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 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.
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