1U 



TERRESTRIAL LIGHT. 



TERRESTRIAL LIGHT. 



isc 



up to this moment clear, grows black. There U a kind of haiy hank 

 or screen produced, which rises gradually and attains an altitude of 

 from 8* to 10*. The colour of the dusky segment panes over into 

 brown or violet. Stan are visible in it, but they are seen as in a 

 portion of the sky obscured with dense smoke ; a broad bright lumi- 

 nous arc or seam, first white, then yellow, bounds the dusky segment. 

 The highest point of the luminous arc, when it has been carefully 

 measured, has been found to be not exactly in the magnetic meridian, 

 but to vary between 6 and 18 degree* from it, towards the side on 

 agnetio declination of the place of observation lies. The 



luminoue bow, in constant motion, flickering and changing it* form 

 incessantly, sometimes remain* visible for hour* before anything like 

 ray* and pencil* of ray* about from it and riae to the senith. The 

 more intenee the discharges of the northern lights, the more vividly do 

 the colours play from violet and bluish-white, through every lhade and 



gradation, to green and purplish -red. The magnetic fiery column* (hoot 

 up, at one time singly from the luminou* arch, even mingled with black 

 ray* like thick smoke ; at another, many columns arise simultaneously 

 from several and opposite points of the horizon, and unit.- in .1 flickering 

 sea of flame, to the iiplendour of which no description can do justice, 

 and whone luminou* waves assume another and a different shape at 

 every instant. The intensity is at times so great that Lowenom per- 

 ceived its oscillation*, in bright sunshine, on the 29th of January, 1786. 

 The motion increases the brilliancy of the phenomenon. Arouu.l tli<< 

 point of the vault of heaven which correspond* with the dim 

 the dipping-needle, the ray* at length collect together, and form the 

 corona or crown. Thia lurround* the summit, ns it were, of a va*t 

 canopy, the dome of heaven, with the mild radiance of iU streaming 

 but not flickering ray*. It is only in rare instances that the pheno- 

 uienon. proceed* the length of forming the corona, completely. With 



its appearance, however, the whole is at an end. The rays now 

 become rarer, shorter, lean intensely coloured. The crown anil the 

 luminou* arches break up. By-and-bye nothing but broad, motionless, 

 and almost ashy-gray pale gleaming fleecy masses appear irregularly 

 dispersed over the whole vault of heaven : those vanish in their turn, 

 and before the last trace of the murky fuliginous segment, which still 

 show* itself deeply on the horizon, ban disappeared. Of the whole 

 brilliant upectorle. nothing at length remain* but a white delicate cloud, 

 feathered at the edges, or broken up, as a cirro-cumulus, into small 

 rounded DMUM or heaps, at equal distances." (' Cosmos; v. i.) 



The two toattun* siren above are selected, for their dissimilarity, 

 from the work of M. de Msiraa. fig. 1 represents an aurora as seen 

 at Breuillepnnt. in Normandy, nearly in the latitude of Paris, on 

 September Mth, 1726. It consisted entirely of streams of litrht, 

 without say darker meteor, fia. 2 shows an aurora as observed at 

 the same piee, and which lasted for several minute*, on Oct. 19, 1 726. 



It has been subject of some dispute whether any sound accom- 

 panies the development of the terrestrial light The Greenland 

 ledgers and Mberian fox-hunter* are positive in their Assertions that 

 there is; and intelligent observers engaged hi the Arctic m., 

 and meteorological nftMUum have recorded their evidence in tl 

 amrmativ*. OB the other hand, Parry, Franklin, Richardson, and 

 !.-. i. i .-.., ;.,,,-,., 



any attendMt mm*. 



According to Dalton, the southern lights have been frequently seen 

 in Knghnd; and the northern light* were seen in 46* 8. lat. on 

 January 14, 1831. Miuntiol.lt remarks, however, that " It is necessary 

 to distinguish between the sphere of a simultaneous apparition of tLo 

 phenomenon and the gone of the earth in which the phenomenon i* 



displayed almost every night of the year. As each observer Bees hi* 

 own rainbow, no also, doubtless, does he nee his own jiolar light. A 

 great portion of the earth engenders the radiating light phenomenon 

 at the Borne t 



The intensity of the terrestrial light diminishes with the decrease of 



latitude, or, more strictly, with the decrease of magnetic latitude. In 



i, (ireeiilaml, ami on the banks of the Slave Lake, it appears at 



certain season* every night. In Italy it is rare. On the shore* of 



Siberia there appear to be " special region* of the northern lights." 



There i* great difficulty in determining the altitude of the terrestrial 

 light, in consequence of the incessant oscillations of the luminous rays ; 

 so that the results of different olwervations veen several 



miles and three or four thousand feet," Moreover, it is jin-KiMo that 

 its altitude differs at different times. 



( Mir observations and I. ' iftl light lead* us to the 



important an.l int. -luminous; 



and it is supposed that the degree of Inn- > itn -than 



that of the in... n in tier first quarter. These i 



with the observed fact that tli. .vi-iwionally with 



a proper |.ln. ^.lion-scent gleam" in those parts which ore not illumi- 

 nated by the sun, lead ns to inquire whether the moon ami planets 

 may not likewise be magnetic, thus keeping u]> a mutual influence 

 betwc . : irth. 



Why the phenomenon -i the terrestrial light should < 

 especially to the polar region* i* readily explained on the supposi- 

 tion that it i* a luminou* discharge of superabundant tnagii- 

 and since heat is destructive of magnetism, the magnetic intensity 

 will be greater in the colder region* of the earth. [TERRESTRIAL 

 MAGNETISM.] An illustration of this may be given by taking a bar- 



