238 AURORA EOREALIS. 



phere is conveyed to the earth. On account of the low conducting power of the 

 medium, the neutralization of the opposite electricities would not be eifected 

 iustantaneousl3^ but by successive discharges, more or less continuous, and va- 

 riable in intensity. These discharges should take place almost simultaneously 

 at the two poles, since the electric tension of the earth should be nearly the 

 same at each pole. 



Figure 5 represents the system of circulation here supposed, the north and 

 south poles of the earth being denoted by the letters N. and S.; and this, as I 

 understand it, is substantially the tlieory of Professor De la Rive. 



10. When electricity from the upper regions of the atmosphere discharges 

 itself to the earth through an imperfectly conducting medium, the flow could 

 not be everywhere uniform, but would take place chiefly along certain lines 

 where the resistance was least ; and if the air be sutliciently rare, this current 

 must develop light, forming thus an auroral beam. It might be supposed that 

 these beams must necessarily have a vertical position, but their position is con- 

 trolled by the earth's magnetism. Professor Plucker, of Bonn, has shown that 

 " when magnetic forces act upon a perfectly flexible conductor through which 

 an electric current passes, equilibrium can only exist when the conductor 

 assumes the form of a magnetic curve."* Now, the axis of the dipping needle at 

 any point on the surface of the earth lies in the magnetic curve passing through 

 that point. Hence the axis of an auroral streamer must lie in the magnetic 

 curve Avhich. passes through its base. 



During the prevalence of a brilliant aurora the inclination of the needle some- 

 times changes to the extent of two or three degrees. ■ Hence the auroral 

 streamers cannot always preserve the same position, but their average inclina- 

 tion should not differ much from the mean dip of the magnetic needle. Hence 

 results an apparent convergence of all the beams towards the magnetic zenith, 

 forming the auroral corona. 



11. Auroral arches assume a position at right angles to the magnetic meridian, 

 in consequence of the influence of the earth's magnetism. Auroral arches gen- 

 erally consist of a collection of auroral beams all nearly parallel to each other. 

 These beams tend to arrange thetuselves upon a curve which is perpendicular 

 to the magnetic meridian, forming thus a ring about the magnetic pole. The 

 same law has b'een discovered to hold true for a stream of electricity under the 

 influence of an artificial magnet. When electricity escapes from a metallic con- 

 ductor under a receiver from which the air has been exhausted, it escapes in 

 streams of diffuse rosy light which appear to diverge from the conductor. But 

 Professor De la Rive has shown that if this conductor be the pole of a powerful 

 magnet, the electric light forms a complete luminous ring around this conductor, 

 and this riug has a movement of rotation around the pole of the magnet, some- 

 times in one direction and sometimes in another, according to the direction of 

 the discharge and the direction of the magnetization.f 



A similar eftect takes place on a grand scale during auroral exhibitions. The 

 auroral arch is a part of a luminous ring sustained everywhere at about the same 

 elevation above the earth, having the north magnetic pole for its centre, and 

 cutting all the magnetic meridians at right angles. The influence of the north 

 magnetic pole of the earth determines this position, as the pole of an artificial 

 magnet determines the electricity which escapes from it to assume the form 

 of a ring. 



12. We have found that auroral arches are not always exactly perpendicular 

 to the magnetic meridian, and that in some places this deviation is pretty uni- 

 form and is considerable in amount. At Bossekop (latitude 70°) the average 

 deviation is lO'^ towards the west of the magnetic meridian. We can ascribe 



*Loncl. Ed. and Dub. Phil. Mag., 4tli ser., v. 18, p. 2. 



t Do la Rive's Elec, vol. 2, p. 24d ; and Lond. Ed. aud Dub. Phil. Mag., June, 1802, p. 2. 



