The
aerostat traversing the clouds completely, and passing by the very
point where the apparition is seen, affords one an opportunity of
ascertaining the state of the cloud. This observation I have been able
to make, and so to offer an explanation of the phenomenon.
As the balloon sails on, borne forward by the wind, its shadow travels
either on the ground or on the clouds. This shadow is, as a rule,
black, like all others; but it frequently happens that it appears
alone on the surface of the ground, and thus appears luminous.
Examining this shadow by the aid of a telescope, I have noticed that
it is often composed of a dark nucleus and a penumbra of the shape of
an aureola. This aureola, frequently very large in proportion to the
diameter of the central nucleus, eclipses it to the naked eye, so that
the whole shadow appears like a nebulous circle projected in yellow
upon the green ground of the woods and meadows. I have noticed, too,
that this luminous shadow is generally all the more strongly marked in
proportion to the greater humidity of the surface of the ground.
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