232 Ml' William Sturgeon on the Aurora Borealis. 



that is, the dipping needle would have to be parallel to one indivi- 

 dual right line, at all places of observation, however wide apart. 

 Now, without taking into consideration the difference of dip at dif- 

 ferent places over which auroral beams often appear at the same 

 time, the different positions of the magnetic meridians, in the planes 

 of which the axis of the dipping needle would repose at those places, 

 would be quite sufficient to shew the fallacy of that part of the hy- 

 pothesis. 



It is somewhat remarkable, that neither Dalton nor any other 

 philosopher that I am aware of, has taken into consideration the 

 electro-magnetic forces of auroral beams or streamers, in disturbing 

 the magnetic needle. These forces are brought into play by every 

 movement of the electric fluid, whether ferruginous or other metal- 

 lic matter be present or not, — as well in the most perfect vacuum 

 as in dense air ; and when such immense floods of the electric fluid 

 are put into motion as constitute a grand aurora borealis, the prin- 

 cipal features of which are extensive groups of streamers, it is to be 

 expected that the electro-magnetic forces of those streamers will dis- 

 turb the compass-needle, causing deflections of different degrees of 

 magnitude, and in different directions, in correspondence with the 

 intensity and direction of the disturbing forces ; and, all other things 

 being the same, the greatest deflections of the horizontal needle 

 would be accomplished by electric streamers that were parallel to it, 

 and, consequently, parallel to the earth's surface, at the place of 

 observation. The supposition of the auroral beams being vertical, 

 or nearly so, and at remote regions above the atmosphere, may pos- 

 sibly have been the cause of the electro-magnetic forces of streamers 

 being so generally overlooked ; but it has long appeared to me that, 

 to their influence the obsei'ved disturbances of the needle are prin- 

 cipally if not solely owing. 



Although the theoretical views which I have taken dispenses en- 

 tirely with the ferruginous effluvium supposed to be floating in the 

 air, I by no means attempt to deny its absolute existence, nor the 

 existence of other metallic effluvia. My motive for contending 

 against the influence of such an agent in producing auroral beams, 

 is to shew that it is quite unnecessary for the purpose it was in- 

 tended, and the manner in which it has been applied preposterous. 

 The manner in which I have attempted to explain the several phe- 

 nomena attending the aurora borealis, requires no other elements 

 nor forces than those well known and understood. The same cause, 

 a sudden depi'ession of temperature, that produces lightning amongst 

 the clouds, would prcduce an aui-ora borealis in a higher region of 

 the air ; a depression of temperature at the earth's surface invari- 

 ably succeeds a lightning-storm, and almost as certainly closely fol- 

 lows an auroi'a borealis ; and very often both of these electrical phe- 

 nomena appear at the same time, — shewing that the disturbance ex- 

 tends to a great height in the atmosphere, and the fall of tempera- 



