Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 1 - By Alexander Von Humboldt And Aime Bonpland.
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On the 18th of
January, and the 15th of February, 1800, the intensity of the
zodiacal light changed in a very perceptible manner, at intervals
of two or three minutes.
Sometimes it was very faint, at others it
surpassed the brilliancy of the Milky Way in Sagittarius. The
changes took place in the whole pyramid, especially toward the
interior, far from the edges. During these variations of the
zodiacal light, the hygrometer indicated considerable dryness. The
stars of the fourth and fifth magnitude appeared constantly to the
naked eye with the same degree of light. No stream of vapour was
visible: nothing seemed to alter the transparency of the
atmosphere. In other years I saw the zodiacal light augment in the
southern hemisphere half an hour before its disappearance. Cassini
admitted "that the zodiacal light was feebler in certain years, and
then returned to its former brilliancy." He thought that these slow
changes were connected with "the same emanations which render the
appearance of spots and faculae periodical on the solar disk." But
this excellent observer does not mention those changes of intensity
in the zodiacal light which I have several times remarked within
the tropics, in the space of a few minutes. Mairan asserts, that in
France it is common enough to see the zodiacal light, in the months
of February and March, mingling with a kind of Aurora Borealis,
which he calls 'undecided,' and the nebulous matter of which
spreads itself all around the horizon, or appears toward the west.
I very much doubt, whether, in the observations I have been
describing, there was any mixture of these two species of light.
The variations in intensity took place at considerable altitudes;
the light was white, and not coloured; steady, and not undulating.
Besides, the Aurora Borealis is so seldom visible within the
tropics, that during five years, though almost constantly sleeping
in the open air, and observing the heavens with unremitting
attention, I never perceived the least traces of that phenomenon.
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