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medium pervading the regions of its orbit. For it is evident that such
a medium must, in retarding the comet's velocity, increase its
centripetal, by weakening its centrifugal force. In other words, the
sun's attraction would be constantly attaining greater power, and the
comet would be drawn nearer at every revolution. Indeed, there is no
other way of accounting for the variation in question. But again. The
real diameter of the same comet's nebulosity is observed to contract
rapidly as it approaches the sun, and dilate with equal rapidity in its
departure towards its aphelion. Was I not justifiable in supposing with
M. Valz, that this apparent condensation of volume has its origin in
the compression of the same ethereal medium I have spoken of before,
and which is only denser in proportion to its solar vicinity? The
lenticular-shaped phenomenon, also called the zodiacal light, was a
matter worthy of attention. This radiance, so apparent in the tropics,
and which cannot be mistaken for any meteoric lustre, extends from the
horizon obliquely upward, and follows generally the direction of the
sun's equator. It appeared to me evidently in the nature of a rare
atmosphere extending from the sun outward, beyond the orbit of Venus at
least, and I believed indefinitely farther.(*2) Indeed, this medium I
could not suppose confined to the path of the comet's ellipse, or to
the immediate neighborhood of the sun. It was easy, on the contrary,
to imagine it pervading the entire regions of our planetary system,
condensed into what we call atmosphere at the planets themselves, and
perhaps at some of them modified by considerations, so to speak, purely
geological.
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