March lo, 1887] 



NA TURE 



435 



needle. These constitute what we are accustomed in the 

 northern hemisphere to call the aurora boreahs. 



Aurora; are, in this view, the polar equivalents of 

 lightning. The same office of relieving the electrical 

 tension of the air is fulfilled by them with innocuous tran- 

 quillity. Not indeed in absolute silence, though the 

 "eerie din" of their rustling streamers has been caught 

 by very few ears. Major Dawson, however, was fortunate 

 enough to hear once, and once only, during his sojourn at 

 Fort Rae, 18S2 S3, a sound " like the swishing of a whip, 

 or the noise pro.luceJ by a sharp sqaall of wmj in the 

 upper rigging of a ship," which accompanied, with its 

 crescendo and diminuendo, the brightening and fading of 

 an aurora visible at the time. This was the first official 

 confirmation of innumerable less authentic reports to the 

 same effect. 



The mutual relations of aurora and thunderstorms are 

 full of significant interest. In point of geographical dis- 

 tribution, they may bs termed complementary. The one 

 kind of phenomenon is not more characteristically of 

 polar than the other is of tropical origin. We take from 

 M. Lemstrom's pages the following concise table, strongly 

 corroborative of Ediun I's theory, showing the dependence 

 upon latitude of storm-frequency : — 



Yet the two kinds of atmospheric luminosity are sepa- 

 rated by profound distinctions. Thunderstorms give no 

 sign of systematic magnetic associations. Sometimes, it 

 is true, the needle may be seen to quiver at the instant 

 of a lightning-flash, but by what seems a casual disturb- 

 ance, quite different from the tumultuous agitation which 

 accompanies or even betrays the darting of northern 

 lights. 



Storms are, nureover (so far as is known), completely 

 e.\empt from the complicated periodicity by which auroral 

 appearances are regulated. Now this, as M. Troinholt 

 discovered in 1882, is exactly inverted in the far north. 

 His discussion of M. Kleinsmidt's observations (1864-S0) 

 at Godthaab in North Greenland made it apparent that, 

 on the polar side of the great auroral zone, a sunspot 

 maximum brings with it an auroral minimum, and vice 

 versA ; that the two equinoctial peaks of the auroral curve 

 in middle latitudes there coalesce into a single elevation 

 at the winter solstice ; and that the evening maximum 

 noted further south is there shifted to the morning. Nor 

 are these curious inversions peculiar to Godthaab. 



Their immediate cause is easily understood. The 

 auroral zone swings to and fro in several superposed 

 periods, over the surface of the earth. .As the sun's activity 

 augments, it travels slowly towards the equator, and re- 

 treats towards the pole as it diminishes, diversifying its 

 progress with minor oscillations, daily, bi-annual, and 

 (perhaps) monthly. The analogy of the closing-in of the 

 solar spot-zones with approach to maximum is striking, 

 but unexplained. Auroral periodicity is thus seen to 

 depend, not upon cessation, but upon removal, and the 



observed reciprocal relation between auror:^! frequency 

 in middle and very high latitudes is completely accounted 

 for. 



We can even go a step further. By Edlund's theory 

 an increase of atmospheric electricity must be attended 

 by a lowering of the latitude in which recomposition takes 

 place. That is to say, the observed shifting in space of 

 the auroral zone implies, and is explained by, a rise and 

 fall of potential in the upper conducting strata of the air, 

 synchronising with the rise and fall of solar disturbance. 

 This is probably in part an indirect effect of the fluctuations 

 in the sun's activity. 



Electricity, in M. Edlund's view, is simply the ether of 

 space, a certain share of which belongs naturally to every 

 ponderable substance. When this normal store is by any 

 means augmented, the body on which the accumulation 

 takes place is positively electrified ; when it is diminished, 

 negative electrification ensues. Hence, the expulsion of 

 this so-called " fluid " from the earth by magnetic in- 

 ductive action leaves it, ipso fac/o, negatively charged, 

 and produces a positive charge in the air. 



A\\ this M. Lemstrom, in the work before us, readily 

 admits ; but he supplements the magnetic forces at work 

 in disturbing our planet's electrical equilibrium with 

 evaporation, of which the enormous capabilities for pro- 

 ducing high potentials have been indicated by Prof. 

 Tait (Nature, vol. xxix. p. 517). But evaporation 

 doubtless proceeds most vigorously when the sun's 

 radiative energy is strongest — that is (presumably), at 

 epochs of spot-maximum, so that an obvious link is thus 

 supplied between the solar and auroral periods. The 

 sequence of cause and effect is as follows : the sun's 

 increased power quickens the development of aqueous 

 vapour ; this, in its turn, gives rise to a more copious 

 supply of atmospheric electricity ; added tension insures 

 more speedy neutralisation ; the zone of gradual recom- 

 position descends towards the equator, and auroras are 

 more frequently visible in middle latitudes. 



Yet this is perhaps not the whole truth. Many circum- 

 stances speak in favour of a direct electrical inductive 

 action of the sun upon the earth. M. Ouet has lately 

 shown {Comptes rendus, t. xcviii. p. 103S) that the exist- 

 ence of a magnetic fluctuation corresponding in period 

 with the sun's rotation on its axis is otherwise inexplicable ; 

 and the instantaneous response of the terrestrial magnetic 

 system to the solar outburst of September i, 1S59, almost 

 compels the same inference, which is strengthened by the 

 undoubted growth of magnetic intensity with solar activity. 

 Hence, as sunspots become numerous, the circulatory 

 process described by M. Ediund must be quickened and 

 strengthened; atmospheric electrical tension will be height- 

 ened ; and although the repellent magnetic power is pro- 

 portionately reinforced, this is more than counterbalanced 

 by the added mutual attraction between the opposite elec- 

 tricities of earth and air. Through this cause also, then, 

 the auroral zone widens its distance from the pole once 

 in eleven years. 



The view that aurora are due to currents of positive 

 electricity illuminating the air on their passage to the 

 earth, has been solidly established by M. Lemstrom's 

 results at Sodankylii. His "discharging apparatus'' 

 served the precise purpose of Franklin's kite. The one 



