known ly the Name of the Aurora Borealis. 179 



a long stream of light straight or slightly curved according as 

 they are situated with regard to the spectator; and their position 

 "is, generally speaking, north and south, deviating oftentinnes 

 more or less- from that direction, according to circumstances. 

 It appears that streamers vary in their form and position ac- 

 cording to tlie relative position of the sun, earth, and the several 

 surfaces by which they are reflected, ami are longer or shorter 

 to the right or left, or in the meridian, according to the angle 

 with which the rays of the sun fall upon the several reflecting 

 planes: in short, tiiey may be considered as an assemblage of 

 lengthened and distorted images of that Inmhiary. The higher 

 or south ends of the streamers point to the zenith, arid the others 

 -towards the northern horizon, extending over a greater or less 

 space, according to circumstances. They may be aptly com- 

 pared to the ribs of a dome roof having a portion of the upper 

 end cut off, but sometimes uniting at the zenith, and even run- 

 ning into a confused mass, exhibiting various colours as before 

 described. Thiv are also curved in a similar manner to those 

 ribs, owing no doubt to the varying density of the atmosphere; 

 •and perhaps it is increased by the attraction of the earth, in the 

 case both of the streamers and the ribs to which they are here 

 compared : those right over head appear straight, and those seen 

 obliquely show the curve. Although both the form and motion 

 of streamers are very irregular, yet I have witnessed only one 

 other form which is a remarkable exception to their general 

 features just described, and that is the beautiful regular arch af- 

 terwards mentioned. There is a kind of luminous clouds some- 

 times seen in unsettled weather, which I believe have been ge- 

 nerally confounded with the aurora bvrealis, but are evidently 

 entirely different in their nature and origin. 



It is now requisite, on the same principle, to explain how the 

 motion of streamers is produced. In order to this, it is only ne- 

 cessary to suppose the bodies of ice by, which in all pvobabiHty 

 they are reflected, to be in motion; and this they may be by 

 floating in detached masses in water, or descending from heights 

 where they had accumulated into the plain, sometimes moving 

 gently, and sometimes vibrating by violent concussions against 

 each other. As the angular motion of the image, or reflected 

 ray, will correspond to that of the body by which it is reflected 

 — if a mass of ice by rolling or falling change its position sixty 

 degrees, it is evident the streamer reflected by it will in the 

 same time move through a space equal to its distance from the 

 burface which reflects it: this distance may be several thousands 

 of miles. A beam of light sweeping through this immense space 

 in an instant, is perhaps the greatest velocity that the eye can 

 \vitr.e8«3 and to produce which no other cause than the one here 



Z 2 assigned 



