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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I must here relate another very singular fact. On the 18th of
January, and the 15th of February, 1800, the - Page 759
Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 1 - By Alexander Von Humboldt And Aime Bonpland. - Page 759 of 779 - First - Home

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I Must Here Relate Another Very Singular Fact.

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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