OPTICAL PHENOMENA. 



523 



tion at all appears. Transported farther towards the south, the dip again discovers itself, 

 but in an opposite direction, the south pole of the needle inclining downwards. " To 

 understand the reason of this dip of the magnetic needle, and of its general direction, we 

 have only to consider that the earth itself operates as a great magnet, the poles of which 

 are situated beneath its surface. The directive property of the needle is owing to these 

 poles ; and when the needle is on the north side of the equator, the north pole of the 

 earth having the greatest effect, the needle is attracted downwards towards the north 

 pole ; hence exactly over the magnetic pole the needle would be vertical. Similar 

 phenomena occur in the southern hemisphere ; but here the south pole predominates, 

 and of course depresses the corresponding pole of the needle ; while at the magnetic 

 equator, from the equal action of both poles, the needle will assume an exactly horizontal 

 position." 



But neither the magnetic equator nor the magnetic poles coincide precisely with 

 the geographical equator and poles, and this difference constitutes what is termed the 

 variation of the needle. From calculation, the north magnetic pole had been fixed in 

 latitude 70, and longitude 98 30' west, a spot which Sir James C. Ross approached 

 within the distance of ten miles, in the year 1830, but was unable to verify the site, for 

 want of the requisite instruments. Upon going through a long series of calculations 

 afterwards himself, he concluded the above position to have been erroneously assigned, 

 and that the real point lay in latitude 70 5' 17" north, and longitude 96 46' 45" west, 

 a spot on the western coast of Boothia, which he prepared to reach. On the 1st of June 

 1831, at eight o'clock in the morning, he arrived at the site to which his calculations 

 pointed, and found the same day the amount of the dip to be 89 59', only one minute 

 less than 90, the vertical position, which would have precisely indicated the polar sta 

 tion ; and the horizontal needles, suspended in the most delicate manner possible, did not 

 betray the slightest movement. The spot was an unattractive level site along the coast, 

 rising into ridges from fifty to sixty feet high, about a mile inland. The wish expressed 

 by the discoverer was natural, that a place so important had possessed more of mark or 

 note, but Nature had erected no monument to denote the spot which she had chosen as 

 the centre of one of her " great and dark powers." A cairn of some magnitude was con 

 structed by the adventurers, upon which the British flag was planted, and underneath, 

 a canister was buried, containing a record of the interesting enterprise. 



The magnetic needle has frequently exhibited violent disturbance when the Aurora 

 Borealis has appeared. This has led to the surmise that these brilliant lights are 

 connected with the electric and magnetic properties of the earth, though in a manner 

 which we cannot explain. It has been remarked that during the appearance of the 

 aurora the electric fluid may often be readily collected from the air. If a current of 

 electricity also be passed through an exhausted receiver, a very correct imitation of the 

 auroral light will be produced, displaying the same variety of colour and intensity, and 

 the same undulating motions. It is highly probable, therefore, that the beautiful and 

 fantastic meteoric display is connected with electricity ; but great obscurity rests upon 

 this department of meteorology. 



Of all optical phenomena, the Aurora Borealis, or the northern day-break, is one of the 

 most striking, especially in the regions where its full glory is revealed. The site of the 

 appearance, in the north part of the heavens, and its close resemblance to the aspect of 

 the sky before sunrise, have originated the name. The " Derwentwater Lights " was 

 long the appellation common in the north of England, owing to their display on the night 

 after the execution of the unfortunate earl of that name. The scene in the illustration 

 is a picture of the auroral light, as observed from the neighbourhood of Loch Leven 

 a scene in itself admirably calculated to exhibit the phenomenon ; and to convey any 



