rom the sun will have the same effect
in increasing the elasticity, as change of density, and the comet will
probably part with its internal ether as long as it is visible to the
earth; and not fully regain it perhaps, until after it arrives at its
aphelion. Suppose that we admit that a comet continues to expand in the
same ratio for all distances, as is laid down for the comet of Encke
when near its perihelion; it would follow, that the comet of 1811, would
have a diameter at its aphelion of fifty millions of millions of miles,
that is, its outside would extend one thousand times further from the
sun, at the opposite side to that occupied by the centre of the comet,
than the distance of the comet's centre from the sun, at its enormous
aphelion distance. Such an absurdity shows us that there is a limit of
expansion due to natural causes, and that if there were no radial stream
the volume of a comet would be greatest when nearest the sun.

But while the comet is shortening its distance and hastening to the sun
in the form of a huge globular mass of diffuse light, it is continually
encountering another force, increasing in a far more rapid ratio than
the law of gravitation. At great distances from the sun, the force of
the radial stream was insufficient to detach any portion of the comet's
atmosphere; presently, however, the globular form is changed to an
ellipsoid, the radial stream begins to strip the comet of that doubly
attenuated atmosphere of which we have spoken, and the diameter of the
comet is diminished, merely because the luminosity of the escaping ether
is terminated at the limit of that atmosphere. Meanwhile the mass of the
comet has suffered only an infinitely small diminution; but if the
perihelion distance be small, the force may become powerful enough to
detach the heavier particles of the nucleus, and thus a comet may suffer
in mass by this denudating process. We regard, therefore, the nucleus of
a comet to represent the mass of the comet and the coma, as auroral rays
passing through a very attenuated envelope of detached particles. The
individual gravitating force of these particles to the comet's centre,
may be therefore considered as inversely as the squares of the
distances, and directly as the density of the particles; and this
density will, according to analogical reasoning, be as the distances or
square roots of the distances;--grant the last ratio, and the
gravitating force of the particles compos