METEOROLOGY. 255 
northern lights appear to have been observed: from 1465 to 1520; from 
1581 to 1600; and from 1721 to 1686. According to Bertholon, there were 
forty-nine years in the seventeenth century without any aurora. Hansteen 
indicates twenty-four aurora periods since 502 A.D., of which the ninth 
between 541 and 608, the twelfth between 823 and 887, the twenty-second 
between 1517 and 1588, and the twenty-fourth between 1707 and 1788, were 
eminently distinguished by the brilliance and frequency of the phenomenon. 
According to this author they are most numerous about the time of the 
equinoxes. Very few instances are on record in which they appeared by 
day, as on September 9th, 1827, in England. 
The northern lights are sometimes visible in the torrid zone, and even in 
the southern hemisphere, just as the southern lights or aurora australis 
have been seen in the northern. In Europe, however, the northern lights 
do not appear often to descend below 37° of latitude. They are rarer in 
Switzerland, South France, and South Germany, than in Holland and 
North Germany. They are more numerous in Great Britain and Ireland, 
increasing to the sixty-fifth or sixty-sixth degree of latitude. Diminishing 
again in number towards the pole, their chief region appears to be in 
Europe, between 60° and 66° N.L. They are far more frequent in North 
America, and are visible further south than in Europe and Asia, The line 
of daily appearance of the aurora commences, according to Horner, at about 
60° N.L., and 70° longitude west of Ferro, and runs thence north-easterly 
through Baffin’s Bay, the point of Greenland, Iceland, and the northern part 
of Spitzbergen, attains its highest point at 60° east longitude, and then 
returns through the Siberian Arctic Ocean, and above Behring’s Straits, to 
its starting point. The estimates of the height of the aurora are mostly 
very uncertain, varying between several miles and 3000 or 4000 feet. The 
distance from the earth is in all probability very different at different times, 
the phenomenon occurring not merely within the limits of the atmosphere, 
but even in the region of clouds. Some of the more recent observers even 
believe that the streamers of the northern light can be moved by winds and 
other aerial currents. 
The connexion between the aurora and magnetism is not at all doubtful, 
considering the remarkable variations produced by the former in the 
magnetic declination, inclination, and intensity, especially since I’araday has 
discovered that light may be produced by magnetic forces. Even the 
morning previous to an aurora the irregular action of the needle indicates a 
disturbance in the equilibrium of terrestrial magnetism, and the needle is 
not seldom affected by the occurrence of this phenomenon in places where 
nothing of it is visible. It is only in the far north, beyond the actual zone 
of auroras, and near the magnetic pole, that there is no longer any influence 
of the northern light on the needle. These polar lights may then be 
considered as magnetic storms, during which the disturbed magnetic 
equilibrium is again restored. 
In ancient times, and even up to the middle of the last century, the 
aurora was explained by supposing that terrestrial exhalations were 
collected in the higher regions of the atmosphere, and there inflamed. 
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