ities and without any unity of
direction. Hence we see causes alike for the streams of shooting stars,
for the solitary shooting stars visible to the naked eye, and for the
telescopic shooting stars a score times more numerous.

Further significant evidence is furnished by the comets of short
periods. Of the thirteen constituting this group, twelve have orbits
falling between those of Mars and Jupiter: one only having its aphelion
beyond the orbit of Jupiter. That is to say, nearly all of them frequent
the same region as the planetoids. By implication, they are similarly
associated in respect of their periods. The periods of the planetoids
range from 3.1 to 8.8 years; and all these twelve comets have periods
falling between these extremes: the least being 3.29 and the greatest
8.86. Once more this family of comets, like the planetoids in the zone
they occupy and like them in their periods, are like them also in the
respect that, as Mr. Lynn has pointed out, their motions are all direct.
How happens this close kinship--how happens there to be this family of
comets so much like the planetoids and so much like one another, but so
unlike comets at large? The obvious suggestion is that they are among
the products of the explosion which originated the planetoids, the
aerolites, and the streams of meteors; and consideration of the probable
circumstances shows us that such products might be expected. If the
hypothetical planet was like its neighbour Jupiter in having an
atmosphere, or like its neighbour Mars in having water on its surface,
or like both in these respects; then these superficial masses of liquid,
of vapour, and of gas, blown into space along with the solid matters,
would yield the materials for comets. There would result, too, comets
unlike one another in constitution. If a fissure opened beneath one of
the seas, the molten metals and metallic gases rushing through it as
above described, would decompose part of the water carried with them;
and the oxygen and hydrogen liberated would be mingled with undecomposed
vapour. In other cases, portions of the atmosphere might be propelled,
probably with portions of vapour; and in yet other cases masses of water
alone. Severally subject to great heat at perihelion, these would behave
more or less differently. Once more, it would ordinarily happen that
detached swarms of meteors projected as implied, would carry with them
masses of vapours and gases; whence would result the