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case the same parts of the milky way are found near it。 But if it were
a reflection and not a genuine affection of these this ought not to be
the case。
Again； we can see the milky way reflected at night in water and
similar mirrors。 But under these circumstances it is impossible for
our sight to be reflected to the sun。
These considerations show that the milky way is not the path of
one of the planets； nor the light of imperceptible stars； nor a
reflection。 And those are the chief theories handed down by others
hitherto。
Let us recall our fundamental principle and then explain our
views。 We have already laid down that the outermost part of what is
called the air is potentially fire and that therefore when the air
is dissolved by motion； there is separated off a kind of matter…and of
this matter we assert that comets consist。 We must suppose that what
happens is the same as in the case of the comets when the matter
does not form independently but is formed by one of the fixed stars or
the planets。 Then these stars appear to be fringed； because matter
of this kind follows their course。 In the same way； a certain kind
of matter follows the sun； and we explain the halo as a reflection
from it when the air is of the right constitution。 Now we must
assume that what happens in the case of the stars severally happens in
the case of the whole of the heavens and all the upper motion。 For
it is natural to suppose that； if the motion of a single star
excites a flame； that of all the stars should have a similar result；
and especially in that region in which the stars are biggest and
most numerous and nearest to one another。 Now the circle of the zodiac
dissolves this kind of matter because of the motion of the sun and the
planets； and for this reason most comets are found outside the
tropic circles。 Again； no fringe appears round the sun or moon： for
they dissolve such matter too quickly to admit of its formation。 But
this circle in which the milky way appears to our sight is the
greatest circle； and its position is such that it extends far
outside the tropic circles。 Besides the region is full of the
biggest and brightest constellations and also of what called
'scattered' stars （you have only to look to see this clearly）。 So
for these reasons all this matter is continually and ceaselessly
collecting there。 A proof of the theory is this： In the circle
itself the light is stronger in that half where the milky way is
divided； and in it the constellations are more numerous and closer
to one another than in the other half； which shows that the cause of
the light is the motion of the constellations and nothing else。 For if
it is found in the circle in which there are most constellations and
at that point in the circle at which they are densest and contain
the biggest and the most stars； it is natural to suppose that they are
the true cause of the affection in question。 The circle and the
constellations in it may be seen in the diagram。 The so…called
'scattered' stars it is not possible to set down in the same way on
the sphere because none of them have an evident permanent position；
but if you look up to the sky the point is clear。 For in this circle
alone are the intervals full of these stars： in the other circles
there are obvious gaps。 Hence if we accept the cause assigned for
the appearance of comets as plausible we must assume that the same
kind of thing holds good of the milky way。 For the fringe which in the
former case is an affection of a single star here forms in the same
way in relation to a whole circle。 So if we are to define the milky
way we may call it 'a fringe attaching to the greatest circle； and due
to the matter secreted'。 This； as we said before； explains why there
are few comets and why they appear rarely； it is because at each
revolution of the heavens this matter has always been and is always
being separated off and gathered into this region。
We have now explained the phenomena that occur in that part of the
terrestrial world which is continuous with the motions of the heavens；
namely； shooting…stars and the burning flame； comets and the milky
way； these being the chief affections that appear in that region。
9
Let us go on to treat of the region which follows next in order
after this and which immediately surrounds the earth。 It is the region
common to water and air； and the processes attending the formation
of water above take place in it。 We must consider the principles and
causes of all these phenomena too as before。 The efficient and chief
and first cause is the circle in which the sun moves。 For the sun as
it approaches or recedes； obviously causes dissipation and
condensation and so gives rise to generation and destruction。 Now
the earth remains but the moisture surrounding it is made to evaporate
by the sun's rays and the other heat from above； and rises。 But when
the heat which was raising it leaves it； in part dispersing to the
higher region； in part quenched through rising so far into the upper
air； then the vapour cools because its heat is gone and because the
place is cold； and condenses again and turns from air into water。
And after the water has formed it falls down again to the earth。
The exhalation of water is vapour： air condensing into water is
cloud。 Mist is what is left over when a cloud condenses into water；
and is therefore rather a sign of fine weather than of rain； for
mist might be called a barren cloud。 So we get a circular process that
follows the course of the sun。 For according as the sun moves to
this side or that； the moisture in this process rises or falls。 We
must think of it as a river flowing up and down in a circle and made
up partly of air； partly of water。 When the sun is near； the stream of
vapour flows upwards； when it recedes； the stream of water flows down：
and the order of sequence； at all events； in this process always
remains the same。 So if 'Oceanus' had some secret meaning in early
writers； perhaps they may have meant this river that flows in a circle
about the earth。
So the moisture is always raised by the heat and descends to the
earth again when it gets cold。 These processes and； in some cases；
their varieties are distinguished by special names。 When the water
falls in small drops it is called a drizzle； when the drops are larger
it is rain。
10
Some of the vapour that is formed by day does not rise high
because the ratio of the fire that is raising it to the water that
is being raised is small。 When this cools and descends at night it
is called dew and hoar…frost。 When the vapour is frozen before it
has condensed to water again it is hoar…frost； and this appea