combustion of the exhalation. When it takes place at a lower level
it is due to the ejection of the exhalation by the condensing and
cooling of the moister evaporation: for this latter as it condenses
and inclines downward contracts, and thrusts out the hot element and
causes it to be thrown downwards. The motion is upwards or downwards
or sideways according to the way in which the evaporation lies, and
its disposition in respect of breadth and depth. In most cases the
direction is sideways because two motions are involved, a compulsory
motion downwards and a natural motion upwards, and under these
circumstances an object always moves obliquely. Hence the motion of
'shooting-stars' is generally oblique.

  So the material cause of all these phenomena is the exhalation,
the efficient cause sometimes the upper motion, sometimes the
contraction and condensation of the air. Further, all these things
happen below the moon. This is shown by their apparent speed, which is
equal to that of things thrown by us; for it is because they are close
to us, that these latter seem far to exceed in speed the stars, the
sun, and the moon.

                                 5

  Sometimes on a fine night we see a variety of appearances that
form in the sky: 'chasms' for instance and 'trenches' and blood-red
colours. These, too, have the same cause. For we have seen that the
upper air condenses into an inflammable condition and that the
combustion sometimes takes on the appearance of a burning flame,
sometimes that of moving torches and stars. So it is not surprising
that this same air when condensing should assume a variety of colours.
For a weak light shining through a dense air, and the air when it acts
as a mirror, will cause all kinds of colours to appear, but especially
crimson and purple. For these colours generally appear when
fire-colour and white are combined by superposition. Thus on a hot
day, or through a smoky, medium, the stars when they rise and set look
crimson. The light will also create colours by reflection when the
mirror is such as to reflect colour only and not shape.

  These appearances do not persist long, because the condensation of
the air is transient.

  'Chasms' get their appearance of depth from light breaking out of
a dark blue or black mass of air. When the process of condensation
goes further in such a case we often find 'torches' ejected. When
the 'chasm' contracts it presents the appearance of a 'trench'.

  In general, white in contrast with black creates a variety of
colours; like flame, for instance, through a medium of smoke. But by
day the sun obscures them, and, with the exception of crimson, the
colours are not seen at night because they are dark.

  These then must be taken to be the causes of 'shooting-stars' and
the phenomena of combustion and also of the other transient
appearances of this kind.

                                 6

  Let us go on to explain the nature of comets and the 'milky way',
after a preliminary discussion of the views of others.

  Anaxagoras and Democritus declare that comets are a conjunction of
the planets approaching one another and so appearing to touch one
another.

  Some of the Italians called Pythagoreans say that the comet is one
of the planets, but that it appears at great intervals of time and
only rises a little above the horizon. This is the case with Mercury
too; because it only rises a little above the horizon it often fails
to be seen and consequently appears at great intervals of time.

  A view like theirs was also expressed by Hippocrates of Chios and
his pupil Aeschylus. Only they say that the tail does not belong to
the comet iself, but is occasionally assumed by it on its course in
certain situations, when our sight is reflected to the sun from the
moisture attracted by the comet. It appears at greater intervals
than the other stars because it is slowest to get clear of the sun and
has been left behind by the sun to the extent of the whole of its
circle before it reappears at the same point. It gets clear of the sun
both towards the north and towards the south. In the space between the
tropics it does not draw water to itself because that region is
dried up by the sun on its course. When it moves towards the south
it has no lack of the necessary moisture, but because the segment of
its circle which is above the horizon is small, and that below it many
times as large, it is impossible for the sun to be reflected to our
sight, either when it approaches the southern tropic, or at the summer
solstice. Hence in these regions it does not develop a tail at all.
But when it is visible in the north it assumes a tail because the
arc above the horizon is large and that below it small. For under
these circumstances there is nothing to prevent our vision from
being reflected to the sun.

  These views involve impossibilities, some of which are common to all
of them, while others are peculiar to some only.

  This is the case, first, with those who say that the comet is one of
the planets. For all the planets appear in the circle of the zodiac,
whereas many comets have been seen outside that circle. Again more
comets than one have often appeared simultaneously. Besides, if
their tail is due to reflection, as Aeschylus and Hippocrates say,
this planet ought sometimes to be visible without a tail since, as
they it does not possess a tail in every place in which it appears.