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7
We consider a satisfactory explanation of phenomena inaccessible
to observation to have been given when our account of them is free
from impossibilities. The observations before us suggest the following
account of the phenomena we are now considering. We know that the
dry and warm exhalation is the outermost part of the
terrestrial world
which falls below the circular motion. It, and a great part of the
air
that is continuous with it below, is carried round the
earth by the
motion of the circular revolution. In the course of this motion it
often ignites wherever it may happen to be of the right consistency,
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
the comet is formed independently it falls behind the motion of the
universe, like the rest of the terrestrial world. It is this fact,
that a comet often forms independently, indeed oftener than round
one of the regular stars, that makes it impossible to maintain that
a comet is a sort of reflection, not indeed, as
Hippocrates and his
school say, to the sun, but to the very star it is alleged to
accompany-in fact, a kind of halo in the pure fuel of fire.
As for the halo we shall explain its cause later.
The fact that comets when frequent foreshadow
wind and
drought
must be taken as an indication of their fiery constitution. For
their origin is plainly due to the plentiful supply of that secretion.
Hence the air is necessarily drier and the moist evaporation is so
dissolved and dissipated by the quantity of the hot exhalation as
not readily to condense into water.-But this phenomenon too shall be
explained more clearly later when the time comes to speak of the
winds.-So when there are many comets and they are dense, it is as we
say, and the years are clearly dry and windy. When they are fewer
and fainter this effect does not appear in the same degree, though
as a rule the is found to be excessive either in duration or strength.
For instance when the stone at
Aegospotami fell out of the air-it
had been carried up by a wind and fell down in the daytime-then too
a comet happened to have appeared in the west. And at the time of
the great comet the winter was dry and north winds prevailed, and
the wave was due to an opposition of winds. For in the gulf a north
wind blew and outside it a violent south wind. Again in the archonship
of
Nicomachus a comet appeared for a few days about the equinoctial
circle (this one had not risen in the west), and simultaneously with
it there happened the storm at Corinth.
That there are few comets and that they appear rarely and outside
the tropic circles more than within them is due to the motion of the
sun and the stars. For this motion does not only cause the hot
principle to be secreted but also dissolves it when it is gathering.
But the chief reason is that most of this stuff collects in the region
of the milky way.
8
Let us now explain the origin, cause, and nature of the milky way.
And here too let us begin by discussing the statements of others on
the subject.
(1) Of the so-called
Pythagoreans some say that this is the path
of one of the stars that fell from heaven at the time of
Phaethon's
downfall. Others say that the sun used once to move in this circle and
that this region was scorched or met with some other affection of this
kind, because of the sun and its motion.
But it is absurd not to see that if this were the reason the
circle of the Zodiac ought to be affected in the same way, and
indeed more so than that of the milky way, since not the sun only
but all the planets move in it. We can see the whole of this circle
(half of it being visible at any time of the night), but it shows no
signs of any such affection except where a part of it touches the
circle of the milky way.
(2)
Anaxagoras,
Democritus, and their schools say that the
milky way
is the light of certain stars. For, they say, when the sun passes
below the earth some of the stars are hidden from it. Now the light of
those on which the sun shines is invisible, being obscured by the of
the sun. But the milky way is the peculiar light of those stars
which are shaded by the earth from the sun's rays.
This, too, is obviously impossible. The milky way is always
unchanged and among the same constellations (for it is clearly a
greatest circle), whereas, since the sun does not remain in the same
place, what is hidden from it differs at different times. Consequently
with the change of the sun's position the milky way ought to change
its position too: but we find that this does not happen. Besides, if
astronomical demonstrations are correct and the size of the sun is
greater than that of the earth and the distance of the stars from
the earth many times greater than that of the sun (just as the sun
is further from the earth than the moon), then the cone made by the
rays of the sun would terminate at no great distance from the earth,
and the shadow of the earth (what we call night) would not reach the
stars. On the contrary, the sun shines on all the stars and the
earth screens none of them.
(3) There is a third theory about the milky way. Some say that it is
a reflection of our sight to the sun, just as they say that the
comet is.
But this too is impossible. For if the eye and the mirror and the
whole of the object were severally at rest, then the same part of
the image would appear at the same point in the mirror. But if the
mirror and the object move, keeping the same distance from the eye
which is at rest, but at different rates of speed and so not always at
the same interval from one another, then it is impossible for the same
image always to appear in the same part of the mirror. Now the
constellations included in the circle of the milky way move; and so
does the sun, the object to which our sight is reflected; but we stand
still. And the distance of those two from us is constant and
uniform, but their distance from one another varies. For the Dolphin
sometimes rises at midnight, sometimes in the morning. But in each
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 appears in
winter and is commoner in cold places. It is dew when the vapour has
condensed into water and the heat is not so great as to dry up the
moisture that has been raised nor the cold sufficient (owing to the
warmth of the climate or season) for the vapour itself to freeze.
For dew is more commonly found when the season or the place is warm,
whereas the opposite, as has been said, is the case with hoar-frost.
For obviously vapour is warmer than water, having still the fire
that raised it: consequently more cold is needed to freeze it.
Both dew and hoar-frost are found when the sky is clear and there is
no wind. For the vapour could not be raised unless the sky were clear,
and if a wind were blowing it could not condense.
The fact that hoar-frost is not found on mountains contributes to
prove that these phenomena occur because the vapour does not rise
high. One reason for this is that it rises from hollow and watery
places, so that the heat that is raising it, bearing as it were too
heavy a burden cannot lift it to a great height but soon lets it
fall again. A second reason is that the motion of the air is more
pronounced at a height, and this dissolves a gathering of this kind.
Everywhere, except in
Pontus, dew is found with south winds and
not with north winds. There the opposite is the case and it is found
with north winds and not with south. The reason is the same as that
which explains why dew is found in warm weather and not in cold. For
the south wind brings warm, and the north, wintry weather. For the
north wind is cold and so quenches the heat of the evaporation. But in
Pontus the south wind does not bring warmth enough to cause
evaporation, whereas the coldness of the north wind concentrates the
heat by a sort of recoil, so that there is more evaporation and not
less. This is a thing which we can often observe in other places
too. Wells, for instance, give off more vapour in a north than in a
south wind. Only the north winds quench the heat before any
considerable quantity of vapour has gathered, while in a south wind
the evaporation is allowed to accumulate.
Water, once formed, does not freeze on the surface of the earth,
in the way that it does in the region of the clouds.
11
From the latter there fall three bodies condensed by cold, namely
rain, snow, hail. Two of these correspond to the phenomena on the
lower level and are due to the same causes, differing from them only
in degree and quantity.
Snow and hoar-frost are one and the same thing, and so are rain
and dew: only there is a great deal of the former and little of the
latter. For rain is due to the cooling of a great amount of vapour,
for the region from which and the time during which the vapour is
collected are considerable. But of dew there is little: for the vapour
collects for it in a single day and from a small area, as its quick
formation and scanty quantity show.
The relation of hoar-frost and snow is the same: when cloud
freezes there is snow, when vapour freezes there is hoar-frost.
Hence snow is a sign of a cold season or country. For a great deal
of heat is still present and unless the cold were overpowering it
the cloud would not freeze. For there still survives in it a great
deal of the heat which caused the moisture to rise as vapour from
the earth.
Hail on the other hand is found in the upper region, but the
corresponding phenomenon in the vaporous region near the earth is
lacking. For, as we said, to snow in the upper region corresponds
hoar-frost in the lower, and to rain in the upper region, dew in the
lower. But there is nothing here to correspond to hail in the upper
region. Why this is so will be clear when we have explained the nature
of hail.
12
But we must go on to collect the facts bearing on the origin of
it, both those which raise no difficulties and those which seem
paradoxical.
Hail is ice, and water freezes in winter; yet hailstorms occur
chiefly in spring and autumn and less often in the late summer, but
rarely in winter and then only when the cold is less intense. And in
general hailstorms occur in warmer, and snow in colder places.
Again, there is a difficulty about water freezing in the upper region.
It cannot have frozen before becoming water: and water cannot remain
suspended in the air for any space of time. Nor can we say that the
case is like that of particles of moisture which are carried up
owing to their small size and rest on the iar (the water swimming on
the air just as small particles of earth and gold often swim on
water). In that case large drops are formed by the union of many
small, and so fall down. This cannot take place in the case of hail,
since solid bodies cannot coalesce like liquid ones. Clearly then
drops of that size were suspended in the air or else they could not
have been so large when frozen.
Some think that the cause and origin of hail is this. The cloud is
thrust up into the upper atmosphere, which is colder because the
reflection of the sun's rays from the earth ceases there, and upon its
arrival there the water freezes. They think that this explains why
hailstorms are commoner in summer and in warm countries; the heat is
greater and it thrusts the clouds further up from the earth. But the
fact is that hail does not occur at all at a great height: yet it
ought to do so, on their theory, just as we see that snow falls most
on high mountains. Again clouds have often been observed moving with a
great noise close to the earth, terrifying those who heard and saw
them as portents of some catastrophe. Sometimes, too, when such clouds
have been seen, without any noise, there follows a violent
hailstorm, and the stones are of incredible size, and angular in
shape. This shows that they have not been falling for long and that
they were frozen near to the earth, and not as that theory would
have it. Moreover, where the hailstones are large, the cause of
their freezing must be present in the highest degree: for hail is
ice as every one can see. Now those hailstones are large which are
angular in shape. And this shows that they froze close to the earth,
for those that fall far are worn away by the length of their fall
and become round and smaller in size.
It clearly follows that the congelation does not take place
because the cloud is thrust up into the cold upper region.
Now we see that warm and cold react upon one another by recoil.
Hence in warm weather the lower parts of the earth are cold and in a
frost they are warm. The same thing, we must suppose, happens in the
air, so that in the warmer seasons the cold is concentrated by the
surrounding heat and causes the cloud to go over into water
suddenly. (For this reason rain-drops are much larger on warm days
than in winter, and showers more violent. A shower is said to be
more violent in proportion as the water comes down in a body, and this
happens when the condensation takes place quickly,-though this is just
the opposite of what Anaxagoras says. He says that this happens when
the cloud has risen into the cold air; whereas we say that it
happens when the cloud has descended into the warm air, and that the
more the further the cloud has descended). But when the cold has
been concentrated within still more by the outer heat, it freezes
the water it has formed and there is hail. We get hail when the
process of freezing is quicker than the descent of the water. For if
the water falls in a certain time and the cold is sufficient to freeze
it in less, there is no difficulty about its having frozen in the air,
provided that the freezing takes place in a shorter time than its
fall. The nearer to the earth, and the more suddenly, this process
takes place, the more violent is the rain that results and the
larger the raindrops and the hailstones because of the shortness of
their fall. For the same reason large raindrops do not fall thickly.
Hail is rarer in summer than in spring and autumn, though commoner
than in winter, because the air is drier in
summer, whereas in
spring it is still moist, and in
autumn it is beginning to grow moist.
It is for the same reason that
hailstorms sometimes occur in the
late summer as we have said.
The fact that the water has previously been warmed contributes to
its freezing quickly: for so it cools sooner. Hence many people,
when they want to cool hot water quickly, begin by putting it in the
sun. So the inhabitants of Pontus when they encamp on the ice to
fish (they cut a hole in the ice and then fish) pour warm water
round their reeds that it may freeze the quicker, for they use the ice
like lead to fix the reeds. Now it is in hot countries and seasons
that the water which forms soon grows warm.
It is for the same reason that rain falls in summer and not in
winter in
Arabia and
Ethiopia too, and that in torrents and repeatedly
on the same day. For the concentration or recoil due to the extreme
heat of the country cools the clouds quickly.
So much for an account of the nature and causes of
rain,
dew,
snow,
hoar-frost, and
hail.
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