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and this we maintain to be the cause of the 'shooting' of scattered

'stars'. We may say, then, that a comet is formed when the upper

motion introduces into a gathering of this kind a fiery principle

not of such excessive strength as to burn up much of the material

quickly, nor so weak as soon to be extinguished, but stronger and

capable of burning up much material, and when exhalation of the

right consistency rises from below and meets it. The kind of comet

varies according to the shape which the exhalation happens to take. If

it is diffused equally on every side the star is said to be fringed,

if it stretches out in one direction it is called bearded. We have

seen that when a fiery principle of this kind moves we seem to have

a shooting-star: similarly when it stands still we seem to have a star

standing still. We may compare these phenomena to a heap or mass of

chaff into which a torch is thrust, or a spark thrown. That is what

a shooting-star is like. The fuel is so inflammable that the fire runs

through it quickly in a line. Now if this fire were to persist instead

of running through the fuel and perishing away, its course through the

fuel would stop at the point where the latter was densest, and then

the whole might begin to move. Such is a comet-like a shooting-star

that contains its beginning and end in itself.

When the matter begins to gather in the lower region independently

the comet appears by itself. But when the exhalation is constituted by

one of the fixed stars or the planets, owing to their motion, one of

them becomes a comet. The fringe is not close to the stars themselves.

Just as haloes appear to follow the sun and the moon as they move, and

encircle them, when the air is dense enough for them to form along

under the sun's course, so too the fringe. It stands in the relation

of a halo to the stars, except that the colour of the halo is due to

reflection, whereas in the case of comets the colour is something that

appears actually on them.

Now when this matter gathers in relation to a star the comet

necessarily appears to follow the same course as the star. But when

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