by Aristotle
Some—Cleidemus is one of them—say that lightning does not exist but appears. They compare it to what happens when you strike the sea with a rod by night and the water is seen to shine. They say that the moisture in the cloud is beaten about in the same way, and that lightning is the appearance of brightness [15] that ensues.
These men were not yet familiar with the theory of reflection, which is the cause of that phenomenon. The water appears to shine when struck because our sight is reflected from it to some bright object; hence the phenomenon occurs mainly by night: the appearance is not seen by day because the daylight is too [20] intense and obscures it.
These are the theories of others about thunder and lightning: some maintaining that lightning is a reflection, the others that lightning is fire shining through the cloud and thunder its extinction, the fire not being generated in each case but [25] existing beforehand. We say that the same stuff is wind on the earth, and earthquake under it, and in the clouds thunder. The substance of all these phenomena is the same: namely, the dry exhalation. If it flows in one way it is wind, in another it causes earthquakes; in the clouds, when they are in a process of change35 and contract and condense into water, it is ejected36 and causes thunder [30] and lightning and the other phenomena of the same nature.
So much for thunder and lightning.
BOOK III
1 · Let us explain the remaining operations of this ejection in the same way [5] as we have treated the rest. When this wind is ejected in small and scattered quantities and frequently, and spreads, and its constitution is rare, it gives rise to thunder and lightning. But if it is ejected in a body and is denser, that is, less rare, we get a hurricane. That is why it is violent: it is due to the rapidity of the ejection. [10] Now when this ejection 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 potentially present. As soon as a stimulus to the development of either potentiality appears, that of which there is the [15] 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 wind 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 [20] 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 [25] same thing happens to the next part, and the next, 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 when the cloud is continually separated off and there is a continuous [30] wind) the cloud follows the exhalation unbroken, and the wind failing to break away from the cloud because of its density, first moves in a circle for the reason given and [371a1] 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 is37 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 [5] phenomena are wind, and wind is a dry and warm exhalation. 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.
[10] So the whirlwind originates in the failure of an incipient hurricane to escape from its cloud: it is due to the resistance of the eddy, and occurs when the spiral 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.
[15] When the cloud burns as it is drawn downwards,—that occurs when the wind 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 wind and it is rare and is squeezed out in the cloud itself we get a thunderbolt. If it is exceedingly rare this rareness prevents the thunderbolt from scorching and the poets call it ‘bright’; if the rareness is less it does [20] scorch and they call it ‘smoky’. The former moves onward 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. That is why resisting substances are affected, unresisting ones are not. For instance, it has happened that the bronze of a [25] shield has been melted while the woodwork remained intact because its texture was so loose that the wind filtered through without affecting it. So it has passed through38 clothes, too, without burning them, and has merely reduced them to shreds.
Such evidence is enough to show that the wind is at work in all these cases, but we sometimes get ocular evidence as well, as in the case of the conflagration of the [30] 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 wind and that smoke burns is certain, and has been stated in another place before; [371b1] but when the flame moves bodily, then it can be seen clearly that it is wind. 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 wind split, and a quantity of it rushed in a body from the place from which it issued forth [5] and went up in a blaze; so that the flame seemed to move away and to fall on the houses. For we must recognize that wind accompanies and precedes thunderbolts though it is colourless and so not seen. Hence, where the thunderbolt is going to strike, the object moves before it is struck, showing that the wind which is its origin [10] falls on the object first. Thunder, too, splits things not by its noise but because the wind that strikes the object and makes the noise is ejected simultaneously. This splits the thing it strikes but does not scorch it at all.
We have now explained thunder and lightning and hurricane, and further fire-winds, whirlwinds, and thunder-bolts, and shown that they are all the same [15] 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 causes account for them all. [20]
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 [25] 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 [30] about midday. There are never more than two rainbows at one time. Each of them is [372a1] 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 [5] 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. Those are the colours of the rainbow, though between the [10] red and the green an orange colour is often see
n.
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 [15] 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 differ in the manner of the reflection and [20] in the reflecting surfaces and according as the reflection to the sun or some other bright object is.
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 [25] 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 [30] air and any object with a smooth surface just as it is from water; also that in some mirrors the shapes of things are reflected, in others only their colours. Of the latter [372b1] kind are those mirrors which are so small as to be indivisible for sense. It is impossible that the shape of a thing should be reflected in them; for if it is the mirror will seem divisible—for every shape is at once a shape and divisible. But since [5] something must be reflected in them and shape 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 [10] investigation 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 [15] 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 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 [20] 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 halos are the darkest in colour. It is a sign of wind when it is broken up because its [25] 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 [30] condense into water, this shows that the moist vapour has not yet separated from the dry and firelike exhalation: and this is a cause of fine weather.
So much for the atmospheric conditions under which the reflection takes place. Sight is reflected from the mist that forms round the sun or the moon, and that is [373a1] 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. [5]
Let ACB and AFB and ADB be lines each of which goes from the point A to the point B and forms an angle. Let the lines AC, AF, AD be equal and those at B—viz. CB, FB, DB— equal too. Draw the line AEB. Then the triangles are equal; [10] for their base AEB is equal. Draw perpendiculars to AEB from the angles; CE from C, FE from F, DE from D. Then these perpendiculars are equal, being in equal triangles and all in one plane; for they are all at right angles to AEB and meet at a [15] 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 CFD the cloud from which the sight is reflected to the sun.
The mirrors must be thought of as continuous: each of them is too small to be visible, but their contiguity makes the whole made up of them all to seem one. The [20] 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 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. Next to this is a dark ring, which seems to be darker because of the [25] brightness of the halo.
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 [30] 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 [373b1] reflection even uncondensed when the sight is weak. Such was the case of a man [5] 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 [10] it back. That is why promontories in the sea loom when there is a south-east wind, and everything seems bigger, and in a mist, too, things seem bigger—as 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 [15] 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 [20] 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 [25] 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. Hence since these conditions are realizable there will be an appearance due to reflection whenever the sun and the cloud are related in the [30] 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 [374a1] 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 nature. Bright light through a dark medium or on a dark surface (it makes no difference) looks red. We [5] can see 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 [10] 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 itself: 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 [15] were not this distinction haloes would be coloured like the rainbow. Actually no complete or circular halo presents this appearance, only small and fragmentary ones 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 [20] find round 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 [25] the flame gives off and which mixes with the air; for a mirror is then formed actually because of the blackness—for soot is smoky. 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 [30] 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 [374b1] 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 [5] its cause is the same, for the sprinkling hand corresponds to the oar.