h magnitude star was stated to have
completely disappeared during the transit over it of Denning's
comet.[303]

From the failure to detect any effects of refraction in the light of
stars occulted by comets, it was inferred (though, as we know now,
erroneously) that their composition is rather that of dust than that of
vapour; that they consist not of any continuous substance, but of
discrete solid particles, very finely divided and widely scattered. In
conformity with this view was the known smallness of their masses.
Laplace had shown that if the amount of matter forming Lexell's comet
had been as much as 1/5000 of that contained in our globe, the effect of
its attraction, on the occasion of its approach within 1,438,000 miles
of the earth, July 1, 1770, must have been apparent in the lengthening
of the year. And that some comets, at any rate, possess masses
immeasurably below this maximum value was clearly proved by the
undisturbed parallel march of the two fragments of Biela's in 1846.

But the discovery in this branch most distinctive of the period under
review is that of "short period" comets, of which four[304] were known
in 1850. These, by the character of their movements, serve as a link
between the planetary and cometary worlds, and by the nature of their
construction, seem to mark a stage in cometary decay. For that comets
are rather transitory agglomerations, than permanent products of
cosmical manufacture, appeared to be demonstrated by the division and
disappearance of one amongst their number, as well as by the singular
and rapid changes in appearance undergone by many, and the seemingly
irrevocable diffusion of their substance visible in nearly all. They
might then be defined, according to the ideas respecting them prevalent
fifty years ago, as bodies unconnected by origin with the solar system,
but encountered, and to some extent appropriated, by it in its progress
through space, owing their visibility in great part, if not altogether,
to light reflected from the sun, and their singular and striking forms
to the action of repulsive forces emanating from him, the penalty of
their evanescent splendour being paid in gradual waste and final
dissipation and extinction.

FOOTNOTES:

[Footnote 241: _Allgemeine Geographische Ephemeriden_, vol. iv., p.
287.]

[Footnote 242: _Astr. Jahrbuch_, 1823, p. 217. The period (1,208 days)
of this body is considerably shorter than that of any other known
comet.]