tail
running "like a road through the constellations." In recent times the
great comet of 1843 may be mentioned as having exactly such an
appearance.

So we cannot expect to find in the Scriptures definite and precise
descriptions that we can recognize as those of comets. At the most we
may find some expressions, some descriptions, that to us may seem
appropriate to the forms and appearances of these objects, and we may
therefore infer that the appearance of a comet may have suggested these
descriptions or expressions.

The head of a great comet is brilliant, sometimes starlike. But its tail
often takes on the most impressive appearance. Donati's comet, in 1858,
assumed the most varied shapes--sometimes its tail was broad, with one
bright and curving edge, the other fainter and finer, the whole making
up a stupendous semi-circular blade-like object. Later, the tail was
shaped like a scimitar, and later again, it assumed a duplex form.

Though the bulk of comets is huge, they contain extraordinarily little
substance. Their heads must contain some solid matter, but it is
probably in the form of a loose aggregation of stones enveloped in
vaporous material. There is some reason to suppose that comets are apt
to shed some of these stones as they travel along their paths, for the
orbits of the meteors that cause some of our greatest "star showers" are
coincident with the paths of comets that have been observed.

But it is not only by shedding its loose stones that a comet diminishes
its bulk; it loses also through its tail. As the comet gets close to the
sun its head becomes heated, and throws off concentric envelopes, much
of which consists of matter in an extremely fine state of division. Now
it has been shown that the radiations of the sun have the power of
repelling matter, whilst the sun itself attracts by its gravitational
force. But there is a difference in the action of the two forces. The
light-pressure varies with the surface of the particle upon which it is
exercised; the gravitational attraction varies with the mass or volume.
If we consider the behaviour of very small particles, it follows that
the attraction due to gravitation (depending on the volume of the
particle) will diminish more rapidly than the repulsion due to
light-pressure (depending on the surface of the particle), as we
decrease continually the size of the particle, since its volume
diminishes more rapidly than its surface. A limit therefore wi