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could not help thinking that the subject admitted room for doubt and
great latitude for speculation.
"But, in point of fact, an ascension being made to any given altitude,
the ponderable quantity of air surmounted in any farther ascension is
by no means in proportion to the additional height ascended (as may
be plainly seen from what has been stated before), but in a ratio
constantly decreasing. It is therefore evident that, ascend as high as
we may, we cannot, literally speaking, arrive at a limit beyond which
no atmosphere is to be found. It must exist, I argued; although it may
exist in a state of infinite rarefaction.
"On the other hand, I was aware that arguments have not been wanting
to prove the existence of a real and definite limit to the atmosphere,
beyond which there is absolutely no air whatsoever. But a circumstance
which has been left out of view by those who contend for such a limit
seemed to me, although no positive refutation of their creed, still
a point worthy very serious investigation. On comparing the intervals
between the successive arrivals of Encke's comet at its perihelion,
after giving credit, in the most exact manner, for all the disturbances
due to the attractions of the planets, it appears that the periods are
gradually diminishing; that is to say, the major axis of the comet's
ellipse is growing shorter, in a slow but perfectly regular decrease.
Now, this is precisely what ought to be the case, if we suppose a
resistance experienced from the comet from an extremely rare ethereal
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