244 Causes of the Aurora Borealis. 



iaries or proximate causes which bring them into action at one time, 

 and not at another, are unknown. The same inconstancy appertains 

 to temperature, drought, rain, hail and snow. 



II. Reasons for attributing the illumination of the Aurora to the 



Solar light. 



1 . It is presumed that this peculiar vapor rests in the atmosphere 

 at no great elevation, perhaps not far higher than the clouds. After 

 the twilight declines, the rays of light which continue to illuminate 

 the upper heavens, fall upon this highly refractive medium, and are 

 thus made visible upon the earth. When it appears immediately af- 

 ter the twilight, it brightens and deepens as the darkness comes on, 

 like the moon and stars. Its commencing occasionally an hour or 

 two after dark, is probably owing to those unknown agencies which 

 collect the vapor at one time and not at another. The intensity of 

 the light seems to depend upon the quantity of the vapor ; and the 

 prismatic colors, on the aqueous particles contained in it. The pre- 

 vailing color of the light in both the frigid and temperate zone, is 

 white or yellowish white ; and when the deep and glowing hues oc- 

 cur, the vapor probably contains more moisture, and approaches near- 

 er to the condition of clouds. 



2. In the temperate zone the phenomenon chiefly occupies the 

 northern heavens, because the sun having so far declined in its diur- 

 nal rotation, that the direct rays are no longer subject to the refrac- 

 tive power of the atmosphere above the horizon, they illuminate the 

 upper heavens at an altitude beyond the angle of vision, opposite to 

 the illuminating source. From the oblique position of the sun to the 

 earth, that light falls on the northern quarter, and is made visible in 

 the temperate zone, by being refracted over from that direction, 

 through a medium of greater refractive power than common air. The 

 same medium could not produce the phenomenon in the south and 

 south-eastern quarter in the same latitudes ; because the light would 

 be intercepted by the spherical figure of the earth. Thus in the 

 temperate zone, the solar light illuminates the refractive vapor, and 

 produces the phenomenon in the northern verge of the hemisphere, 

 and opposite the illuminating source. 



3. But under what law can it be accounted for, that in the arctic 

 regions it commonly occupies the southern heavens ? That portion 

 of the globe receives the solar rays in an increasing obliquity of dj- 



