loudly that we have, at last, found out the
veritable and undeniable and final truth.

Monsieur Encke, on comparing the intervals
of time between several complete circuits
of his comet and the sun, discovered that the
length of the ellipse described by its orbit
was shortened in a slow but regular manner;
at every successive return, from eighteen
hundred and nineteen to eighteen hundred
and thirty-two, its actual position has been
remarked to anticipate, ceaselessly and
uniformly, its calculated position by about two
days; that is, its return happened two days
sooner than it should have done according to
the strictest calculations. Its orbit, therefore,
is diminishing; its mean distance from
the sun is constantly decreasing, and it must
finally fall into that luminary, were it not for
the repulsion exercised by incandescent
surfaces, which repulsion will probably shoot it
off again in the form of an excessively rarified
vapour.

The perturbation experienced by the comet
could only be attributed to the existence in
the celestial space which it traverses of a
highly-divided very subtle matter which
constantly impedes the rapidity of its progress.
The resistance which this rare medium
opposes to the progress of the comet, would
also diminish its centrifugal tendency by the
very act of diminishing its velocity, and
would therefore increase the sun's power of
drawing it towards itself.

From the ever-abbreviated course pursued
by Encke's short-perioded comet, Arago
argued that a new element ought
henceforward to be taken into consideration:
namely, the resistance which an excessively
rare gaseous substance which fills celestial
space (and which it has been agreed to
denominate The Ether, and which, of course, is
perfectly distinct from the ether of the
chemists) offers to the passage of bodies
which traverse it. This resistance produces
no appreciable effect on the planets, on
account of their considerable density; but the
comets being, for the most part, mere heaps
of the lightest vapours, may be notably
retarded in their progress through space. To
prove the justness of the distinction here
made between dense and rare bodies, in
respect to resistance, it is only necessary to
compare the inequality of the distances
traversed through the air by three balls of
lead, of cork, and of eider-down, even in the
case when projected from a gun-barrel by
equal charges of powder they would have
the same initial velocity.

In the last century, the presence of the
ether in the midst of the celestial spaces was
strongly suspected; at the present day, it is
considered impossible to maintain the
Newtonian theory, that the heavenly bodies
perform their orbits in the isolation of an
enormous vacuum. Mr. Grove, in his able
Correlation of Physical Forces, remarks
that the tendency of matter to diffuse
itself is so great, as to have given rise to
the adage, Nature abhors a vacuum; and
that the aphorism, which has been made the
butt of a considerable amount of witticisms,
nevertheless contains a profound truth,
precisely enunciated, although in a metaphorical
form.

We may now very naturally inquire,—
what, in short, is this wonderful ether ? Is
it a fluid, transparent, impalpable body,
which penetrates throughout and
everywhere ? Is it composed of matter which is
equally subtle and rarified at all the points
which it occupies? Is it exactly the same in
the neighbourhood of a voluminous planet as
in the midst of an immense open space
entirely empty of solid bodies ? In a word,
does it differ essentially from the most rarified
portion of the planetary atmospheres? All
these points are open to controversy. In the
opinion of learned men, whose expressed belief
merits deference and attention, the ether
differs only in its extreme subtilty, from the
much more highly condensed matter which
constitutes the atmospheres of the planets,—
a definition that has been ventured is, the
ether is the simple of which atmospheres are
the compound; in other words, atmospheric
matter results from the condensation of a
certain amount of etherial matter; or, finally,
ether is the elementary matter of which all
other things are formed.

This notion is not very far removed from
that entertained by Mr. Grove, who believes
that the ether possesses all the qualities of
ordinary gross matter, and particularly the
quality of weight. If this matter, on account
of its extreme rarification, can only manifest
the properties with which it is endowed on a
scale of infinite minuteness,—on the other
hand, at the surface of the earth it attains a
degree of density which we are able to
measure by experiment. The ether, or the
extremely rarified matter which fills the
interplanetary spaces, is thus believed to be an
expansion of all or several of the atmospheres
of the planets, or of their most volatile
elements, and would thus furnish the material
necessary for the transmission of those
modifications of motion which we designate by the
names of light, heat, and so forth. And it is
held to be far from impossible that attenuated
portions of these atmospheres, by gradual
changes, may pass from one planet to
another, thus forming a link of material
communication between the distant monads
of the universe.

The ether, then, is an imponderable, or
unweighable, or, rather, an unweighed fluid,
endued with perfect elasticity. It fills not
only the planetary spaces, but also the intervals
between the elementary molecules of
solid bodies, and even the molecules
themselves, as those of the gases which are
assumed to be hollow and spherical. In
short, the ether pervades everything, and is
everywhere; in the most elaborately-formed