AURORA BOREALIS. 117 



even it will produce the irabs, or beam, provided the illuminated 

 cloud is tranquil, but if not it will produce the sagitta, or faces 

 (pencil rays, or torch) ; and if the rays are conveyed from thence by 

 a second reflection, which is very often the case, the bothinia, or 

 cave, will appear with the pencil rays ascending ; the corona and 

 pithifB depend also on the shape of the clouds as well as their 

 positive and negative qualities. 



The colours of the Aurora depend on several circumstances : First, 

 on the colours of the objects which originally receive the sun's rays 

 at the incidental point. Secondly, on the state and qualities of the 

 atmosphere, through which the reflected rays pass before they 

 reach the clouds which they are to illuminate, and thereby render 

 visible to the spectator not only the clouds themselves, but the 

 various colours which the rays have then assumed. Thirdly, by 

 the nature and composition of the cloud itself; however, it is most 

 probable that the colour depends oftener or more materially on the 

 colour of the objects which first receive the sun's rays at the 

 incidental point, from whence they proceed by the laws of reflec- 

 tion, according to the various oblique directions of that surface ; 

 since the observations of Captain Cook, and other antarctic 

 navigators, represent that the Aurora Australis has always " a clear 

 white light," and that no coloured ice has been observed in the 

 antarctic regions; while, on the contrary, ice of every colour has 

 been observed by myself and others to exist in great abundance in 

 the Arctic Regions. 



The reason that the Aurora is not always visible, is evidently 

 because the sky near the pole is often cloudy and foggy, particu- 



