i86 



SCIENCE 



[Vol. XXI. No. 531 



case a very different thing from a sham biology which is princi- 

 pally, or all, zoology. 



THE AURORA. 



BT W. A. ASHE, F.B.A.S. (RBTIKED) , QUEBEC, CANADA. 



Some notes resulting from a study of the Aurora extending over 

 many years, and pointing out how some of the better known 

 theories fail to account for known conditions of the phenomena, 

 may interest the readers of Science. 



I regret, that after having endeavored to -show how the present 

 theories fail, that I have no theory of my own to advance. I 

 have done a good deal of theorizing on different subjects, at in- 

 tervals in a somewhat busy life, so that there are few who have 

 a better opportunity of knowing how deceptive evidence is which 

 is sought for to support a theory; in other words, how faulty — 

 yet how plausible — the result, when the observed facts are (un- 

 consciously) made to fit the theory, instead of the theory the 

 facts. Argument with such a theorist is futile. To use Profes- 

 sor Swift's words, in Science of Dec. 9, "... aurorge frequently 

 occur when no spots are visible on the sun, . . . sun-spots are often 

 seen when auroral exhibitions . . . are entirely absent, . . . the 

 advocates of the theory . . . answer to the former, that sun-spots 

 may have been on the other side of the sun, and, to the latter ob- 

 jection, that there may have been auroras visible in the Arctic or 

 Antarctic regions, or in both." I do not credit those who pin 

 their faith to a connection between the two classes of phenomena, 

 with having to go so far for an excuse, as they generally utterly 

 ignore the want of coincidence, and instead of discrediting their 

 theory (and I need not add that one failure should have very 

 many times greater weight than one coincidence) calmly ignore 

 it, and proceed with their cumulation. I do not wish to be un- 

 derstood as thinking that there are not dispassionate investiga- 

 tors in this matter; I am only pointing out what I believe to be 

 a very common human peculiarity, and one which I believe does 

 much harm in so far as permitting of the propagation of theories 

 which had else died, still-born, on their authors' hands. 



"The evidence of the correctness of a theory or hypothesis in- 

 creases with the number of facts it is capable of satisfactorily 

 explaining. It diminishes with the number of facts it does not 

 ■explain, and with the number of different ways in which sim- 

 ilar phenomena can be explained. A single fact, inconsistent 

 with any theory or hypothesis, is sufficient to overthrow it," is a 

 statement of fact that will be most useful to us in theorizing, and 

 serve to measui'e some existing theories with. 



Any theory of the Aurora must account for the following, 

 amongst other, peculiarities, which seem to me to be characteristic 

 of the same. A. — That they most frequently occur in the colder 

 half of the year, being limited, approximately, by the same iso- 

 thermal lines as far as the southern limit, in the northern hemi- 

 sphere, of their visibility is concerned, and not depending in this 

 on latitude. It would seem, then, that temperature is a factor in 

 the required theory. B. — Auroral displays do occur in the sum- 

 mer season, when their situation is more equatorial, and, per- 

 haps as a rule, they cover a larger area than the average winter 

 display. It would seem, then, that on the transference of the 

 maximum winter displays from one hemisphere to the other, 

 these displays may take place in intermediate situations. C. — 

 From my experience in these latitudes, summer displays of lim- 

 ited extent seem to be concurrent with a drop in the temperature 

 considerably below that corresponding to the average of the date 

 in question. D. — My experience has been that auroral displays 

 do not occur during generally unsettled weather, requiring (al- 

 though the particular locality of the display may be largely over- 

 cast, permitting only of the aurora being seen behind the clouds 

 or through the interstices) that generally elsewhere the weather 

 should be clear. As though clouds on the horizon of the display 

 {not of the observer) intercepted the influence producing the 

 same. E. — The typical aurora, from which are many departures 

 as pointed out by Professor Swift in the communication men- 

 tioned, is a narrow circular arch in that part of the heavens away 

 from the sun, the concave side of which is usually well defined, 



and beneath which is absolute darkness, into which streamers do 

 )iof descend; the convex side of this arch is, generally, illy de- 

 fined, from which streamers proceed and the light of which is 

 very much less intense than that of the concave side; conveying 

 to me the impression of the light, the visible effect of the influ- 

 ence, being completely cut off by the interposition of the solid 

 mass of the earth, it being assumed to be the intercepting horizon 

 at the altitude of the display. F. —(Speaking still of the typical 

 auroral arch). It is on the lower and brighter side where the 

 greatest horizontal movements and the greatest contrasts in the 

 intensity of its light (forming, amongst other outlines, so called 

 " curtain-folds ") are seen. As though at the horizon of the dis- 

 play, our atmosphere, acting as a lense, concentrated the light 

 (the visible effect of the auroral influence) in just such a way as 

 a spherical, atmospherical, lense would, having its centre 

 "stopped" out by such a body as our earth, in which the densest 

 part being next the earth, the greatest relative variation in 

 its homogeneity would exist and the greatest variation in the 

 transmitted light (the visible effect of the auroral influence), re- 

 sulting in just such movements as we have seen in the typical 

 arch. G. — It has been constantly noted, that two or more ob- 

 servers, situated, say, 100 miles apart, view occasionally, if not 

 always, totally distinct auroral outlines, differing, at times, radic- 

 ally ; so that one observer may report a display differing entirely 

 in class and details from the other at the same instant, or even 

 reporting the entire absence of a display when the local conditions 

 were such as would have permitted its being seen had it existed. 

 From this, it appears to me, we must conclude that the light 

 (the visible effect of the auroral influence) has no material exist- 

 ence in that part of the heavens in which it is seen, else, all ob- 

 servers, so situated on the earth that the point of display is above 

 their horizon and this particular point not obscured by clouds, 

 should see the same display, modified only in detail owing to the 

 effects of perspective attributable to the different points of view. 

 H. — There is an intimate relation between the aurora and mag- 

 netic storms; not sufHcient to permit of our concluding the one is 

 Cause and the other Effect, but suiBcient, I think, to permit of 

 the supposition that both are Effects of a common Cause. These 

 appear to me to be some of the more self-evident peculiarities of 

 the typical Aurora. 



The theory in connection with the aurora which appears to 

 have the greatest hold on the investigator and the general pub- 

 lic, is one which supposes a connection between these displays 

 and certain disturbed — sun-spot — areas of the sun. If one 

 were to accept the evidence that is brought forward to support 

 this supposition, without taking into account the evidence which 

 has, unintentionally, been suppi'essed, or perhaps it would be 

 better to say, "not advanced," it would be a very hardened 

 sceptic who would not admit that this question had been settled 

 for all time. In Astronomy and Astro-Physics^ it is concluded 

 that auroral displays recur at intervals which exactly correspond 

 with that of the solar rotation, and at the instant when this dis- 

 turbed area is at the eastern "limb" of the sun. Dropping for 

 a moment the discussion of the cumulative evidence, it is inter- 

 esting to note the peculiar nature of the force which proceeds 

 from the solar area in this case. If this influence is at its maxi- 

 mum on the appearance of the area on the eastern limb, and not 

 continuous to the western limb, it is evident that the maximum 

 effects are produced horizontally and in one direction only from 

 the sun's surface. It is not impossible that this is so, but it is an 

 unfair assumption to make, apart from any knowledge of a sim- 

 ilarly acting force in nature, and in direct opposition to what ex- 

 perience, in other matters, would suggest as the direction in which 

 such a source of energy would produce maximum results. As to 

 the fact of maximum auroral displays occurring at the instant 

 when the disturbed solar area has reached the eastern limb, the 

 coincidence cannot be as great as claimed, or else the occasions 

 on which this has happened have been given undue prominence 

 in collecting facts to suit the theory, for in a communication to 

 the Royal Astronomical Society-, the Astronomer Royal states, in 



' Eeprint No. 113. 



2 "Monthly Notices," March, If 92. 



