WHAT IS SHEET-LIGHTNING ? 



This electric phenomenon is unaccompanied by thunder, or 

 too distant to be heard : when it appears, the whole sky, but 

 particularly the horizon, is suddenly illuminated with a flicker- 

 ing flash. Philosophers differ much as to its cause. Mat- 

 teucci supposes it to be produced either during evaporation, or 

 evolved (according to Pouillet's theory) in the process of vege- 

 tation ; or generated by chemical action in the great laboratory 

 of nature, the earth, and accumulated in the lower strata of the 

 air in consequence of the ground being an imperfect conductor. 



Arago and Kamtz, however, consider sheet-lightning as reflections 



