Mr William Sturgeon on the Aurora Borealis. 229 



The frequent position of the auroral arch with respect to the mag- 

 netic meridian, and the occasional disturbance of the magnetic needle 

 during an auroral display, are well calculated to associate terrestrial 

 magnetism with the theory of the meteor ; but it would be difficult 

 to imagine how the extravagant appendages of Dalton could be re- 

 quired to give " an irregular oscillation to the horizontal needle," 

 which amounted to no more than " half a degree" on each side of 

 its "mean daily position;"* especially as the principles of electro- 

 magnetism were as well known at the time the last edition of the 

 Hypothesis was published (1836), as they are at the present day; 

 and would as easily have accounted for the needle's movements in- 

 dependently of those appendages as with them ; and although other 

 observers have met with much greater movements of the magnetic 

 needle during an aurora, I can see no reason for supposing that they 

 were due to ferruginous matter, floating in the atmosphere, because 

 the well-known principles of electro-magnetism are, independently 

 of any such ferruginous elements, quite sufficient to accomplish their 

 production. 



The hypothesis of Dalton, with some slight modifications, being 

 that in most repute at the present day, and favoured by the views 

 of some of our most illustrious philosophers, require more than an 

 ordinary consideration ; and, fortunately, being expressed in terms 

 that cannot well be misunderstood, it may be examined without 

 any apprehensions of mistaking the principles on which it is founded. 



In order that our author might not be obscure in his views, he 

 particularly states the difference between the magnetic effluvia of 

 Halley, and i\ie ferruginous matter of which he constructs his cylin- 

 drical magnetic beams. " It may perhaps be necessary here, before 

 the subject is dismissed,'' says Dalton, " to caution my readers not 

 to form an idea, that the elastic Jluid of magnetic matter, which I 

 have all along conceived to exist in the higher regions of the atmo- 

 sphere, is the same thing as the magnetic fluid or effluvia of most 

 writers on the subject of magnetism. This last they consider as 

 the efficient cause of all the magnetic phenomena ; but it is a mere 

 hypothesis, and the existence of the effluvia has never been proved. 

 My fluid of magnetic matter is, like magnetic steel, a substance 

 possessed of the properties of magnetism, or, if these writers please, 

 a substance capable of being acted upon by the magnetic effluvia, and 

 not the magnetic effluvia themselves." 



It is somewhat remarkable that, after such an abrupt dismissal of 

 all preceding attempts at explanation, the hypothesis of Dalton should 

 appear the most extravagant that has hitherto appeared in the his- 

 tory of the aurora borealis. We have no knowledge whatever of the 

 existence of this imaginary ferruginous effluvium ; nor would any 



* Aleteorological Essays, p. 171. 



