The regularity of the change of volume,
according to the different degrees of distance from the Sun, appears very
striking. The physical explanation of the phenomenon can not, however, be
sought in the condensed layers of cosmical vapor occurring in the vicinity
of the Sun, since it is difficult to imagine the nebulous envelope of the
nucleus of the comet to be vesicular and impervious to the other.*
[footnote] *Arago, in the 'Annuaire', 1832, p. 217-220. Sir John Herschel,
'Astron.', 488.
The dissimilar eccentricity of the orbits of comets has, in recent times
(1819), in the most brilliant manner enriched our knowledge of the solar
system. Encke has discovered the existence of a comet of so short a period
of revolution that it remains entirely within the limits of our planetary
system, attaining its aphelion between the orbits of the smaller planets and
that of Jupiter. Its eccentricity must be assumed at 0.845, that of Juno
(which has the greatest eccentricity of any of the planets) being 0.255.
Encke's comet has several times, although with difficulty, been observed by
the naked eye, as in Europe in 1819, and according to Rumker, in New Holland
in 1822. Its period of revolution is about 3 1/3d years; but, from a
careful comparison of the epochs of its return to its perihelion, the
remarkable fact has been discovered that these periods have diminished in
the most regular manner between the years 1786 and 1838, the diminution
amounting, in the course of 52 years, to about 1 3/10th days.
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