PART. II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. IV. 607 



The auroral curtain may have characteristic undulating folds and eddies; and from one fold luminous 

 waves may pass along the curtain eastwards and westwards simultaneously ( ! ). 



We will now try to combine these facts with the experimental results at which we arrived through 

 our terrella experiments. 



First we must suppose that the auroral rays do not exactly follow the magnetic lines of force, 

 but that, in what we call negative precipitation, they form a small angle towards the east with the 

 lines of force, while in what we in analogy with the polar storms call positive precipitation, they 

 form a small angle towards the west with the lines of force. We shall subsequently show how these 

 angles towards the east and the west are to be understood. The angles, however, are very small, 

 because the auroral rays are only formed by those rays from space which fall as vertically as possible 

 along the lines of force, and they penetrate, therefore, most deeply into the atmosphere and create 

 the auroral rays. 



There are unfortunately not many observations which can be referred to with regard to this sup- 

 posed inclination between auroral rays and magnetic lines of force, but in the well-known work of PAUL 

 GAIMARD, "Voyages en Scandinavie, en Laponie etc : Aurores boreales", page 505, we note the following 

 remark: "We certainly are justified in stating that the rays are not always strictly parallel with the line 

 of inclination". 



In the same work BRAVAIS makes the following remarks: "We admitted one of the two following 

 hypotheses: either the average orientation of the auroral arcs is not perpendicular to the magnetic 

 meridian, or the average direction of the rays is not strictly parallel with the line of inclination". 



We shall now see that both these hypotheses must be assumed at the same time. 



CARLHEIM GYLLENSKIOLD recapitulates, 1. c., page 69, his result as follows: 



"The disagreement in our observations is rather great. When not taking into consideration the 

 doubtful positions, the difference of the average position is, in two cases, 22 54' and 20 31'; it 

 exceeds 10 degrees in eight others. The average difference is 6 34' and the probable error of the 

 average is + 42'.!. The members of the French expedition on board the corvette "La Recherche" 

 have made, at Bossekop, 43 observations of the centre of the corona; the average difference is 5 and 

 the probable error of the average is o 30'. The greatest difference is 15; it exceeds 12 in two 

 other cases. Our observations consequently agree less with each other than those of the French 

 expedition. However, our observations are probably not in reality less exact than those made at 

 Bossekop; we are inclined to believe that the position of the corona is subject to greater variation in 

 a latitude of 78 degrees than in Finmark". 



Mr. SIRKS OF DEVENTER( 2 ) arrives, through 16 observations made in Europe during the great aurora 

 borealis on February 4th, 1872, at the result that "the corona in almost all places was some degrees 

 inferior to the magnetic inclination; the azimuth of the corona was also less than the magnetic decli- 

 nation". 



When discussing the angle made by the auroral rays with the magnetic lines of force, the angle 

 always meant is that between the tangents of the magnetic line of force and the axis of the auroral ray 

 through the foot-point of its orbit. 



Such an angle will generally have a projection on the plane of the magnetic meridian, through the 

 foot-point, and on a plane through the tangent of the line of force perpendicularly on the meridian. 



(') See CARLHEIM GYLLENSKIOLD: Aurores boreales. Observations faites au Cap Thordsen, Spitzberg, 1882 1883. Stock- 

 holm, 1886, Vol. II: i, p. 136. 

 ( 2 ) POGGENDORFF'S Annalen, Band 149. 



