Proceedings of the Polyteciixic Association. 897 



aiul the sound licard, ^xe. can tell how distant it is. When it is a 

 mile off, it will take five seconds, and at five miles we cannot hear 

 the sound, the telegraph has proved this. The motion of the columns 

 of light is very different from that produced by moving clouds, as 

 every one will testify who has ever in his life seen such an aurora in 

 its full brilliant display, with corona and luminous arcs ; and that it 

 can only be harmonized with electric action is known to all who are 

 acquainted with the experiments of electricity ioi vacuo. The bows 

 spanning the north, often seen, cannot possibly be explained as 

 reflected light, or as the rainbow ; this is beyond a doubt to all those 

 who really understand the laws of reflection and refraction, which 

 originate the rainbow, so well defined by Newton. 



The columns of light sometimes seen shooting up in the west just 

 after sundown, and in the east just before sunrise, and proceeding 

 from the sun, are utterly dissimilar from the auroral beams. The 

 gradual increase of auroral lights from nightfall till midnight, and 

 their gradual decrease from midnight till morning, are not in. har- 

 mony with reflections from the sun ; Mdiile at twelve, midnight, the 

 sun is the lowest under the northern horizon. If it is asserted that 

 in the latter case the light is stronger, it is in contradiction with the 

 increase of auroras in frequency and brilliancy as we go north. That 

 they are unknown at the tropics, is an erroneous statement. 



The fact that they produce no report shows only that they are not 

 electric discharges like strokes of lightning ; but that electricity 

 noiselessly discharges through rarefied air, or through a so-called 

 vacuum, is well known to all students of this branch of science. The 

 asserted fact that in summer when the sun is further north, auroras 

 should be more frequent and brilliant than in winter, when the sun 

 is further south, is in contradiction with the argument of tlie reflec- 

 tion theory ; and it is asserted that at twelve, midnight, the light is 

 strongest, while the sun is lowest. 



In regard to the relation of the aurora to thunder storms in sum- 

 mer, it must be observed that at the magnetic pole, where the aurora 

 borealis always originates, it is nearly always winter. But the direct 

 proof that the aurora borealis is by no means derived from the sun, 

 is produced by the spectroscope, which does not show the lines pecu- 

 liar to solar light, and in also to every object illuminated by the sun, 

 but which in the aurora shows lines not seen in any other known 

 luminous object. Moreover, Biot has proved that reflected light is 

 always partially or wholly polarized ; the auroral light, being not 



[Inst.] 57 



