nces, even should its orbit be
elliptical. This unexpected confirmation of the theory by the
observation of Capt. Ray, cannot easily be surmounted.

We must now endeavor to explain the physical peculiarities of comets, in
accordance with the principles laid down. The most prominent phenomenon
of this class is the change of diameter of the visible nebulosity. It is
a most singular circumstance, but well established as a fact, that a
comet contracts in its dimensions on approaching the sun, and expands on
leaving it. In 1829, accurate measures were taken on different days, of
the diameter of Encke's comet, and again in 1838. The comet of 1618 was
also observed by Kepler with this very object, and also the comet of
1807; but without multiplying instances, it may be asserted that it is
one of those facts in cometary phenomena, to which there are no
exceptions. According to all analogy, the very reverse of this ought to
obtain. If a comet is chiefly vaporous, (as this change of volume would
seem to indicate,) its approach to the sun ought to be attended by a
corresponding expansion by increase of temperature. When the contrary is
observed, and invariably so, it ought to be regarded as an index of the
existence of other forces besides gravitation, increasing rapidly in the
neighborhood of the sun; for the disturbing power of the sun's
attraction would be to enlarge the diameter of a comet in proportion to
its proximity. Now, the force of the radial stream, as we have shown, is
as the 2.5th power of the distances inversely. If this alternate
contraction and expansion be due to the action of this force, there
ought to be an approximate correspondence of the law of the effect with
the law of the cause. Arago, in speaking of the comet of 1829, states,
"that between the 28th of October and the 24th of December, the volume
of the comet was reduced as 16000 to 1, the change of distance in the
meantime only varying about 3 to 1." To account for this, a memoir was
published on the subject by M. Valz, in which he supposes an atmosphere
around the sun, whose condensation increases rapidly from superincumbent
pressure; so that the deeper the comet penetrates into this atmosphere
the greater will be the pressure, and the less the volume. In this it is
evident, that the ponderous nature of a resisting medium is not yet
banished from the schools. In commenting on this memoir, Arago justly
observes, that "there would be no difficulty in this