THE AURORA BOREALIS. 



95 



little clouds, and designated by the name of auroral plates (plaques aurorales}. 

 Their milky light frequently undergoes striking changes in its brightness, like 

 motions of dilatation and contraction, which are propagated reciprocally be- 

 tween the centre and the circumference, like those which are observed in ma- 

 rine animals called Medusae. The phenomena become gradually more faint, 

 and generally disappear altogether on the appearance of twilight. Sometime?, 

 however, the aurora continues after the commencement of daybreak, when the 

 light is so strong that a printed book may be read. It then disappears, some- 

 times suddenly ; but it often happens that, as the daylight augments, the aurora 

 becomes gradually vague and undefined, takes a whitish color, and is ultimately 

 so mingled with the cirrho-stratus clouds that it is impossible to distinguish it 

 from them. 



Among the various theories and hypotheses which have been proposed to 

 explain auroras, that which appears most entitled to attention has been suggested 

 by M. Biot. 



The first question which naturally urges itself upon the consideration of the 

 scientific inquirer is, whether the phenomenon is to be regarded as meteoro- 

 logical or astronomical ; in other words, whether it takes place within the limits 

 of our atmosphere, and partakes in common with that fluid in the diurnal motion 

 of the earth, or is situate in a region beyond the limits of the atmosphere, being 

 seen through it, like the stars, planets, comets, and other celestial objects. The 

 relation which the form of aurora invariably bears to the direction of the mag- 

 netic meridian raises a prima facie presumption in favor of the phenomenon be- 

 ing atmospheric ; but all doubt on this question has been removed by the obser- 

 vations of M. Biot, from which it appears that the apparent place of the aurora 

 in relation to celestial objects is not fixed ; that its altitude and azimuth do no-t 

 undergo those hourly changes to which celestial objects are subject ; and that 

 they undergo no motion, in reference to the zenith or horizon, such as would be 

 produced by the diurnal rotation of the earth. It must then be taken as demon- 

 strated, that the aurora borealis is a phenomenon placed within the limits of 

 our atmosphere, and that it is connected with the atmosphere or with some mat- 

 ter suspended in it, partaking of the diurnal motion common to the atmosphere 

 and the globe. 



The fact that the rays or columns of light are always paralled to the dipping 

 needle, and that the bows, coronse, and other visible forms which the phenom- 

 ena afiect, are always symmetrically placed with respect to the magnetic me- 

 ridian, demonstrate that the cause of the phenomena, whatever it may be, has 

 an intimate relation with that of terrestrial magnetism. 



M. Biot conceives that the luminous columns composing the aurora have not 

 in reality the position or form which they appear to the eye to have ; but that 

 their apparent form is merely the result of perspective. He considers that the 

 phenomenon is produced by an infinite number of luminous columns, parallel 

 to the dipping needle and to each other, arranged side by side at nearly the 

 same height from the surface of the earth ; these systems of columns being 

 placed at unequal distances from the eye, and see" Mirier different v?3'?s of 

 obliquity, are projected into various figures, which are subject to variation 

 arising "from the varying splendor of their component rays. 



It has been attempted, on various occasions, to determine the height of auroras 

 by the same method which has been applied with such accurate results to the 

 determination of the distances of the sun, moon, and other celestial objects. 

 This method consists in the comparison of two observations of the exact ap- 

 parent place in the heavens observed at the same moment in distant parts of the 

 earth. Many causes, however, conspire to render this method inapplicable to 

 auroras ; among which may be mentioned the difficulty of making the two ob- 



