AURORAS. 



179 





winds (or anti-trades) toward the polar regions, 

 they form in those regions a sort of positive 

 envelope to the earth, the surface of which is 

 itself usually or ahvays negative. Now the 

 earth being an almost perfect conductor, and 

 the highly rarefied air of the upper regions being 

 also such, while the lower atmosphere is less 

 conductive or highly non-conducting, the result 

 is an arrangement that precisely and on a very 

 grand scale represents the two coatings and in- 

 tervening insulating layer of the electrical jar 

 or the condenser. The antagonistic electricities 

 most strongly intensify each other where their 

 . positions approach the nearest, and this, though 

 not ahvays, will be in a general way near to 

 the two poles; in such positions, when their 

 intensities have passed a limit depending on the 

 insulating power of the lower air. more or less 

 frequent discharges will occur : these discharges, 

 taking place in the lo\ver latitudes and in sum- 

 mer more frequently in form of lightning, will, 

 through difference of meteorological conditions, 

 in the higher latitudes and in winter more fre- 

 quently appear as displays of auroral light. 

 While, through high conducting power of the. 

 earth, the auroral discharge must usually take 

 place almost or quite simultaneously at the two 

 poles, there can evidently exist such differences 

 in the charge at the two poles, in the condition 

 of the intervening air, and otherwise, as shall 

 produce not only differences in the display from 

 moment to moment in either hemisphere, but 

 also inequalities of time and brilliancy in the 

 two, or the appearance of the light at only one 

 of the poles. Owing, moreover, to the low con- 

 ducting power of the medium, the neutralizing 

 of the opposite electricities cannot be instan- 

 taneous, but must be by successive and more 

 or less continuous discharges. 



The magnetic phenomena in our hemisphere 

 attending the aurora consist in an augmenta- 

 tion of the westerly deflection, with often more 

 or less oscillation, followed and occasionally pre- 

 ceded by a feebler easterly deflection. Among 

 the observed electrical phenomena are currents, 

 frequently very intense, in lines of telegraphic 

 wire. The observations of Dr. Walker in Eng- 

 land, and of Prof. Loomis in America, have 

 shown that during the appearance of an aurora 

 these currents vary every moment, and that in 

 direction as well as intensify, flowing more 

 generally from N. to S., but being at irregular 

 intervals reversed. These variations are in 

 part to be ascribed to the location and relative 

 intensity of the discharges ; these taking place 

 at both poles or chiefly at the north pole, the 

 current along the earth will, in the northern 

 hemisphere, flow southward; and the well- 

 known electro-magnetic effect in all such cases 

 will be to produce westerly deflection of the 

 needle. But when the only or stronger dis- 

 charge is at the south pole, the current in the 

 earth will move northward; and those familiar 

 with the character and production of induced 

 currents will see that the plates at the end of 

 the telegraph wires becoming, when charged 



in either direction, rapidly polarized by the 

 current which they transmit, speedily give rise 

 to an induced current, moving in the opposite 

 direction, which will suffice either to diminish 

 or momentarily replace the original one : 

 through such electrical variations, the oscilla- 

 tions and occasional easterly deflection of the 

 needle are to be explained. 



Essentially M. De la Rive's apparatus for re- 

 producing in miniature the auroral display, and 

 showing the fact and influence of the attendant 

 circumstances now considered, consists of a 

 wooden sphere of about 12 inches diameter, 

 to represent the earth, supported at the ends 

 of its horizontal axis by upright soft-iron rods, 

 the magnetic poles being represented by two 

 cylindrical soft-iron rods, at the ends of the 

 axis, projecting each about 2 inches beyond 

 the supports, insulated at all parts save the free 

 ends, and which can be rendered magnetic by 

 action of coils or of the poles of an electro-mag- 

 net, when desired, while the principal portion 

 of the length of each outside the body of the 

 sphere, is situated within and in the middle 

 line of a glass tube of larger diameter (about 

 4 inches), and greater length ; these tubes 

 being hermetically closed at the two ends with 

 metallic disks, as well as furnished with stop- 

 cocks through which the air within them can 

 be exhausted to any rarity desired. It is with- 

 in these partially vacuous tubes that the minia- 

 ture auroras are to be produced. Through the 

 inner metal disk of each, of course, the soft iron 

 (magnetic) pole projects, terminating about 

 mid-length of the tube; immediately about 

 and concentric with each of these poles, is a soft- 

 iron ring, of such size that it lies just within 

 the glass tube, and leaves an annular space be- 

 tween itself and the pole : the ring plays the 

 part of the upper electrified air ; the intervening 

 space, that of the stratum in which the dis- 

 charge becomes visible. To obtain the effect 

 of the earth's conduction between the poles, a 

 band of blotting paper is extended from one of 

 the soft-iron poles to the other ; while a similar 

 band is extended equatorially about the sphere ; 

 and during experiments both these are kept 

 moist by capillary action from a little capsule 

 containing salt water. On the opposite sides 

 of the equator of the sphere, small copper 

 plates are fixed to the wood upon the meridian 

 conducting band of paper, and wires proceed 

 from these to the coil of a galvanometer needle 

 about 36 feet distant, so as to be beyond the 

 influence of the magnetic action upon the poles 

 of the sphere. The electrical current used is 

 that of a Ruhmkorff 's coil, one pole of which, 

 through the conducting paper bands, is made 

 to communicate with the soft-iron poles of the 

 sphere, while the other divides into two 

 branches, and by wires passing through the 

 outer disks of the glass tubes, terminates in the 

 two rings. Having the tubes properly exhaust- 

 ed, let the positive pole of the electrical appa- 

 ratus communicate with and terminate in the 

 poles of the sphere, while the negative termi- 



