or Aurora Borealis and Aiistralis. 339 



the crown in his magnetic zenith — and it is therefore, not the 

 same crown which is seen by different observers in different 

 places — as little as two observers see the same rainbow. It is 

 therefore incorrect to try to fix the perpendicular height of 

 the crown above the surface of the earth from observations 

 made in two places lying far apart. 2. When the arch of 

 ■polar light rises so high as to reach the magnetic zenith, this 

 arch seems also interrupted in this place ; whence we may con- 

 clude that it also consists of short rays parallel to each other. 

 3. The rays often form themselves into a regular ring (DE, 

 fig. 3.), i.e. they dart forth from a small zone of the surface 

 of the earth, the centre of which lies somewhere north of 

 Hudson's Bay. Thus the regular arches are formed. If 

 BC is the horizon for the observer in C, then FCB is the 

 elevation of the lowest edge of the arch, and ECB the highest. 

 If the arch keeps for some time at rest in the same elevation, 

 and another observer in the same magnetic meridian, but 

 north or south of C, has also observed at the same time the 

 elevation of the point F or E, the perpendicular height EG 

 of the arch above the surface of the earth may be determined 

 from it. Mairan, from a great number of such parallactic 

 observations, found this height to be above 100 geographical 

 miles. This must of course also refer to the rays, of which 

 the whole remainder of the aurora consists. The substance 

 of this light, therefore, does not receive its luminous property 

 till it is far beyond the atmosphere. 4. Whilst the substance 

 of the aurora runs through the atmosphere it is not luminous ; 

 for I have never known the aurora to have been seen before 

 a mountain or any other elevated object. On tlie contrary, it 

 seems to have the opposite effect of making the atmosphere 

 opatjue. For when the aurora shows itself", the sky is often 

 within a few minutes overcast by an opaque veil, and as ra- 

 pidly clears up again. The cause is probably this : that the 

 substance of the aurora condenses the aqueous vapours ex- 

 isting in the air in a transparent state. Whence we may also 

 explain the dark segment underneath the arch, and the black 

 rays of the aurora borealis, which have been described by va- 

 rious older Northern observers, and which I have frequently 

 noticed myself. The line of vision runs in the direction CF, 

 or CA (fig. 1.) through a long region of the atmosphere which 

 has become opaque by the rays of the substance of the aurora. 

 Yf, Ee' having passeil through, whence the whole space FCA 

 below the aurora has a dark colour approaching to black. 

 The nearer the place of the observer C is to the northern 

 limity of the aurora, the less dark this obscure segment will 

 2X2 be 



