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1
LET us explain the remaining operations of this secretion in the
same way as we have treated the rest. When this exhalation is secreted
in small and scattered quantities and frequently, and is transitory,
and its constitution rare, it gives rise to
thunder and
lightning. But
if it is secreted in a body and is denser, that is, less rare, we
get a
hurricane. The fact that it issues in body explains its
violence: it is due to the rapidity of the secretion. Now when this
secretion issues in a great and continuous current the result
corresponds to what we get when the opposite development takes place
and
rain and a quantity of water are produced. As far as the matter
from which they are developed goes both sets of phenomena are the
same. As soon as a stimulus to the development of either
potentiality appears, that of which there is the greater quantity
present in the cloud is at once secreted from it, and there results
either rain, or, if the other exhalation prevails, a hurricane.
Sometimes the exhalation in the cloud, when it is being secreted,
collides with another under circumstances like those found when a
wind
is forced from an open into a narrow space in a gateway or a road.
It often happens in such cases that the first part of the moving
body is deflected because of the resistance due either to the
narrowness or to a contrary current, and so the wind forms a circle
and
eddy. It is prevented from advancing in a straight line: at the
same time it is pushed on from behind; so it is compelled to move
sideways in the direction of least resistance. The same thing
happens to the next part, and the next, and so on, till the series
becomes one, that is, till a circle is formed: for if a figure is
described by a single
motion that figure must itself be one. This is
how eddies are generated on the
earth, and the case is the same in the
clouds as far as the beginning of them goes. Only here (as in the case
of the hurricane which shakes off the cloud without cessation and
becomes a continuous wind) the cloud follows the exhalation
unbroken, and the exhalation, failing to break away from the cloud
because of its density, first moves in a circle for the reason given
and then descends, because clouds are always densest on the side where
the heat escapes. This phenomenon is called a whirlwind when it is
colourless; and it is a sort of undigested hurricane. There is never a
whirlwind when the weather is northerly, nor a hurricane when there is
snow. The reason is that all these phenomena are 'wind', and wind is a
dry and warm
evaporation. Now frost and cold prevail over this
principle and quench it at its birth: that they do prevail is clear or
there could be no snow or northerly rain, since these occur when the
cold does prevail.
So the whirlwind originates in the failure of an incipient hurricane
to escape from its cloud: it is due to the resistance which
generates the eddy, and it consists in the spiral which descends to
the earth and drags with it the cloud which it cannot shake off. It
moves things by its wind in the direction in which it is blowing in
a straight line, and whirls round by its circular motion and
forcibly snatches up whatever it meets.
When the cloud burns as it is drawn downwards, that is, when the
exhalation becomes rarer, it is called a
fire-wind, for its fire
colours the neighbouring
air and inflames it.
When there is a great quantity of exhalation and it is rare and is
squeezed out in the cloud itself we get a thunderbolt. If the
exhalation is exceedingly rare this rareness prevents the
thunderbolt from scorching and the poets call it 'bright': if the
rareness is less it does scorch and they call it 'smoky'. The former
moves rapidly because of its rareness, and because of its rapidity
passes through an object before setting fire to it or dwelling on it
so as to blacken it: the slower one does blacken the object, but
passes through it before it can actually burn it. Further, resisting
substances are affected, unresisting ones are not. For instance, it
has happened that the bronze of a shield has been melted while the
woodwork remained intact because its texture was so loose that the
exhalation filtered through without affecting it. So it has passed
through clothes, too, without burning them, and has merely reduced
them to shreds.
Such evidence is enough by itself to show that the exhalation is
at work in all these cases, but we sometimes get direct evidence as
well, as in the case of the conflagration of the temple at
Ephesus
which we lately witnessed. There independent sheets of flame left
the main fire and were carried bodily in many directions. Now that
smoke is exhalation and that smoke burns is certain, and has been
stated in another place before; but when the flame moves bodily,
then we have ocular proof that smoke is exhalation. On this occasion
what is seen in small fires appeared on a much larger scale because of
the quantity of matter that was burning. The beams which were the
source of the exhalation split, and a quantity of it rushed in a
body from the place from which it issued forth and went up in a blaze:
so that the flame was actually seen moving through the air away and
falling on the houses. For we must recognize that exhalation
accompanies and precedes
thunderbolts though it is colourless and so
invisible. Hence, where the thunderbolt is going to strike, the object
moves before it is struck, showing that the exhalation leads the way
and falls on the object first. Thunder, too, splits things not by
its noise but because the exhalation that strikes the object and
that which makes the noise are ejected simultaneously. This exhalation
splits the thing it strikes but does not scorch it at all.
We have now explained thunder and lightning and hurricane, and
further
firewinds,
whirlwinds, and thunderbolts, and shown that they
are all of them forms of the same thing and wherein they all differ.
2
Let us now explain the
nature and cause of halo,
rainbow, mock suns,
and rods, since the same account applies to them all.
We must first describe the phenomena and the circumstances in
which each of them occurs. The halo often appears as a complete
circle: it is seen round the sun and the
moon and bright
stars, by
night as well as by day, and at midday or in the afternoon, more
rarely about sunrise or sunset.
The rainbow never forms a full circle, nor any segment greater
than a semicircle. At sunset and sunrise the circle is smallest and
the segment largest: as the sun rises higher the circle is larger
and the segment smaller. After the
autumn equinox in the shorter
days it is seen at every hour of the day, in the
summer not about
midday. There are never more than two rainbows at one time. Each of
them is three-coloured; the colours are the same in both and their
number is the same, but in the outer rainbow they are fainter and
their position is reversed. In the inner rainbow the first and largest
band is red; in the outer rainbow the band that is nearest to this one
and smallest is of the same colour: the other bands correspond on
the same principle. These are almost the only colours which painters
cannot manufacture: for there are colours which they create by mixing,
but no mixing will give red, green, or purple. These are the colours
of the rainbow, though between the red and the green an orange
colour is often seen.
Mock suns and rods are always seen by the side of the sun, not above
or below it nor in the opposite quarter of the sky. They are not
seen at night but always in the neighbourhood of the sun, either as it
is rising or setting but more commonly towards sunset. They have
scarcely ever appeared when the sun was on the meridian, though this
once happened in Bosporus where two mock suns rose with the sun and
followed it all through the day till sunset.
These are the facts about each of these phenomena: the cause of them
all is the same, for they are all reflections. But they are
different varieties, and are distinguished by the surface from which
and the way in which the reflection to the sun or some other bright
object takes place.
The rainbow is seen by day, and it was formerly thought that it
never appeared by night as a moon rainbow. This opinion was due to the
rarity of the occurrence: it was not observed, for though it does
happen it does so rarely. The reason is that the colours are not so
easy to see in the dark and that many other conditions must
coincide, and all that in a single day in the month. For if there is
to be one it must be at full moon, and then as the moon is either
rising or setting. So we have only met with two instances of a moon
rainbow in more than fifty years.
We must accept from the theory of optics the fact that
sight is
reflected from air and any object with a smooth surface just as it
is from water; also that in some mirrors the forms of things are
reflected, in others only their colours. Of the latter kind are
those mirrors which are so small as to be indivisible for sense. It is
impossible that the figure of a thing should be reflected in them, for
if it is the mirror will be sensibly divisible since divisibility is
involved in the notion of figure. But since something must be
reflected in them and figure cannot be, it remains that colour alone
should be reflected. The colour of a bright object sometimes appears
bright in the reflection, but it sometimes, either owing to the
admixture of the colour of the mirror or to
weakness of sight, gives
rise to the appearance of another colour.
However, we must accept the account we have given of these things in
the theory of sensation, and take some things for granted while we
explain others.
3
Let us begin by explaining the shape of the halo; why it is a circle
and why it appears round the sun or the moon or one of the other
stars: the explanation being in all these cases the same.
Sight is reflected in this way when air and
vapour are condensed
into a cloud and the condensed matter is uniform and consists of small
parts. Hence in itself it is a sign of rain, but if it fades away,
of fine weather, if it is broken up, of wind. For if it does not
fade away and is not broken up but is allowed to attain its normal
state, it is naturally a sign of rain since it shows that a process of
condensation is proceeding which must, when it is carried to an end,
result in rain. For the same reason these haloes are the darkest. It
is a sign of wind when it is broken up because its breaking up is
due to a wind which exists there but has not reached us. This view
finds support in the fact that the wind blows from the quarter in
which the main division appears in the halo. Its fading away is a sign
of fine weather because if the air is not yet in a state to get the
better of the heat it contains and proceed to condense into water,
this shows that the moist vapour has not yet separated from the dry
and firelike exhalation: and this is the cause of fine weather.
So much for the atmospheric conditions under which the reflection
takes place. The reflection is from the mist that forms round the
sun or the moon, and that is why the halo is not seen opposite the sun
like the rainbow.
Since the reflection takes place in the same way from every point
the result is necessarily a circle or a segment of a circle: for if
the lines start from the same point and end at the same point and
are equal, the points where they form an angle will always lie on a
circle.
Let AGB and AZB and ADB be lines each of which goes from the point A
to the point B and forms an angle. Let the lines AG, AZ, AD be equal
and those at B, GB, ZB, DB equal too. (See diagram.)
Draw the line AEB. Then the triangles are equal; for their base
AEB is equal. Draw perpendiculars to AEB from the angles; GE from G,
ZE from Z, DE from D. Then these perpendiculars are equal, being in
equal triangles. And they are all in one plane, being all at right
angles to AEB and meeting at a single point E. So if you draw the line
it will be a circle and E its centre. Now B is the sun, A the eye, and
the circumference passing through the points GZD the cloud from
which the line of sight is reflected to the sun.
The mirrors must be thought of as contiguous: each of them is too
small to be visible, but their contiguity makes the whole made up of
them all to seem one. The bright band is the sun, which is seen as a
circle, appearing successively in each of the mirrors as a point
indivisible to sense. The band of cloud next to it is black, its
colour being intensified by contrast with the brightness of the
halo. The halo is formed rather near the earth because that is calmer:
for where there is wind it is clear that no halo can maintain its
position.
Haloes are commoner round the moon because the greater heat of the
sun dissolves the condensations of the air more rapidly.
Haloes are formed round stars for the same reasons, but they are not
prognostic in the same way because the condensation they imply is so
insignificant as to be barren.
4
We have already stated that the rainbow is a reflection: we have now
to explain what sort of reflection it is, to describe its various
concomitants, and to assign their causes.
Sight is reflected from all smooth surfaces, such as are air and
water among others. Air must be condensed if it is to act as a mirror,
though it often gives a reflection even uncondensed when the sight
is weak. Such was the case of a man whose sight was faint and
indistinct. He always saw an image in front of him and facing him as
he walked. This was because his sight was reflected back to him. Its
morbid condition made it so weak and delicate that the air close by
acted as a mirror, just as distant and condensed air normally does,
and his sight could not push it back. So promontories in the
sea
'loom' when there is a south-east wind, and everything seems bigger,
and in a mist, too, things seem bigger: so, too, the sun and the stars
seem bigger when rising and setting than on the meridian. But things
are best reflected from water, and even in process of formation it
is a better mirror than air, for each of the particles, the union of
which constitutes a raindrop, is necessarily a better mirror than
mist. Now it is obvious and has already been stated that a mirror of
this kind renders the colour of an object only, but not its shape.
Hence it follows that when it is on the point of raining and the air
in the clouds is in process of forming into raindrops but the rain
is not yet actually there, if the sun is opposite, or any other object
bright enough to make the cloud a mirror and cause the sight to be
reflected to the object then the reflection must render the colour
of the object without its shape. Since each of the mirrors is so small
as to be invisible and what we see is the continuous magnitude made up
of them all, the reflection necessarily gives us a continuous
magnitude made up of one colour; each of the mirrors contributing
the same colour to the whole. We may deduce that since these
conditions are realizable there will be an appearance due to
reflection whenever the sun and the cloud are related in the way
described and we are between them. But these are just the conditions
under which the rainbow appears. So it is clear that the rainbow is
a reflection of sight to the sun.
So the rainbow always appears opposite the sun whereas the halo is
round it. They are both reflections, but the rainbow is
distinguished by the variety of its colours. The reflection in the one
case is from water which is dark and from a distance; in the other
from air which is nearer and lighter in colour. White light through
a dark medium or on a dark surface (it makes no difference) looks red.
We know how red the flame of green wood is: this is because so much
smoke is mixed with the bright white firelight: so, too, the sun
appears red through smoke and mist. That is why in the rainbow
reflection the outer circumference is red (the reflection being from
small particles of water), but not in the case of the halo. The
other colours shall be explained later. Again, a condensation of
this kind cannot persist in the neighbourhood of the sun: it must
either turn to rain or be dissolved, but opposite to the sun there
is an interval during which the water is formed. If there were not
this distinction haloes would be coloured like the rainbow. Actually
no complete or circular halo presents this colour, only small and
fragmentary appearances called 'rods'. But if a haze due to water or
any other dark substance formed there we should have had, as we
maintain, a complete rainbow like that which we do find lamps. A
rainbow appears round these in winter, generally with southerly
winds.
Persons whose eyes are moist see it most clearly because their sight
is weak and easily reflected. It is due to the moistness of the air
and the soot which the flame gives off and which mixes with the air
and makes it a mirror, and to the blackness which that mirror
derives from the smoky nature of the soot. The light of the lamp
appears as a circle which is not white but purple. It shows the
colours of the rainbow; but because the sight that is reflected is too
weak and the mirror too dark, red is absent. The rainbow that is
seen when oars are raised out of the sea involves the same relative
positions as that in the sky, but its colour is more like that round
the lamps, being purple rather than red. The reflection is from very
small particles continuous with one another, and in this case the
particles are fully formed water. We get a rainbow, too, if a man
sprinkles fine drops in a room turned to the sun so that the sun is
shining in part of the room and throwing a shadow in the rest. Then if
one man sprinkles in the room, another, standing outside, sees a
rainbow where the sun's rays cease and make the shadow. Its nature and
colour is like that from the oars and its cause is the same, for the
sprinkling hand corresponds to the oar.
That the colours of the rainbow are those we described and how the
other colours come to appear in it will be clear from the following
considerations. We must recognize, as we have said, and lay down:
first, that white colour on a black surface or seen through a black
medium gives red; second, that sight when strained to a distance
becomes weaker and less; third, that black is in a sort the negation
of sight: an object is black because sight fails; so everything at a
distance looks blacker, because sight does not reach it. The theory of
these matters belongs to the account of the senses, which are the
proper subjects of such an inquiry; we need only state about them what
is necessary for us. At all events, that is the reason why distant
objects and objects seen in a mirror look darker and smaller and
smoother, why the reflection of clouds in water is darker than the
clouds themselves. This latter is clearly the case: the reflection
diminishes the sight that reaches them. It makes no difference whether
the change is in the object seen or. in the sight, the result being in
either case the same. The following fact further is worth noticing.
When there is a cloud near the sun and we look at it does not look
coloured at all but white, but when we look at the same cloud in water
it shows a trace of rainbow colouring. Clearly, then, when sight is
reflected it is weakened and, as it makes dark look darker, so it
makes white look less white, changing it and bringing it nearer to
black. When the sight is relatively strong the change is to red; the
next stage is green, and a further degree of weakness gives violet. No
further change is visible, but three completes the series of colours
(as we find three does in most other things), and the change into
the rest is imperceptible to sense. Hence also the rainbow appears
with three colours; this is true of each of the two, but in a contrary
way. The outer band of the primary rainbow is red: for the largest
band reflects most sight to the sun, and the outer band is largest.
The middle band and the third go on the same principle. So if the
principles we laid down about the appearance of colours are true the
rainbow necessarily has three colours, and these three and no
others. The appearance of yellow is due to contrast, for the red is
whitened by its juxtaposition with green. We can see this from the
fact that the rainbow is purest when the cloud is blackest; and then
the red shows most yellow. (Yellow in the rainbow comes between red
and green.) So the whole of the red shows white by contrast with the
blackness of the cloud around: for it is white compared to the cloud
and the green. Again, when the rainbow is fading away and the red is
dissolving, the white cloud is brought into contact with the green and
becomes yellow. But the moon rainbow affords the best instance of this
colour contrast. It looks quite white: this is because it appears on
the dark cloud and at night. So, just as fire is intensified by
added fire, black beside black makes that which is in some degree
white look quite white. Bright dyes too show the effect of contrast.
In woven and embroidered stuffs the appearance of colours is
profoundly affected by their juxtaposition with one another (purple,
for instance, appears different on white and on black wool), and
also by differences of illumination. Thus embroiderers say that they
often make mistakes in their colours when they work by lamplight,
and use the wrong ones.
We have now shown why the rainbow has three colours and that these
are its only colours. The same cause explains the double rainbow and
the faintness of the colours in the outer one and their inverted
order. When sight is strained to a great distance the appearance of
the distant object is affected in a certain way: and the same thing
holds good here. So the reflection from the outer rainbow is weaker
because it takes place from a greater distance and less of it
reaches the sun, and so the colours seen are fainter. Their order is
reversed because more reflection reaches the sun from the smaller,
inner band. For that reflection is nearer to our sight which is
reflected from the band which is nearest to the primary rainbow. Now
the smallest band in the outer rainbow is that which is nearest, and
so it will be red; and the second and the third will follow the same
principle. Let B be the outer rainbow, A the inner one; let R stand
for the red colour, G for green, V for violet; yellow appears at the
point Y. Three rainbows or more are not found because even the
second is fainter, so that the third reflection can have no
strength
whatever and cannot reach the sun at all. (See diagram.)
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