230 Mr William Sturgeon on the Aurora Borealis. 



magnetist ever suppose that such a fluid, even were it admitted to 

 have an existence, would put on the form of cylindrical beams, and 

 at the same time adapt itself into rings round the magnetic poles of 

 the earth. Moreover, as this imaginary fluid is supposed to be float- 

 ing within the atmosphere, the hypothesis is left in a state of im- 

 perfection fi'oni a want of information respecting the author's mode 

 of expanding the shell of air to the thickness of 150 miles, the 

 height at which he has placed the aurora borealis. 



The same hypothesis supposes that the auroral beams " are simi- 

 lar and equal in their real dimensions to one another," an assertion 

 by no means sanctioned by observation ; but, on the contrary, perfectly 

 at variance with the appearances generally. The hypothesis also 

 supposes that the auroral beams are all " parallel to the difping 

 needle at the places over which they appear;'' and that " the point 

 in the heavens to which the beams of the aurora appear to converge 

 at any place, is the same as that to which the south pole of the 

 dipping needle points at that place.'' With all due respect for the 

 philosophical ability and skill of Dr Dalton, the cause of science has 

 a predominating claim to our regards over all other considerations 

 in discussions of this nature ; there can, therefore, be no impropriety 

 in stating that, were there no other observations to discountenance 

 this part of the hypothesis, those on the aurora of the 17th Novem- 

 ber last would be suflicient to prove its inaccuracy. 



That the auroral arches, when they appear in the north of these 

 latitudes, cross the magnetic meridian at nearly right angles, is a 

 fact very frequently observed, though it is by no means its universal 

 position. The highest point of the arch is probably as frequently 

 in other positions as in the magnetic north : it is sometimes several 

 degrees eastward of the true north, at other times due north ; and, 

 on many occasions, it never appears at all. To admit that the arch 

 is a visible part of a complete ring that surrounds the magnetic pole 

 of the earth, and that at the same time it crosses, at right angles, 

 the magnetic meridians of every place of observation, would be to 

 admit a complete system of confusion — in fact, an absurdity. Ac- 

 cording to Hansteen's and Barlow's maps, the curve of equal vaca- 

 tion that passes through Great Britain, passes also a little north of 

 the Western Isles, through Newfoundland and into Hudson's Bay ; 

 and in the other direction, it passes through the Noi-th Sea, the 

 Shetland Islands, and thence almost direct north past Spitzbergen. 

 No circular ring that could possibly be imagined to surround the 

 north-western magnetic pole of the earth, would answer the other 

 parts of the hypothesis for all the magnetic meridians of that parti- 

 cular curve of equal variation. In Hudson's Bay, the magnetic 

 meridian would be at right angles to the magnetic meridan of Great 

 Britain ; and in many parts of the curve there would be such obli- 

 quities of the magnetic meridians to each other, that but very few 

 of them would cross tangents to the supposed ring, at right angles, 



