390 Analyses of Books. [May, 



meteorologists ; they are nevertheless facts. We have sometimes 

 seen an attenuated Aurora flashing across 100° of the sky in a 

 single second ; a quickness of motion inconsistent with the 

 height of sixty or seventy miles, the least of which has hitherto 

 been ascribed to it. This kind of Aurora is not brighter than 

 the milky way, and resembles sheet-lightning in its motions." 



" For the sake of perspicuity, I shall describe the several parts 

 of the Aurora, which I term beams, flashes, and arches. The 

 beams are little conical pencils of light, ranged in parallel lines, 

 with their pointed extremities towards the earth, generally in 

 the direction of the dipping needle. The flashes seem to be 

 scattered beams approaching nearer to the earth, because they 

 are similarly shaped, and infinitely larger. I have called them 

 flashes, because their appearance is sudden, and seldom conti- 

 nues long. When the Aurora first becomes visible, it is formed 

 like a rainbow, the light of which is faint, and the motion of the 

 beams undistinguishable. It is then in the horizon. As it 

 approaches the zenith, it resolves itself at intervals, into beams, 

 which, by a quick undulating motion, project themselves into 

 wreaths, afterwards fading away, and again brightening, without 

 any visible expansion or concentration of matter. Numerous 

 flashes attend in different parts of the sky. That this mass, 

 from its short distance above the earth, would appear like an 

 arch to a person situated at the horizon, may be demonstrated 

 by the rules of perspective, supposing its parts to be nearly equi- 

 distant from the earth. An undeniable proof of it, however, 

 is afforded by the observations of the 6th and 7th of April, when 

 the Aurora which filled the sky at Cumberland House, from the 

 northern horizon to the zenith, with wreaths and flashes, assumed 

 the shape of arches at some distance to the southward." 



" But the Aurora does not always make its first appearance 

 as an arch. It sometimes rises from a confused mass of light in 

 the east or west, and crosses the sky towards the opposite point, 

 exhibiting wreaths of beams, or coromc boreales in its way. An 

 arch, also, which is pale and uniform at the horizon, passes the 

 zenith without displaying any irregularity or additional brilliancy; 

 and we have seen three arches together, very near the northern 

 horizon, one of which exhibited beams and even colours, but the 

 other two were faint and uniform." 



" On the 7th of April, an arch was visible to the southward, 

 exactly similar to that in the north, and it disappeared in fifteen 

 minutes. It had probably passed the zenith before sunset. The 

 motion of the whole body of Aurora is from the northward to 

 the southward, at angles not more than 20° from the magnetic 

 meridian. The centres of the arches were as often in the mag- 

 netic as in the true, meridian." 



" The colours do not seem to depend on the presence of any 

 luminary, but to be generated by the motion of the beams, and 

 then only when that motion is rapid, and the light brilliant. 



