54 Proceedings. 



from the equator, and the same applies iu a lessening degree 

 until the line through Edinburgh is reached. But as tlie 

 Aurora gains an unusual height, so it may he seen from a 

 point lower down on the globe's surface. The height of the 

 Aurora is one of the great problems of the age. It is very 

 variously estimated, and the circum-polar obseiTers having 

 made diligent observations, believe that we may safely place 

 it at between 62 and 124 miles ; but it had previously been 

 estimated as low as 7 miles, and as high as 825. 



The intensity of the Auroral light is not great ; only in its 

 most brilliant displays does it give sufficient to enable large 

 print to be read, and it is insufficient for photography. It is 

 by no means sufficient to light the peasants in their work in 

 the total absence of the sun during the long winter. 



The hissing and crackling sound, of which much has been 

 ■written, is reported on the authority of one scientific party 

 alone. All others record that it is absolutely silent, and 

 this is now generally accepted. 



The times and frequency of the display have been carefully 

 recorded for many years. The daily maximum of intensity 

 occurs about 9 p.m. There are two annual maxima, which 

 are found at the equinoxes, while there are two minima, which 

 occur at the solstices. From each equinox there is a gi-adual 

 decrease until midsummer or midwinter, and then a gradual 

 increase again. 



Besides these annual periods, there are found to be periods 

 of eleven years which give a clear maximum and minimum. 

 These periods are found to correspond with the maximum of 

 magnetic disturbances, and both to correspond with the 

 maximum and minimum of sun-spots and solar coronae. It 

 would thus seem safe to ascribe a common origin — and that 

 a magnetic one — to all three phenomena. 



The resemblance between certain forms of the Aurora and 

 an electric discharge in a chamber of rarefied air is very 

 striking, and the electrical theory which seems to deserve 

 most adherence is that of Prof. Edlund, of Stockholm. He 

 refers it to unipolar induction. 



The globe is regarded as a rotating magnet, with a good 



