relation to this: "But I think that the certainty that the theory based
upon this law, _perfectly_ explains all the observations, is not
correctly inferred." We will not here enumerate the cases to which
suspicion might be directed, neither will we more than just allude to
the fact, that the Theory of Newton requires a vacuum, in order that the
planetary motions may be mathematically exact, and permanent in their
stability.


A VACUUM REQUIRED BY MODERN SYSTEMS.

Whatever may be the practical belief of the learned, their fundamental
principles forbid the avowal of a plenum, although the undulatory theory
of light renders a plenum necessary, and is so far virtually recognized
by them, and a correction for resistance is applied to the Comet of
Encke. Yet there has been no attempt made to reconcile these opposing
principles, other than by supposing that the celestial regions are
filled with an extremely rare and elastic fluid. That no definite view
has been agreed on, is not denied, and Sir John Herschel speculates on
the reality of a resisting medium, by suggesting questions that will
ultimately have to be considered, as: "What is the law of density of the
resisting medium which _surrounds_ the sun? Is it in rest or in motion?
If the latter, in what direction does it move?" In these queries he
still clings to the idea of Encke, that the resistance is confined to
the neighborhood of the sun and planets, like a ponderable fluid. But
the most profound analyst the world has ever boasted, speaks less
cautiously, (Poisson Rech.) "It is difficult to attribute, as is usually
done, the incandescence of aërolites to friction against the molecules
of the atmosphere, at an elevation above the earth where the density of
the air is almost null. May we not suppose that the electric fluid, in a