268 THE AUROKA BOREALIS. 



which bear a great resemblance in color and fo.rni to the vapors which 

 usually rise from a sudiJen break in the ice of the sea." This is no 

 doubt the dark bank of vapor-like matter so generally seen even in mid- 

 dle latitudes. M. Lotten, a French naval officer, and member of a sci- 

 entific commission sent to the north seas, who during the winter of 

 1838-9, observed upwards of one hundred auroras, at the bay of Alten, 

 observes that "a light sea-fog^ extending to the altitude of from four to 

 six degrees, became colored on its upper border, or rather was fringed 

 with the light of the aurora, which was then behind it; this border be- 

 came gradually more regular, and took the form of an arc of a pale yel- 

 low color. This bow swelled upward more or less slowly, its vertex 



being constantly on the magnetic meridian, or nearly so." His de- 

 scription of the bow, shows it to be very like in character to that wit- 

 nessed in lower latitudes, with its "oblique fleeces," or "snow-drift" 

 forms, so much resembling an illuminated cirrus-cloud, but to which so 

 extravagant a height has been assigned by some. This furnishes a strong 

 presumption in favor of aqueous vapor forming the luminous particles 

 which constitute all the visible parts of the auroral display. 



6. It is influenced by local circumstances. Wrangell says, "Auroras 

 are more frequent and brilliant on the sea coast than at a distance from 

 it," whilst "latitude does not otherwise influence them." This seems 

 to be corroborated by the statements of Capt. Bonnycastle concerning 

 the auroras of the lakes. It shows that the existence of vapor, other 

 things being the same, is favorable to their development. 



7. It is affected by the state of the weather. "The finest auroras," 

 says the same authority, "always appear at the setting in of strong gales 

 in November and Jirtiuary ; when the cold is intense they are more 

 rare." 



8. And finally, that the aurora "is usually nearer the surface of the 

 earth," "than the higher regions of the atmosphere," "is shown by the 

 visible injlucnce of the lower current on its beams?'' "We have fre- 

 quently seen the eflect of the wind which is blov/ing at the surface of 

 the earth, on the streamers as distinctly as on the clouds." 



V. Its nature. 



Tiiat this is a most difficult point to determine is shown by the 

 great number of theories which have been offered concerning it. 



]. It was once maintained that the aurora was occasioned by "fiery 

 and sulphurous vapors exhaled from the bowels of the earth, which, 

 rising into the region of the air," there became phosphorescent, or were 

 ignited. Thitj theory has nothing in the discoveries of modern science 



