The arc varies in elevation, but is seldom more than ninety miles above the terrestrial surface. Its diameter, however, must be enormous, for it has been known to extend southward to Italy, and has been simultaneously visible in Sardinia, Connecticut, and at New Orleans.
According to some authorities, the phenomenon is accompanied by noises resembling the discharge of fireworks, or the crackling of silk when one piece is folded over another; but this statement is discredited by the most trustworthy observers.
Mrs. Somerville’s description is worth quoting, as taking up more emphatically some points to which we have already alluded:—
The aurora, she says, is decidedly an electrical (or, more strictly speaking, a magneto-electrical) phenomenon. It generally appears soon after sunset in the form of a luminous arc stretching more or less from east to west, the most elevated point being always in the magnetic meridian of the place of the observer; across the arc the coruscations are rapid, vivid, and of various colours, darting like lightning to the zenith, and at the same time flitting laterally with incessant velocity. The brightness of the rays varies in an instant; they sometimes surpass the splendour of stars of the first magnitude, and often exhibit colours of admirable transparency,—blood-red at the base, emerald-green in the middle, and clear yellow towards their extremity. Sometimes one, and sometimes a quick succession of luminous currents run from one end of the arc or bow to the other, so that the rays rapidly increase in brightness; but it is impossible to say whether the coruscations themselves are actually affected by a horizontal motion of translation, or whether the more vivid light is conveyed from ray to ray. The rays occasionally dart far past the zenith, vanish, suddenly reappear, and, being joined by others from the arc, form a magnificent corona or immense dome of light. The segment of the sky below the arc is quite black, as if formed by dense clouds; yet M. Struve is said to have seen stars in it, and so it would appear that the blackness of which several observers speak must be the effect of contrast. The lower edge of the arc is evenly defined; its upper margin is fringed by the coruscations, their convergence towards the north, and that of the arc itself, being probably an effect of perspective.
The aurora exercises a remarkable influence on the magnetic needle, even in places where the display is not visible. Its vibrations seem to be slower or quicker according as the auroral light is quiescent or in motion, and the variations of the compass during the day show that the aurora is not peculiar to night. It has been ascertained by careful observations that the disturbances of the magnetic needle and the auroral displays were simultaneous at Toronto, in Canada, on thirteen days out of twenty-four, the remaining days having been clouded; and contemporaneous observations show that in these thirteen days there were also magnetic disturbances at Prague and Tasmania; so that the occurrence of auroral phenomena at Toronto on these occasions may be viewed as a local manifestation connected with magnetic effects, which, whatever may have been their origin, probably prevailed on the same day over the whole surface of the globe.
Among the atmospheric phenomena of the outer world we are justified in reckoning the Winds, which are remarkable for their variability. Their force is considerably diminished when they pass over a wide surface of ice; sometimes the ice seems even to beat back the breeze, and turn it in a contrary direction. The warm airs from the south grow cool as they sweep across the frozen expanse, and give up their moisture in the form of snow. In a region so bleak and chill it is not often that clouds are created, the atmospheric vapours being condensed into snow or hail without passing through any intermediate condition.
Whirlwinds of frozen snow are formidable enemies to the seaman forced to traverse the ice on foot, or in a sledge drawn by Eskimo dogs. Dense showers lash and sting the unfortunate traveller’s face, penetrate his mouth and nostrils, freeze together his very eyelids, and almost blind him. His skin assumes a bluish tint, and burns as if scarred by the keen thongs of a knout.
An optical illusion of frequent occurrence in the Polar Regions makes objects appear of dimensions much larger than they really possess. A fox assumes the proportions of a bear; low banks of ice are elevated into lofty mountains. The eye is fatigued by dwelling upon the horizon of lands which are never approached. Just as in the sandy deserts of the Sahara the distances of real objects are apparently diminished, so the Arctic explorer, misled by the aërial illusion, advances towards a goal which seems always near at hand, but is never attained.
Another source of error, common both to the Arctic and the Tropical deserts, is the mirage, a phenomenon of refraction, which represents as suspended in air the images of remote objects, and thus gives rise to the most curious illusions and fantastic scenes. Dr. Scoresby one day perceived in the air the reversed representation of a ship which he recognized as the Fame, commanded by his father. He afterwards discovered that it had been lying moored in a creek about ten leagues from the point where the mirage had played with his imagination.
Again, in approaching a field of ice or snow, the traveller invariably descries a belt of resplendent white immediately above the horizon. This is known as the “ice-blink,” and it reveals to the Arctic navigator beforehand the character of the ice he is approaching. At times, too, a range of icebergs, or of broken masses of ice, will be reflected in colossal shadows on the sky, with a strange and even weird effect.