Page:Philosophical Transactions - Volume 029.djvu/436

 ſtill more and more oblique, as the Diſtance from the Poles is greater. To this we beg leave to ſuppoſe, that this ſubtile Matter, no otherways diſcovering it ſelf but by its Effects on the Magnetick Needle, wholly imperceptible and at other times inviſible, may now and then, by the Concourſe of ſeveral Cauſes very rarely coincident, and to us as yet unknown, be capable of producing a ſmall Degree of Light; perhaps from the greater Denſity of the Matter, or the greater Velecity of its Motion: after the ſame manner as we ſee the Affluvia of Electrick Bodies by a ſtrong and quick Friction emit Light in the Dark: to which ſort of Light this ſeems to have a great Affinity.

This being allowed me, I think we may readily aſſign a Cauſe for many of the 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 Groenland, 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 Groenland; as appears by the Variation of the Needle this Year obſerved, full twelve Degrees at London to the Weſt.

The erect Poſition of the luminous Beams or Stri ſo often repeated that Night, was occaſioned by the riſing of the Vapour or lucid Matter nearly perpendicular to the Earth’s Surface. For that any Line erected perpendicularly upon the Surface of the Globe, will appear erect to the Horizon of an Eye placed any where in the ſame ſpherical Superſicies; as Euclid demonſtrates in a Plain, that any Line erected at Right Angles to it, will appear to be perpendicular to that Plain from any Point thereof, That it ſhould be ſo in the Sphere is a very pretty Propoſition,