248 EEPOET— 1891. 



the labours of Dr. Sophus Tromholdt, who has spent a year 

 within the Arctic Circle studying them, we now know that their 

 movements are not as eccentric as they have hitherto appeared 

 to be. He tells us that the Aurora Borealis, with its crown of 

 many lights, encircles the Pole obliquely, and that it has its 

 lower edge suspended above the earth at a height of from fifty 

 to a hundred miles ; the mean of 18 trigonometrical measure- 

 ments, taken with a base-line of 50 miles, being 75 miles. 

 The aurora forms a ring round the Pole, which changes its 

 latitude four times a year. At the equinoxes it attains its 

 greatest distance from its Pole, and at midsummer and mid- 

 winter it approaches it most closely ; and it has a zone of 

 maximum intensity which is placed obliquely between the 

 parallels of 60° and 70° N. The length of its meridional ex- 

 cursion varies from year to year, decreasing and increasing 

 through tolerably regular periods, and reaching a maximum 

 about every eleven years, when, also, its appearance simul- 

 taneously attains to its greatest brilliancy. Again, it has its 

 regular yearly and daily movements or periods. At the 

 winter solstice it reaches its maximum annual intensity ; and 

 it has its daily maximum at from 8 p.m. and 2 a.m., accord- 

 ing to the latitude. Thus at Prague, in 50° N. lat., the 

 lights appear at about 8.45 p.m. ; at Upsala, 60° N. lat., at 

 9.30 p.m.; at Bosskop, 70° N. lat., at 1.30 a.m. Now, while 

 these data may be true for the Northern Hemisphere, it remains 

 to be proved how far they apply to the Southern. Indeed, 

 seeing that the atmosphere of the latter region is moister and 

 shallower than that of the former, it is probable that the 

 phenomena would be modified. A systematic observation of 

 the Aurora Australis at a number of stations in high latitudes 

 is therefore desirable. 



Whether or not there is any connection between am'oral 

 exhibitions and the weather is a disputed point. Tromholdt 

 believes that such a relationship is probable (" Under the 

 Eays," vol. i., p. 283). He says that, " however clear the sky, 

 it always became overcast immediately after a vivid exhibition, 

 and it generally cleared again as quickly " (" Under the 

 Eays," vol. i., p. 235). Payer declares that brilliant auroras were 

 generally succeeded by bad weather (" Voyage of Tegethoff," 

 vol. i., p. 324), but that those which had a low altitude and 

 little mobility appeared to precede calms. Koss remarks of a 

 particular display, " that it was followed by a fall of snow, as 

 usual" (Eoss's Voyage, vol. i., p. 312). Scoresby appears 

 to have formed the opinion that there is a relationship indi- 

 cated by his experience. It is therefore allowable to regard 

 the ultimate establishment of some connection between these 

 two phenomena as a possible contingency. If, then, we look 

 at the eleven-year cycle of auroral intensity from the meteoro- 



