120 AURORA BOREALIS. 



EXPLANATION OF THE PLATE. 



A — The sun as at the Equinox. 



B B — The points of incidence. 



D D — Positions of the spectators. 



C C — Clouds rendered visible by the reflected rays. 



E — The earth as at the Equinox. 



F F — ^The rays of the sun. 



This diagram represents the sun's rays acting on the icy or snow- 

 covered part of the earth, and being reflected over the poles reach 

 clouds which are rendered visible by illumination; and having 

 themselves reflecting qualities, distribute them upwards, down- 

 wards, or in any other direction, and owing to the rotation of the 

 earth, are changing or steadfast according to the nature of the 

 surface at the incidental point which first receives the rays. 



