Table of Contents |
Next section
WE have already discussed the first causes of
nature, and all
natural motion, also the stars ordered in the motion of the heavens,
and the physical element-enumerating and specifying them and showing
how they change into one another-and becoming and perishing in
general. There remains for consideration a part of this inquiry
which all our predecessors called
meteorology. It is concerned with
events that are natural, though their order is less perfect than
that of the first of the elements of bodies. They take place in the
region nearest to the motion of the
stars. Such are the
milky way, and
comets, and the movements of
meteors. It studies also all the
affections we may call common to air and water, and the kinds and
parts of the earth and the affections of its parts. These throw
light on the causes of
winds and
earthquakes and all the
consequences the motions of these kinds and parts involve. Of these
things some puzzle us, while others admit of explanation in some
degree. Further, the inquiry is concerned with the falling of
thunderbolts and with
whirlwinds and
fire-winds, and further, the
recurrent affections produced in these same bodies by concretion. When
the inquiry into these matters is concluded let us consider what
account we can give, in accordance with the method we have followed,
of animals and plants, both generally and in detail. When that has
been done we may say that the whole of our original undertaking will
have been carried out.
After this introduction let us begin by discussing our immediate
subject.
2
We have already laid down that there is one physical element which
makes up the system of the bodies that move in a circle, and besides
this four bodies owing their existence to the four principles, the
motion of these latter bodies being of two kinds: either from the
centre or to the centre. These four bodies are
fire, air, water,
earth. Fire occupies the highest place among them all, earth the
lowest, and two elements correspond to these in their relation to
one another, air being nearest to fire, water to earth. The whole
world surrounding the earth, then, the affections of which are our
subject, is made up of these bodies. This world necessarily has a
certain continuity with the upper motions: consequently all its
power and order is derived from them. (For the originating principle
of all motion is the first cause. Besides, that clement is eternal and
its motion has no limit in space, but is always complete; whereas
all these other bodies have separate regions which limit one another.)
So we must treat fire and earth and the elements like them as the
material causes of the events in this world (meaning by material
what is subject and is affected), but must assign causality in the
sense of the originating principle of motion to the influence of the
eternally moving bodies.
3
Let us first recall our original principles and the distinctions
already drawn and then explain the 'milky way' and comets and the
other phenomena akin to these.
Fire, air, water, earth, we assert, originate from one another,
and each of them exists potentially in each, as all things do that can
be resolved into a common and ultimate
substrate.
The first difficulty is raised by what is called the air. What are
we to take its nature to be in the world surrounding the earth? And
what is its position relatively to the other physical elements. (For
there is no question as to the relation of the bulk of the earth to
the size of the bodies which exist around it, since astronomical
demonstrations have by this time proved to us that it is actually
far smaller than some individual stars. As for the water, it is not
observed to exist collectively and separately, nor can it do so
apart from that volume of it which has its seat about the earth: the
sea, that is, and rivers, which we can see, and any subterranean water
that may be hidden from our observation.) The question is really about
that which lies between the earth and the nearest stars. Are we to
consider it to be one kind of body or more than one? And if more
than one, how many are there and what are the bounds of their regions?
We have already described and characterized the first element, and
explained that the whole world of the upper motions is full of that
body.
This is an opinion we are not alone in holding: it appears to be
an old assumption and one which men have held in the past, for the
word ether has long been used to denote that element.
Anaxagoras, it
is true, seems to me to think that the word means the same as fire.
For he thought that the upper regions were full of fire, and that
men referred to those regions when they spoke of ether. In the
latter point he was right, for men seem to have assumed that a body
that was eternally in motion was also divine in nature; and, as such a
body was different from any of the terrestrial elements, they
determined to call it '
ether'.
For the um opinions appear in cycles among men not once nor twice,
but infinitely often.
Now there are some who maintain that not only the bodies in motion
but that which contains them is pure fire, and the interval between
the earth and the stars air: but if they had considered what is now
satisfactorily established by mathematics, they might have given up
this puerile opinion. For it is altogether childish to suppose that
the moving bodies are all of them of a small size, because they so
to us, looking at them from the earth.
This a matter which we have already discussed in our treatment of
the upper region, but we may return to the point now.
If the intervals were full of fire and the bodies consisted of
fire every one of the other elements would long ago have vanished.
However, they cannot simply be said to be full of air either; for
even if there were two elements to fill the space between the earth
and the heavens, the air would far exceed the quantitu required to
maintain its proper proportion to the other elements. For the bulk
of the earth (which includes the whole volume of water) is
infinitesimal in comparison with the whole world that surrounds it.
Now we find that the excess in volume is not proportionately great
where water dissolves into air or air into fire. Whereas the
proportion between any given small quantity of water and the air
that is generated from it ought to hold good between the total
amount of air and the total amount of water. Nor does it make any
difference if any one denies that the elements originate from one
another, but asserts that they are equal in power. For on this view it
is certain amounts of each that are equal in power, just as would be
the case if they actually originated from one another.
So it is clear that neither air nor fire alone fills the
intermediate space.
It remains to explain, after a preliminary discussion of
difficulties, the relation of the two elements air and fire to the
position of the first element, and the reason why the stars in the
upper region impart heat to the earth and its neighbourhood. Let us
first treat of the air, as we proposed, and then go on to these
questions.
Since water is generated from air, and air from water, why are
clouds not formed in the upper air? They ought to form there the more,
the further from the earth and the colder that region is. For it is
neither appreciably near to the heat of the stars, nor to the rays
relected from the earth. It is these that dissolve any formation by
their heat and so prevent clouds from forming near the earth. For
clouds gather at the point where the reflected rays disperse in the
infinity of space and are lost. To explain this we must suppose either
that it is not all air which water is generated, or, if it is produced
from all air alike, that what immediately surrounds the earth is not
mere air, but a sort of vapour, and that its vaporous nature is the
reason why it condenses back to water again. But if the whole of
that vast region is vapour, the amount of air and of water will be
disproportionately great. For the spaces left by the heavenly bodies
must be filled by some element. This cannot be fire, for then all
the rest would have been dried up. Consequently, what fills it must be
air and the water that surrounds the whole earth-vapour being water
dissolved.
After this exposition of the difficulties involved, let us go on
to lay down the truth, with a view at once to what follows and to what
has already been said. The upper region as far as the moon we affirm
to consist of a body distinct both from fire and from air, but varying
degree of purity and in kind, especially towards its limit on the side
of the air, and of the world surrounding the earth. Now the circular
motion of the first element and of the bodies it contains dissolves,
and inflames by its motion, whatever part of the lower world is
nearest to it, and so generates heat. From another point of view we
may look at the motion as follows. The body that lies below the
circular motion of the heavens is, in a sort, matter, and is
potentially hot, cold, dry, moist, and possessed of whatever other
qualities are derived from these. But it actually acquires or
retains one of these in virtue of motion or rest, the cause and
principle of which has already been explained. So at the centre and
round it we get earth and water, the heaviest and coldest elements, by
themselves; round them and contiguous with them, air and what we
commonly call fire. It is not really fire, for fire is an excess of
heat and a sort of ebullition; but in reality, of what we call air,
the part surrounding the earth is moist and warm, because it
contains both vapour and a dry exhalation from the earth. But the next
part, above that, is warm and dry. For vapour is naturally moist and
cold, but the exhalation warm and dry; and vapour is potentially
like water, the exhalation potentially like fire. So we must take
the reason why clouds are not formed in the upper region to be this:
that it is filled not with mere air but rather with a sort of fire.
However, it may well be that the formation of clouds in that upper
region is also prevented by the circular motion. For the air round the
earth is necessarily all of it in motion, except that which is cut off
inside the circumference which makes the earth a complete sphere. In
the case of winds it is actually observable that they originate in
marshy districts of the earth; and they do not seem to blow above
the level of the highest mountains. It is the revolution of the heaven
which carries the air with it and causes its circular motion, fire
being continuous with the upper element and air with fire. Thus its
motion is a second reason why that air is not condensed into water.
But whenever a particle of air grows heavy, the warmth in it is
squeezed out into the upper region and it sinks, and other particles
in turn are carried up together with the fiery exhalation. Thus the
one region is always full of air and the other of fire, and each of
them is perpetually in a state of change.
So much to explain why clouds are not formed and why the air is
not condensed into water, and what account must be given of the
space between the stars and the earth, and what is the body that fills
it.
As for the heat derived from the sun, the right place for a
special and scientific account of it is in the treatise about sense,
since heat is an affection of sense, but we may now explain how it can
be produced by the heavenly bodies which are not themselves hot.
We see that motion is able to dissolve and inflame the air;
indeed, moving bodies are often actually found to melt. Now the
sun's motion alone is sufficient to account for the origin of
terrestrial warmth and heat. For a motion that is to have this
effect must be rapid and near, and that of the stars is rapid but
distant, while that of the moon is near but slow, whereas the sun's
motion combines both conditions in a sufficient degree. That most heat
should be generated where the sun is present is easy to understand
if we consider the analogy of terrestrial phenomena, for here, too, it
is the air that is nearest to a thing in rapid motion which is
heated most. This is just what we should expect, as it is the
nearest air that is most dissolved by the motion of a solid body.
This then is one reason why heat reaches our world. Another is
that the fire surrounding the air is often scattered by the
motion
of the heavens and driven downwards in spite of itself.
Shooting-stars further suffix to prove that the celestial sphere
is not hot or fiery: for they do not occur in that upper region but
below: yet the more and the faster a thing moves, the more apt it is
to take fire. Besides, the sun, which most of all the stars is
considered to be hot, is really white and not fiery in colour.
4
Having determined these principles let us explain the cause of the
appearance in the sky of burning flames and of shooting-stars, and
of '
torches', and '
goats', as some people call them. All these
phenomena are one and the same thing, and are due to the same cause,
the difference between them being one of degree.
The explanation of these and many other phenomena is this. When
the sun warms the earth the evaporation which takes place is
necessarily of two kinds, not of one only as some think. One kind is
rather of the nature of vapour, the other of the nature of a windy
exhalation. That which rises from the moisture contained in the
earth and on its surface is vapour, while that rising from the earth
itself, which is dry, is like smoke. Of these the windy exhalation,
being warm, rises above the moister vapour, which is heavy and sinks
below the other. Hence the world surrounding the earth is ordered as
follows. First below the circular motion comes the warm and dry
element, which we call fire, for there is no word fully adequate to
every state of the fumid evaporation: but we must use this terminology
since this element is the most inflammable of all bodies. Below this
comes air. We must think of what we just called fire as being spread
round the terrestrial sphere on the outside like a kind of fuel, so
that a little motion often makes it burst into flame just as smoke
does: for flame is the ebullition of a dry exhalation. So whenever the
circular motion stirs this stuff up in any way, it catches fire at the
point at which it is most inflammable. The result differs according to
the disposition and quantity of the combustible material. If this is
broad and long, we often see a flame burning as in a field of stubble:
if it burns lengthwise only, we see what are called 'torches' and
'goats' and shooting-stars. Now when the inflammable material is
longer than it is broad sometimes it seems to throw off sparks as it
burns. (This happens because matter catches fire at the sides in small
portions but continuously with the main body.) Then it is called a
'goat'. When this does not happen it is a 'torch'. But if the whole
length of the exhalation is scattered in small parts and in many
directions and in breadth and depth alike, we get what are called
shooting-stars.
The cause of these shooting-stars is sometimes the motion which
ignites the exhalation. At other times the air is condensed by cold
and squeezes out and ejects the hot element; making their motion
look more like that of a thing thrown than like a running fire. For
the question might be raised whether the 'shooting' of a 'star' is the
same thing as when you put an exhalation below a lamp and it lights
the lower lamp from the flame above. For here too the flame passes
wonderfully quickly and looks like a thing thrown, and not as if one
thing after another caught fire. Or is a 'star' when it 'shoots' a
single body that is thrown? Apparently both cases occur: sometimes
it is like the flame from the lamp and sometimes bodies are
projected by being squeezed out (like
fruit stones from one's fingers)
and so are seen to fall into the
sea and on the dry land, both by
night and by day when the sky is clear. They are thrown downwards
because the condensation which propels them inclines downwards.
Thunderbolts fall downwards for the same reason: their origin is never
combustion but ejection under pressure, since naturally all heat tends
upwards.
When the phenomenon is formed in the upper region it is due to the
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.
But, as a matter of fact, no planet has been observed besides the
five. And all of them are often visible above the horizon together
at the same time. Further, comets are often found to appear, as well
when all the planets are visible as when some are not, but are
obscured by the neighbourhood of the sun. Moreover the statement
that a comet only appears in the north, with the sun at the summer
solstice, is not true either. The great comet which appeared at the
time of the earthquake in
Achaea and the tidal wave rose due west; and
many have been known to appear in the south. Again in the archonship
of
Euclees, son of
Molon, at
Athens there appeared a comet in the
north in the month
Gamelion, the sun being about the winter
solstice. Yet they themselves admit that reflection over so great a
space is an impossibility.
An objection that tells equally against those who hold this theory
and those who say that comets are a coalescence of the planets is,
first, the fact that some of the fixed stars too get a tail. For
this we must not only accept the authority of the
Egyptians who assert
it, but we have ourselves observed the fact. For a star in the thigh
of the Dog had a tail, though a faint one. If you fixed your sight
on it its light was dim, but if you just glanced at it, it appeared
brighter. Besides, all the comets that have been seen in our day
have vanished without setting, gradually fading away above the
horizon; and they have not left behind them either one or more
stars. For instance the great comet we mentioned before appeared to
the west in winter in frosty weather when the sky was clear, in the
archonship of
Asteius. On the first day it set before the sun and
was then not seen. On the next day it was seen, being ever so little
behind the sun and immediately setting. But its light extended over
a third part of the sky like a leap, so that people called it a
'path'. This comet receded as far as
Orion's belt and there dissolved.
Democritus however, insists upon the truth of his view and affirms
that certain stars have been seen when comets dissolve. But on his
theory this ought not to occur occasionally but always. Besides, the
Egyptians affirm that conjunctions of the planets with one another,
and with the fixed stars, take place, and we have ourselves observed
Jupiter coinciding with one of the stars in the Twins and hiding it,
and yet no comet was formed. Further, we can also give a rational
proof of our point. It is true that some stars seem to be bigger
than others, yet each one by itself looks indivisible. Consequently,
just as, if they really had been indivisible, their conjunction
could not have created any greater magnitude, so now that they are not
in fact indivisible but look as if they were, their conjunction will
not make them look any bigger.
Enough has been said, without further argument, to show that the
causes brought forward to explain comets are false.
Table of Contents |
Next section