254 PHYSICS. 
of it has yet been suggested. In high latitudes it is far more fr equent than 
in lower, occurring aliiogs every = during certain seasons in the far 
north. 
The aurora, mostly observed in winter, generally begins with a bright 
glow, at first white and then yellowish, in the northern part of the heavens; 
its shape is that of an arch inclosing a dark nebulous cloud, previously 
formed in the otherwise clear heavens, and appearing as a segment of a 
circle from eight to ten degrees in height, which does not obscure the stars. 
In the far north this segment appears brighter than with us, or is entirely 
wanting. The highest point of the arc generally deviates from five to 
eighteen degrees from the magnetic meridian, towards that side to which 
the magnetic declination of the place is directed. Sometimes there arise 
two or three bright arches. From one of these, generally the uppermost, 
there subsequently ascend streaks of light and groups of rays of different 
colors, alternately appearing and vanishing, and changing their place with 
greater or less rapidity, so that the entire mass of light appears to be in 
incessant motion, the whole heavens being sometimes filled with a flickering, 
tremulous light. The colors, in particular instances, pass from violet and 
bluish white through all shades into green and purplish red; black streaks, 
even, resembling a thick smoke, occasionally occur. These streaks of light 
sometimes ascend from the arch alone, sometimes simultaneously from 
many opposite points of the horizon. The rays at times converge towards 
that point of the heavens corresponding to the direction of the dipping 
needle, thus forming the so-called crown of the northern light (pl. 26, fig. 13), 
resembling the lantern of a dome, and only rarely coming to perfection, but 
always constituting the culminating point of the whole phenomenon. This 
soon after begins gradually to decrease, flames up a few times more, and 
then vanishes, leaving either a whitish gleam in the north, which lasts some 
time longer, or a light white cloud. 
This phenomenon, mentioned by Aristotle and Pliny, was first called 
aurora borealis by Gassendi, in consequence of the one observed by him on 
the 12th of September, 1621. The fact that the aurora appears more 
frequently at certain times than at others, is well ascertained, without om 
being able to determine a regular periodical alternation. According to 
Mairan, the following is the rate of their occurrence in earlier times: 26 
between 583 and 1354 A.D.; from 1446 to 1560, 34; from 1561 to 1592, 
69; from 1593 to 1633, 70; frath 1634 to 1684, 34; from 1685 to 1721, 
219; from 1722 to 1745, 961; from 1746 to 1751, 28. From 1716 to about 
1790 they were so frequent, that the Dutch philosopher, Muschenbroek, 
observed no less than 720 at Leyden and Utrecht; in Leyden 750 were 
observed in twenty-nine years, while in 1730, 116 in all were seen in 
different parts of the earth.. After 1790 they became rarer, and it is only 
since about 1825 that they have been observed more frequently. The 
auroras of modern times. most conspicuous for their beauty, their wide 
distribution, and long duration, were those of the 7th of January, 1831, and 
24th and 25th of October. 1847 (seen even in the southern latitudes of Italy 
and Spain). Mairan gives the following as periods during which no 
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