VOL. XXIX.] PHILOSOPHICAL TRANSACTIONS. 223 



Strange appearances we have been treating of, and for some of the most difficult 

 to account for otherwise; as, why these lights are rarely seen any where else 

 but in the north, and never, that we hear of, near the equator: as also why 

 they are more frequently seen in Iceland and Greenland, than in Norway, 

 though nearer the pole of the world. For the magnetical poles, in this age, 

 are to the westward of our meridian, and more so of that of Norway, and not 

 far from Greenland; as appears by the variation of the needle this year ob- 

 served, full 12 degrees at London to the west. 



The erect position of the luminous beams or striae so often repeated that 

 night, was occasioned by the rising of the vapour or lucid matter nearly per- 

 pendicular to the earth's surface. For any line erected perpendicularly on the 

 surface of the globe, will appear erect to the horizon of an eye placed any 

 where in the same spherical superficies ; as Euclid demonstrates in a plane, that 

 any line erected at right angles to it, will appear to be perpendicular to that 

 plane from any point of it. That it should be so in the sphere is a very pretty 

 proposition, not very obvious, but demonstrated from Prop. 5, Lib. i, Theodosii 

 Sphaeric. For by it all lines erected on the surface pass through the centre, 

 where meeting with those from the eye, they form the planes of vertical circles 

 to it. And by the converse hereof it is evident, that this luminous matter 

 arose nearly perpendicular to the earth's surface, because it appeared in this 

 erect position. And whereas in this appearance (and perhaps in all others of 

 the kind) those beams which arose near the east and west, as l, m, n, were 

 farthest from the perpendicular, on both sides inclining towards the south, 

 while those in the north were directly upright : the cause of which may well be 

 explained by the obliquity of the magnetical curves, making still obtuser angles 

 with the meridians of the terrella, as they are farther from its poles. 



Hence also it is manifest how that wonderful corona, that was seen to the 

 southward of the vertex, in the beginning of the night, and so very remark- 

 able for its tremulous and vibrating light, was produced; viz. by the concourse 

 of many of those beams rising very high out of the circumjacent regions, and 

 meeting near the zenith : their effluvia mixing and interfering with one another, 

 and so occasioning a much stronger but uncertain wavering light. And since 

 it is agreed by all our accounts that this corona was tinged with various colours, 

 it is more than probable that these vapours were carried up to such a height, as 

 to emerge out of the shadow of the earth, and to be illuminated by the direct 

 beams of the sun : whence it might come to pass that this first corona was 

 seen coloured, and much brighter, than what appeared afterwards in some 

 places, where the sight of it was more than once repeated, after the sun was 

 gone down much lower under the horizon. Hence also it will he easily under- 



