1
WE have already discussed the first causes of nature, and allnatural motion, also the stars ordered in the motion of the heavens,and the physical element-enumerating and specifying them and showinghow they change into one another-and becoming and perishing ingeneral. There remains for consideration a part of this inquirywhich all our predecessors called meteorology. It is concerned withevents that are natural, though their order is less perfect thanthat of the first of the elements of bodies. They take place in theregion nearest to the motion of the stars. Such are the milky way, andcomets, and the movements of meteors. It studies also all theaffections we may call common to air and water, and the kinds andparts of the earth and the affections of its parts. These throwlight on the causes of winds and earthquakes and all theconsequences the motions of these kinds and parts involve. Of thesethings some puzzle us, while others admit of explanation in somedegree. Further, the inquiry is concerned with the falling ofthunderbolts and with whirlwinds and fire-winds, and further, therecurrent affections produced in these same bodies by concretion. Whenthe inquiry into these matters is concluded let us consider whataccount we can give, in accordance with the method we have followed,of animals and plants, both generally and in detail. When that hasbeen done we may say that the whole of our original undertaking willhave been carried out.
After this introduction let us begin by discussing our immediatesubject.
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We have already laid down that there is one physical element whichmakes up the system of the bodies that move in a circle, and besidesthis four bodies owing their existence to the four principles, themotion of these latter bodies being of two kinds: either from thecentre or to the centre. These four bodies are fire, air, water,earth. Fire occupies the highest place among them all, earth thelowest, and two elements correspond to these in their relation toone another, air being nearest to fire, water to earth. The wholeworld surrounding the earth, then, the affections of which are oursubject, is made up of these bodies. This world necessarily has acertain continuity with the upper motions: consequently all itspower and order is derived from them. (For the originating principleof all motion is the first cause. Besides, that clement is eternal andits motion has no limit in space, but is always complete; whereasall these other bodies have separate regions which limit one another.)So we must treat fire and earth and the elements like them as thematerial causes of the events in this world (meaning by materialwhat is subject and is affected), but must assign causality in thesense of the originating principle of motion to the influence of theeternally moving bodies.
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Let us first recall our original principles and the
distinctionsalready drawn and then explain the ’milky way’ and comets and theother 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 canbe resolved into a common and ultimate substrate.
The first difficulty is raised by what is called the air. What arewe to take its nature to be in the world surrounding the earth? Andwhat is its position relatively to the other physical elements. (Forthere is no question as to the relation of the bulk of the earth tothe size of the bodies which exist around it, since astronomicaldemonstrations have by this time proved to us that it is actuallyfar smaller than some individual stars. As for the water, it is notobserved to exist collectively and separately, nor can it do soapart from that volume of it which has its seat about the earth: thesea, that is, and rivers, which we can see, and any subterranean waterthat may be hidden from our observation.) The question is really aboutthat which lies between the earth and the nearest stars. Are we toconsider it to be one kind of body or more than one? And if morethan one, how many are there and what are the bounds of their regions?
We have already described and characterized the first element, andexplained that the whole world of the upper motions is full of thatbody.
This is an opinion we are not alone in holding: it appears to bean old assumption and one which men have held in the past, for theword ether has long been used to denote that element. Anaxagoras, itis 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 thatmen referred to those regions when they spoke of ether. In thelatter point he was right, for men seem to have assumed that a bodythat was eternally in motion was also divine in nature; and, as such abody was different from any of the terrestrial elements, theydetermined to call it ’ether’.
For the same 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 motionbut that which contains them is pure fire, and the interval betweenthe earth and the stars air: but if they had considered what is nowsatisfactorily established by mathematics, they might have given upthis puerile opinion. For it is altogether childish to suppose thatthe moving bodies are all of them of a small size, because they soto us, looking at them from the earth.
This a matter which we have already discussed in our treatment ofthe upper region, but we may return to the point now.
If the intervals were full of fire and the bodies consisted offire every one of the other elements would long ago have vanished.
However, they cannot simply be said to be full of air either; foreven if there were two elements to fill the space between the earthand the heavens, the air would far exceed the quantitu required tomaintain its proper proportion to the other elements. For the bulkof the earth (which includes the whole volume of water) isinfinitesimal in comparison with the whole world that surrounds it.Now we find that the excess in volume is not proportionately greatwhere water dissolves into air or air into fire. Whereas theproportion between any given small quantity of water and the airthat is generated from it ought to hold good between the totalamount of air and the total amount of water. Nor does it make anydifference if any one denies
that the elements originate from oneanother, but asserts that they are equal in power. For on this view itis certain amounts of each that are equal in power, just as would bethe case if they actually originated from one another.
So it is clear that neither air nor fire alone fills theintermediate space.
It remains to explain, after a preliminary discussion ofdifficulties, the relation of the two elements air and fire to theposition of the first element, and the reason why the stars in theupper region impart heat to the earth and its neighbourhood. Let usfirst treat of the air, as we proposed, and then go on to thesequestions.
Since water is generated from air, and air from water, why areclouds 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 isneither appreciably near to the heat of the stars, nor to the raysrelected from the earth. It is these that dissolve any formation bytheir heat and so prevent clouds from forming near the earth. Forclouds gather at the point where the reflected rays
disperse in theinfinity of space and are lost. To explain this we must suppose eitherthat it is not all air which water is generated, or, if it is producedfrom all air alike, that what immediately surrounds the earth is notmere air, but a sort of vapour, and that its vaporous nature is thereason why it condenses back to water again. But if the whole ofthat vast region is vapour, the amount of air and of water will bedisproportionately great. For the spaces left by the heavenly bodiesmust be filled by some element. This cannot be fire, for then allthe rest would have been dried up. Consequently, what fills it must beair and the water that surrounds the whole earth-vapour being waterdissolved.
After this exposition of the difficulties involved, let us go onto lay down the truth, with a view at once to what follows and to whathas already been said. The upper region as far as the moon we affirmto consist of a body distinct both from fire and from air, but varyingdegree of purity and in kind, especially towards its limit on the sideof the air, and of the world surrounding the earth. Now the circularmotion of the first element and of the bodies it contains dissolves,and inflames by its motion, whatever part of the lower world isnearest to it, and so generates heat. From another point of view wemay look at the motion as follows. The body that lies below thecircular motion of the heavens is, in a sort, matter, and ispotentially hot, cold, dry, moist, and possessed of whatever otherqualities are derived from these. But it actually acquires orretains one of these in virtue of motion or rest, the cause andprinciple of which has already been explained. So at the centre andround it we get earth and water, the heaviest and coldest elements, bythemselves; round them and contiguous with them, air and what wecommonly call fire. It is not really fire, for fire is an excess ofheat and a sort of ebullition; but in reality, of what we call air,the part surrounding the earth is moist and warm, because itcontains both vapour and a dry exhalation from the
earth. But the nextpart, above that, is warm and dry. For vapour is naturally moist andcold, but the exhalation warm and dry; and vapour is potentiallylike water, the exhalation potentially like fire. So we must takethe 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 upperregion is also prevented by the circular motion. For the air round theearth is necessarily all of it in motion, except that which is cut offinside the circumference which makes the earth a complete sphere. Inthe case of winds it is actually observable that they originate inmarshy districts of the earth; and they do not seem to blow abovethe level of the highest mountains. It is the revolution of the heavenwhich carries the air with it and causes its circular motion, firebeing continuous with the upper element and air with fire. Thus itsmotion is a second reason why that air is not condensed into water.
But whenever a particle of air grows heavy, the warmth in it issqueezed out into the upper region and it sinks, and other particlesin turn are carried up together with the fiery exhalation. Thus theone region is always full of air and the other of fire, and each ofthem is perpetually in a state of change.
So much to explain why clouds are not formed and why the air isnot condensed into water, and what account must be given of thespace between the stars and the earth, and what is the body that fillsit.
As for the heat derived from the sun, the right place for aspecial 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 canbe 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 thesun’s motion alone is sufficient to account for the origin ofterrestrial warmth and heat. For a motion that is to have thiseffect must be rapid and near, and that of the stars is rapid butdistant, while that of the moon is near but slow, whereas the sun’smotion combines both conditions in a sufficient degree. That most heatshould be generated where the sun is present is easy to understandif we consider the analogy of terrestrial phenomena, for here, too, itis the air that is nearest to a thing in rapid motion which isheated most. This is just what we should expect, as it is thenearest air that is most dissolved by the motion of a solid body.
This then is one reason why heat reaches our world. Another isthat the fire surrounding the air is often scattered by the motionof the heavens and driven downwards in spite of itself.
Shooting-stars further suffix to prove that the celestial sphereis not hot or fiery: for they do not occur in that upper region butbelow: yet the more and the faster a thing moves, the more apt it isto take fire. Besides, the sun, which most of all the stars isconsidered to be hot, is really white and not fiery in colour.
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Having determined these principles let us explain the cause of theappearance in the sky of burning flames and of shooting-stars, andof ’torches’, and ’goats’, as some people call them. All thesephenomena 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. Whenthe sun warms the earth the evaporation which takes place isnecessarily of two kinds, not of one only as some think. One kind israther of the nature of vapour,
the other of the nature of a windyexhalation. That which rises from the moisture contained in theearth and on its surface is vapour, while that rising from the earthitself, which is dry, is like smoke. Of these the windy exhalation,being warm, rises above the moister vapour, which is heavy and sinksbelow the other. Hence the world surrounding the earth is ordered asfollows. First below the circular motion comes the warm and dryelement, which we call fire, for there is no word fully adequate toevery state of the fumid evaporation: but we must use this terminologysince this element is the most inflammable of all bodies. Below thiscomes air. We must think of what we just called fire as being spreadround the terrestrial sphere on the outside like a kind of fuel, sothat a little motion often makes it burst into flame just as smokedoes: for flame is the ebullition of a dry exhalation. So whenever thecircular motion stirs this stuff up in any way, it catches fire at thepoint at which it is most inflammable. The result differs according tothe disposition and quantity of the combustible material. If this isbroad 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 islonger than it is broad sometimes it seems to throw off sparks as itburns. (This happens because matter catches fire at the sides in smallportions but continuously with the main body.) Then it is called a’goat’. When this does not happen it is a ’torch’. But if the wholelength of the exhalation is scattered in small parts and in manydirections and in breadth and depth alike, we get what are calledshooting-stars.
The cause of these shooting-stars is sometimes the motion whichignites the exhalation. At other times the air is condensed by coldand squeezes out and ejects the hot element; making their motionlook more like that of a thing thrown than like a running fire. Forthe question might be raised
whether the ’shooting’ of a ’star’ is thesame thing as when you put an exhalation below a lamp and it lightsthe lower lamp from the flame above. For here too the flame passeswonderfully quickly and looks like a thing thrown, and not as if onething after another caught fire. Or is a ’star’ when it ’shoots’ asingle body that is thrown? Apparently both cases occur: sometimesit is like the flame from the lamp and sometimes bodies areprojected 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 bynight and by day when the sky is clear. They are thrown downwardsbecause the condensation which propels them inclines downwards.Thunderbolts fall downwards for the same reason: their origin is nevercombustion but ejection under pressure, since naturally all heat tendsupwards.
When the phenomenon is formed in the upper region it is due to thecombustion of the exhalation. When it takes place at a lower levelit is due to the ejection of the exhalation by the condensing andcooling of the moister evaporation: for this latter as it condensesand inclines downward contracts, and thrusts out the hot element andcauses it to be thrown downwards. The motion is upwards or downwardsor sideways according to the way in which the evaporation lies, andits disposition in respect of breadth and depth. In most cases thedirection is sideways because two motions are involved, a compulsorymotion downwards and a natural motion upwards, and under thesecircumstances 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 thecontraction and condensation of the air. Further, all these thingshappen below the moon. This is shown by their apparent speed, which isequal to that of things thrown by us; for it is because they are closeto us,
that these latter seem far to exceed in speed the stars, thesun, and the moon.
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Sometimes on a fine night we see a variety of appearances thatform in the sky: ’chasms’ for instance and ’trenches’ and blood-redcolours. These, too, have the same cause. For we have seen that theupper air condenses into an inflammable condition and that thecombustion sometimes takes on the appearance of a burning flame,sometimes that of moving torches and stars. So it is not surprisingthat 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 actsas a mirror, will cause all kinds of colours to appear, but especiallycrimson and purple. For these colours generally appear whenfire-colour and white are combined by superposition. Thus on a hotday, or through a smoky, medium, the stars when they rise and set lookcrimson. The light will also create colours by reflection when themirror is such as to reflect colour only and not shape.
These appearances do not persist long, because the condensation ofthe air is transient.
’Chasms’ get their appearance of depth from light breaking out ofa dark blue or black mass of air. When the process of condensationgoes further in such a case we often find ’torches’ ejected. Whenthe ’chasm’ contracts it presents the appearance of a ’trench’.
In general, white in contrast with black creates a variety ofcolours; like flame, for instance, through a medium of smoke. But byday the sun obscures them, and, with the exception of crimson, thecolours are not seen at night because they are dark.
These then must be taken to be the causes of ’shooting-stars’ andthe phenomena of combustion and also of the other transientappearances of this kind.
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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 ofthe planets approaching one another and so appearing to touch oneanother.
Some of the Italians called Pythagoreans say that the comet is oneof the planets, but that it appears at great intervals of time andonly rises a little above the horizon. This is the case with Mercurytoo; because it only rises a little above the horizon it often failsto be seen and consequently appears at great intervals of time.
A view like theirs was also expressed by Hippocrates of Chios andhis pupil Aeschylus. Only they say that the tail does not belong tothe comet iself, but is occasionally assumed by it on its course incertain situations, when our sight is reflected to the sun from themoisture attracted by the comet. It appears at greater intervalsthan the other stars because it is slowest to get clear of the sun andhas been left behind by the sun to the extent of the whole of itscircle before it reappears at the same point. It gets clear of the sunboth towards the north and towards the south. In the space between thetropics it does not draw water to itself because that region isdried up by the sun on its course. When it moves towards the southit has no lack of the necessary moisture, but because the segment ofits circle which is above the horizon is small, and that below it manytimes as large, it is impossible for the sun to be reflected to oursight, either when it approaches the southern tropic, or at the summersolstice. 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 thearc above the horizon is large and that below it
small. For underthese circumstances there is nothing to prevent our vision frombeing reflected to the sun.
These views involve impossibilities, some of which are common to allof them, while others are peculiar to some only.
This is the case, first, with those who say that the comet is one ofthe planets. For all the planets appear in the circle of the zodiac,whereas many comets have been seen outside that circle. Again morecomets than one have often appeared simultaneously. Besides, iftheir tail is due to reflection, as Aeschylus and Hippocrates say,this planet ought sometimes to be visible without a tail since, asthey 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 thefive. And all of them are often visible above the horizon togetherat the same time. Further, comets are often found to appear, as wellwhen all the planets are visible as when some are not, but areobscured by the neighbourhood of the sun. Moreover the statementthat a comet only appears in the north, with the sun at the summersolstice, is not true either. The great comet which appeared at thetime of the earthquake in Achaea and the tidal wave rose due west; andmany have been known to appear in the south. Again in the archonshipof Euclees, son of Molon, at Athens there appeared a comet in thenorth in the month Gamelion, the sun being about the wintersolstice. Yet they themselves admit that reflection over so great aspace is an impossibility.
An objection that tells equally against those who hold this theoryand 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. Forthis we must not only accept the authority of the Egyptians who assertit, but we have ourselves observed the fact. For a star in the thighof the Dog had a tail, though a faint one. If you fixed your sighton
it its light was dim, but if you just glanced at it, it appearedbrighter. Besides, all the comets that have been seen in our dayhave vanished without setting, gradually fading away above thehorizon; and they have not left behind them either one or morestars. For instance the great comet we mentioned before appeared tothe west in winter in frosty weather when the sky was clear, in thearchonship of Asteius. On the first day it set before the sun andwas then not seen. On the next day it was seen, being ever so littlebehind the sun and immediately setting. But its light extended overa 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 affirmsthat certain stars have been seen when comets dissolve. But on histheory this ought not to occur occasionally but always. Besides, theEgyptians affirm that conjunctions of the planets with one another,and with the fixed stars, take place, and we have ourselves observedJupiter coinciding with one of the stars in the Twins and hiding it,and yet no comet was formed. Further, we can also give a rationalproof of our point. It is true that some stars seem to be biggerthan others, yet each one by itself looks indivisible. Consequently,just as, if they really had been indivisible, their conjunctioncould not have created any greater magnitude, so now that they are notin fact indivisible but look as if they were, their conjunction willnot make them look any bigger.
Enough has been said, without further argument, to show that thecauses brought forward to explain comets are false.
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We consider a satisfactory explanation of phenomena inaccessibleto observation to have been given when our account of them is freefrom impossibilities. The observations before us suggest the followingaccount of the
phenomena we are now considering. We know that thedry and warm exhalation is the outermost part of the terrestrial worldwhich falls below the circular motion. It, and a great part of the airthat is continuous with it below, is carried round the earth by themotion of the circular revolution. In the course of this motion itoften 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 uppermotion introduces into a gathering of this kind a fiery principlenot of such excessive strength as to burn up much of the materialquickly, nor so weak as soon to be extinguished, but stronger andcapable of burning up much material, and when exhalation of theright consistency rises from below and meets it. The kind of cometvaries according to the shape which the exhalation happens to take. Ifit 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 haveseen that when a fiery principle of this kind moves we seem to havea shooting-star: similarly when it stands still we seem to have a starstanding still. We may compare these phenomena to a heap or mass ofchaff into which a torch is thrust, or a spark thrown. That is whata shooting-star is like. The fuel is so inflammable that the fire runsthrough it quickly in a line. Now if this fire were to persist insteadof running through the fuel and perishing away, its course through thefuel would stop at the point where the latter was densest, and thenthe whole might begin to move. Such is a comet-like a shooting-starthat contains its beginning and end in itself.
When the matter begins to gather in the lower region independentlythe comet appears by itself. But when the exhalation is constituted byone of the fixed stars or the planets, owing to their motion, one ofthem 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, andencircle them, when the air is dense enough for them to form alongunder the sun’s course, so too the fringe. It stands in the relationof a halo to the stars, except that the colour of the halo is due toreflection, whereas in the case of comets the colour is something thatappears actually on them.
Now when this matter gathers in relation to a star the cometnecessarily appears to follow the same course as the star. But whenthe comet is formed independently it falls behind the motion of theuniverse, like the rest of the terrestrial world. It is this fact,that a comet often forms independently, indeed oftener than roundone of the regular stars, that makes it impossible to maintain thata comet is a sort of reflection, not indeed, as Hippocrates and hisschool say, to the sun, but to the very star it is alleged toaccompany-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 droughtmust be taken as an indication of their fiery constitution. Fortheir origin is plainly due to the plentiful supply of that secretion.Hence the air is necessarily drier and the moist evaporation is sodissolved and dissipated by the quantity of the hot exhalation asnot readily to condense into water.-But this phenomenon too shall beexplained more clearly later when the time comes to speak of thewinds.-So when there are many comets and they are dense, it is as wesay, and the years are clearly dry and windy. When they are fewerand fainter this effect does not appear in the same degree, thoughas 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-ithad been carried up by
a wind and fell down in the daytime-then tooa comet happened to have appeared in the west. And at the time ofthe great comet the winter was dry and north winds prevailed, andthe wave was due to an opposition of winds. For in the gulf a northwind blew and outside it a violent south wind. Again in the archonshipof Nicomachus a comet appeared for a few days about the equinoctialcircle (this one had not risen in the west), and simultaneously withit there happened the storm at Corinth.
That there are few comets and that they appear rarely and outsidethe tropic circles more than within them is due to the motion of thesun and the stars. For this motion does not only cause the hotprinciple to be secreted but also dissolves it when it is gathering.But the chief reason is that most of this stuff collects in the regionof the milky way.
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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 onthe subject.
(1) Of the so-called Pythagoreans some say that this is the pathof one of the stars that fell from heaven at the time of Phaethon’sdownfall. Others say that the sun used once to move in this circle andthat this region was scorched or met with some other affection of thiskind, because of the sun and its motion.
But it is absurd not to see that if this were the reason thecircle of the Zodiac ought to be affected in the same way, andindeed more so than that of the milky way, since not the sun onlybut 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 nosigns of any such affection except where a part of it touches thecircle of the milky way.
(2) Anaxagoras, Democritus, and their schools say that
the milky wayis the light of certain stars. For, they say, when the sun passesbelow the earth some of the stars are hidden from it. Now the light ofthose on which the sun shines is invisible, being obscured by the ofthe sun. But the milky way is the peculiar light of those starswhich are shaded by the earth from the sun’s rays.
This, too, is obviously impossible. The milky way is alwaysunchanged and among the same constellations (for it is clearly agreatest circle), whereas, since the sun does not remain in the sameplace, what is hidden from it differs at different times. Consequentlywith the change of the sun’s position the milky way ought to changeits position too: but we find that this does not happen. Besides, ifastronomical demonstrations are correct and the size of the sun isgreater than that of the earth and the distance of the stars fromthe earth many times greater than that of the sun (just as the sunis further from the earth than the moon), then the cone made by therays 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 thestars. On the contrary, the sun shines on all the stars and theearth screens none of them.
(3) There is a third theory about the milky way. Some say that it isa reflection of our sight to the sun, just as they say that thecomet is.
But this too is impossible. For if the eye and the mirror and thewhole of the object were severally at rest, then the same part ofthe image would appear at the same point in the mirror. But if themirror and the object move, keeping the same distance from the eyewhich is at rest, but at different rates of speed and so not always atthe same interval from one another, then it is impossible for the sameimage always to appear in the same part of the mirror. Now theconstellations included in the circle of the milky way move; and sodoes the sun, the object to which our sight is reflected; but we standstill. And the distance
of those two from us is constant anduniform, but their distance from one another varies. For the Dolphinsometimes rises at midnight, sometimes in the morning. But in eachcase the same parts of the milky way are found near it. But if it werea reflection and not a genuine affection of these this ought not to bethe case.
Again, we can see the milky way reflected at night in water andsimilar mirrors. But under these circumstances it is impossible forour sight to be reflected to the sun.
These considerations show that the milky way is not the path ofone of the planets, nor the light of imperceptible stars, nor areflection. And those are the chief theories handed down by othershitherto.
Let us recall our fundamental principle and then explain ourviews. We have already laid down that the outermost part of what iscalled the air is potentially fire and that therefore when the airis dissolved by motion, there is separated off a kind of matter-and ofthis matter we assert that comets consist. We must suppose that whathappens is the same as in the case of the comets when the matterdoes not form independently but is formed by one of the fixed stars orthe planets. Then these stars appear to be fringed, because matterof this kind follows their course. In the same way, a certain kindof matter follows the sun, and we explain the halo as a reflectionfrom it when the air is of the right constitution. Now we mustassume that what happens in the case of the stars severally happens inthe case of the whole of the heavens and all the upper motion. Forit is natural to suppose that, if the motion of a single starexcites a flame, that of all the stars should have a similar result,and especially in that region in which the stars are biggest andmost numerous and nearest to one another. Now the circle of the zodiacdissolves this kind of matter because of the motion of the sun and theplanets,
and for this reason most comets are found outside thetropic circles. Again, no fringe appears round the sun or moon: forthey dissolve such matter too quickly to admit of its formation. Butthis circle in which the milky way appears to our sight is thegreatest circle, and its position is such that it extends faroutside the tropic circles. Besides the region is full of thebiggest and brightest constellations and also of what called’scattered’ stars (you have only to look to see this clearly). Sofor these reasons all this matter is continually and ceaselesslycollecting there. A proof of the theory is this: In the circleitself the light is stronger in that half where the milky way isdivided, and in it the constellations are more numerous and closerto one another than in the other half; which shows that the cause ofthe light is the motion of the constellations and nothing else. For ifit is found in the circle in which there are most constellations andat that point in the circle at which they are densest and containthe biggest and the most stars, it is natural to suppose that they arethe true cause of the affection in question. The circle and theconstellations in it may be seen in the diagram. The so-called’scattered’ stars it is not possible to set down in the same way onthe 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 circlealone are the intervals full of these stars: in the other circlesthere are obvious gaps. Hence if we accept the cause assigned forthe appearance of comets as plausible we must assume that the samekind of thing holds good of the milky way. For the fringe which in theformer case is an affection of a single star here forms in the sameway in relation to a whole circle. So if we are to define the milkyway we may call it ’a fringe attaching to the greatest circle, and dueto the matter secreted’. This, as we said before, explains why thereare few comets and why they appear rarely; it is because at eachrevolution of the heavens this matter has always been and is alwaysbeing separated off and gathered into this region.
We have now explained the phenomena that occur in that part of theterrestrial world which is continuous with the motions of the heavens,namely, shooting-stars and the burning flame, comets and the milkyway, these being the chief affections that appear in that region.
9
Let us go on to treat of the region which follows next in orderafter this and which immediately surrounds the earth. It is the regioncommon to water and air, and the processes attending the formationof water above take place in it. We must consider the principles andcauses of all these phenomena too as before. The efficient and chiefand first cause is the circle in which the sun moves. For the sun asit approaches or recedes, obviously causes dissipation andcondensation and so gives rise to generation and destruction. Nowthe earth remains but the moisture surrounding it is made to evaporateby the sun’s rays and the other heat from above, and rises. But whenthe heat which was raising it leaves it, in part dispersing to thehigher region, in part quenched through rising so far into the upperair, then the vapour cools
because its heat is gone and because theplace 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 iscloud. Mist is what is left over when a cloud condenses into water,and is therefore rather a sign of fine weather than of rain; formist might be called a barren cloud. So we get a circular process thatfollows the course of the sun. For according as the sun moves tothis side or that, the moisture in this process rises or falls. Wemust think of it as a river flowing up and down in a circle and madeup partly of air, partly of water. When the sun is near, the stream ofvapour flows upwards; when it recedes, the stream of water flows down:and the order of sequence, at all events, in this process alwaysremains the same. So if ’Oceanus’ had some secret meaning in earlywriters, perhaps they may have meant this river that flows in a circleabout the earth.
So the moisture is always raised by the heat and descends to theearth again when it gets cold. These processes and, in some cases,their varieties are distinguished by special names. When the waterfalls in small drops it is called a drizzle; when the drops are largerit is rain.
10
Some of the vapour that is formed by day does not rise highbecause the ratio of the fire that is raising it to the water thatis being raised is small. When this cools and descends at night itis called dew and hoar-frost. When the vapour is frozen before ithas condensed to water again it is hoar-frost; and this appears inwinter and is commoner in cold places. It is dew when the vapour hascondensed into water and the heat is not so great as to dry up themoisture that has been raised nor the cold sufficient (owing to thewarmth 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 firethat raised it: consequently more cold is needed to freeze it.
Both dew and hoar-frost are found when the sky is clear and there isno 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 toprove that these phenomena occur because the vapour does not risehigh. One reason for this is that it rises from hollow and wateryplaces, so that the heat that is raising it, bearing as it were tooheavy a burden cannot lift it to a great height but soon lets itfall again. A second reason is that the motion of the air is morepronounced at a height, and this dissolves a gathering of this kind.
Everywhere, except in Pontus, dew is found with south winds andnot with north winds. There the opposite is the case and it is foundwith north winds and not with south. The reason is the same as thatwhich explains why dew is found in warm weather and not in cold. Forthe south wind brings warm, and the north, wintry weather. For thenorth wind is cold and so quenches the heat of the evaporation. But inPontus the south wind does not bring warmth enough to causeevaporation, whereas the coldness of the north wind concentrates theheat by a sort of recoil, so that there is more evaporation and notless. This is a thing which we can often observe in other placestoo. Wells, for instance, give off more vapour in a north than in asouth wind. Only the north winds quench the heat before anyconsiderable quantity of vapour has gathered, while in a south windthe 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, namelyrain, snow, hail. Two of these correspond to the phenomena on thelower level and are due to the same causes, differing from them onlyin degree and quantity.
Snow and hoar-frost are one and the same thing, and so are rainand dew: only there is a great deal of the former and little of thelatter. 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 iscollected are considerable. But of dew there is little: for the vapourcollects for it in a single day and from a small area, as its quickformation and scanty quantity show.
The relation of hoar-frost and snow is the same: when cloudfreezes there is snow, when vapour freezes there is hoar-frost.Hence snow is a sign of a cold season or country. For a great dealof heat is still present and unless the cold were overpowering itthe cloud would not freeze. For there still survives in it a greatdeal of the heat which caused the moisture to rise as vapour fromthe earth.
Hail on the other hand is found in the upper region, but thecorresponding phenomenon in the vaporous region near the earth islacking. For, as we said, to snow in the upper region correspondshoar-frost in the lower, and to rain in the upper region, dew in thelower. But there is nothing here to correspond to hail in the upperregion. Why this is so will be clear when we have explained the natureof hail.
12
But we must go on to collect the facts bearing on the origin ofit, both those which raise no difficulties and those which seemparadoxical.
Hail is ice, and water freezes in winter; yet hailstorms occurchiefly in spring and autumn and less often in the late summer, butrarely in winter and then only when the cold is less intense. And ingeneral 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 remainsuspended in the air for any space of time. Nor can we say that thecase is like that of particles of moisture which are carried upowing to their small size and rest on the iar (the water swimming onthe air just as small particles of earth and gold often swim onwater). In that case large drops are formed by the union of manysmall, and so fall down. This cannot take place in the case of hail,since solid bodies cannot coalesce like liquid ones. Clearly thendrops of that size were suspended in the air or else they could nothave been so large when frozen.
Some think that the cause and origin of hail is this. The cloud isthrust up into the upper atmosphere, which is colder because thereflection of the sun’s rays from the earth ceases there, and upon itsarrival there the water freezes. They think that this explains whyhailstorms are commoner in summer and in warm countries; the heat isgreater and it thrusts the clouds further up from the earth. But thefact is that hail does not occur at all at a great height: yet itought to do so, on their theory, just as we see that snow falls moston high mountains. Again clouds have often been observed moving with agreat noise close to the earth, terrifying those who heard and sawthem as portents of some catastrophe. Sometimes, too, when such cloudshave been seen, without any noise, there follows a violenthailstorm, and the stones are of incredible size, and angular inshape. This shows that they have not been falling for long and thatthey were frozen near to the earth, and not as that theory wouldhave it. Moreover, where the hailstones are large, the cause oftheir freezing must be present in the highest degree: for hail isice as every one can see. Now those hailstones are large which areangular 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 falland become round and smaller in size.
It clearly follows that the congelation does not take
placebecause 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 afrost they are warm. The same thing, we must suppose, happens in theair, so that in the warmer seasons the cold is concentrated by thesurrounding heat and causes the cloud to go over into watersuddenly. (For this reason rain-drops are much larger on warm daysthan in winter, and showers more violent. A shower is said to bemore violent in proportion as the water comes down in a body, and thishappens when the condensation takes place quickly,-though this is justthe opposite of what Anaxagoras says. He says that this happens whenthe cloud has risen into the cold air; whereas we say that ithappens when the cloud has descended into the warm air, and that themore the further the cloud has descended). But when the cold hasbeen concentrated within still more by the outer heat, it freezesthe water it has formed and there is hail. We get hail when theprocess of freezing is quicker than the descent of the water. For ifthe water falls in a certain time and the cold is sufficient to freezeit in less, there is no difficulty about its having frozen in the air,provided that the freezing takes place in a shorter time than itsfall. The nearer to the earth, and the more suddenly, this processtakes place, the more violent is the rain that results and thelarger the raindrops and the hailstones because of the shortness oftheir fall. For the same reason large raindrops do not fall thickly.Hail is rarer in summer than in spring and autumn, though commonerthan in winter, because the air is drier in summer, whereas inspring it is still moist, and in autumn it is beginning to grow moist.It is for the same reason that hailstorms sometimes occur in thelate summer as we have said.
The fact that the water has previously been warmed
contributes toits freezing quickly: for so it cools sooner. Hence many people,when they want to cool hot water quickly, begin by putting it in thesun. So the inhabitants of Pontus when they encamp on the ice tofish (they cut a hole in the ice and then fish) pour warm waterround their reeds that it may freeze the quicker, for they use the icelike lead to fix the reeds. Now it is in hot countries and seasonsthat the water which forms soon grows warm.
It is for the same reason that rain falls in summer and not inwinter in Arabia and Ethiopia too, and that in torrents and repeatedlyon the same day. For the concentration or recoil due to the extremeheat 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.
13
Let us explain the nature of winds, and all windy vapours, also ofrivers and of the sea. But here, too, we must first discuss thedifficulties involved: for, as in other matters, so in this notheory has been handed down to us that the most ordinary man could nothave thought of.
Some say that what is called air, when it is in motion and flows, iswind, and that this same air when it condenses again becomes cloud andwater, implying that the nature of wind and water is the same. So theydefine wind as a motion of the air. Hence some, wishing to say aclever thing, assert that all the winds are one wind, because theair that moves is in fact all of it one and the same; they maintainthat the winds appear to differ owing to the region from which the airmay happen to flow on each occasion, but really do not differ atall. This is just like thinking that all rivers are one and the sameriver, and the ordinary unscientific view is better than ascientific theory like this. If all rivers flow from one source, andthe same is true in the case of the winds, there might be some truthin this theory; but if it is no more true in the one case than inthe other, this ingenious idea is plainly
false. What requiresinvestigation is this: the nature of wind and how it originates, itsefficient cause and whence they derive their source; whether one oughtto think of the wind as issuing from a sort of vessel and flowinguntil the vessel is empty, as if let out of a wineskin, or, aspainters represent the winds, as drawing their source from themselves.
We find analogous views about the origin of rivers. It is thoughtthat the water is raised by the sun and descends in rain and gathersbelow the earth and so flows from a great reservoir, all the riversfrom one, or each from a different one. No water at all isgenerated, but the volume of the rivers consists of the water thatis gathered into such reservoirs in winter. Hence rivers are alwaysfuller in winter than in summer, and some are perennial, others not.Rivers are perennial where the reservoir is large and so enoughwater has collected in it to last out and not be used up before thewinter rain returns. Where the reservoirs are smaller there is lesswater in the rivers, and they are dried up and their vessel emptybefore the fresh rain comes on.
But if any one will picture to himself a reservoir adequate to thewater that is continuously flowing day by day, and consider the amountof the water, it is obvious that a receptacle that is to contain allthe water that flows in the year would be larger than the earth, or,at any rate, not much smaller.
Though it is evident that many reservoirs of this kind do exist inmany parts of the earth, yet it is unreasonable for any one torefuse to admit that air becomes water in the earth for the samereason as it does above it. If the cold causes the vaporous air tocondense into water above the earth we must suppose the cold in theearth to produce this same effect, and recognize that there not onlyexists in it and flows out of it actually formed water, but that wateris continually forming in it too.
Again, even in the case of the water that is not being formed fromday to day but exists as such, we must not suppose as some do thatrivers have their source in definite subterranean lakes. On thecontrary, just as above the earth small drops form and these joinothers, till finally the water descends in a body as rain, so too wemust suppose that in the earth the water at first trickles togetherlittle by little, and that the sources of the rivers drip, as it were,out of the earth and then unite. This is proved by facts. When menconstruct an aqueduct they collect the water in pipes and trenches, asif the earth in the higher ground were sweating the water out.Hence, too, the head-waters of rivers are found to flow frommountains, and from the greatest mountains there flow the mostnumerous and greatest rivers. Again, most springs are in theneighbourhood of mountains and of high ground, whereas if we exceptrivers, water rarely appears in the plains. For mountains and highground, suspended over the country like a saturated sponge, make thewater ooze out and trickle together in minute quantities but in manyplaces. They receive a great deal of water falling as rain (for itmakes no difference whether a spongy receptacle is concave andturned up or convex and turned down: in either case it will containthe same volume of matter) and, they also cool the vapour that risesand condense it back into water.
Hence, as we said, we find that the greatest rivers flow from thegreatest mountains. This can be seen by looking at itineraries: whatis recorded in them consists either of things which the writer hasseen himself or of such as he has compiled after inquiry from thosewho have seen them.
In Asia we find that the most numerous and greatest rivers flow fromthe mountain called Parnassus, admittedly the greatest of allmountains towards the south-east. When you have crossed it you see theouter ocean, the further limit of which is unknown to the dwellersin our world.
Besides other rivers there flow from it the Bactrus, theChoaspes, the Araxes: from the last a branch separates off and flowsinto lake Maeotis as the Tanais. From it, too, flows the Indus, thevolume of whose stream is greatest of all rivers. From the Caucasusflows the Phasis, and very many other great rivers besides. Now theCaucasus is the greatest of the mountains that lie to the northeast,both as regards its extent and its height. A proof of its height isthe fact that it can be seen from the so-called ’deeps’ and from theentrance to the lake. Again, the sun shines on its peaks for a thirdpart of the night before sunrise and again after sunset. Its extent isproved by the fact that thought contains many inhabitable regionswhich are occupied by many nations and in which there are said to begreat lakes, yet they say that all these regions are visible up to thelast peak. From Pyrene (this is a mountain towards the west inCeltice) there flow the Istrus and the Tartessus. The latter flowsoutside the pillars, while the Istrus flows through all Europe intothe Euxine. Most of the remaining rivers flow northwards from theHercynian mountains, which are the greatest in height and extent aboutthat region. In the extreme north, beyond furthest Scythia, are themountains called Rhipae. The stories about their size are altogethertoo fabulous: however, they say that the most and (after the Istrus)the greatest rivers flow from them. So, too, in Libya there flowfrom the Aethiopian mountains the Aegon and the Nyses; and from theso-called Silver Mountain the two greatest of named rivers, theriver called Chremetes that flows into
the outer ocean, and the mainsource of the Nile. Of the rivers in the Greek world, the Achelousflows from Pindus, the Inachus from the same mountain; the Strymon,the Nestus, and the Hebrus all three from Scombrus; many rivers,too, flow from Rhodope.
All other rivers would be found to flow in the same way, but we havementioned these as examples. Even where rivers flow from marshes,the marshes in almost every case are found to lie below mountains orgradually rising ground.
It is clear then that we must not suppose rivers to originate fromdefinite reservoirs: for the whole earth, we might almost say, wouldnot be sufficient (any more than the region of the clouds would be) ifwe were to suppose that they were fed by actually existing wateronly and it were not the case that as some water passed out ofexistence some more came into existence, but rivers always drewtheir stream from an existing store. Secondly, the fact that riversrise at the foot of mountains proves that a place transmits thewater it contains by gradual percolation of many drops, little bylittle, and that this is how the sources of rivers originate. However,there is nothing impossible about the existence of such placescontaining a quantity of water like lakes: only they cannot be bigenough to produce the supposed effect. To think that they are isjust as absurd as if one were to suppose that rivers drew all theirwater from the sources we see (for most rivers do flow fromsprings). So it is no more reasonable to suppose those lakes tocontain the whole volume of water than these springs.
That there exist such chasms and cavities in the earth we are taughtby the rivers that are swallowed up. They are found in many parts ofthe earth: in the Peloponnesus, for instance, there are many suchrivers in Arcadia. The reason is that Arcadia is mountainous and thereare no channels from its valleys to the sea. So these places getfull of water, and this, having no outlet, under the pressure of thewater that is added above, finds a way out for itself
underground.In Greece this kind of thing happens on quite a small scale, but thelake at the foot of the Caucasus, which the inhabitants of these partscall a sea, is considerable. Many great rivers fall into it and it hasno visible outlet but issues below the earth off the land of theCoraxi about the so-called ’deeps of Pontus’. This is a place ofunfathomable depth in the sea: at any rate no one has yet been able tofind bottom there by sounding. At this spot, about three hundredstadia from land, there comes up sweet water over a large area, notall of it together but in three places. And in Liguria a river equalin size to the Rhodanus is swallowed up and appears again elsewhere:the Rhodanus being a navigable river.
14
The same parts of the earth are not always moist or dry, but theychange according as rivers come into existence and dry up. And sothe relation of land to sea changes too and a place does not alwaysremain land or sea throughout all time, but where there was dry landthere comes to be sea, and where there is now sea, there one day comesto be dry land. But we must suppose these changes to follow some orderand cycle. The principle and cause of these changes is that theinterior of the earth grows and decays, like the bodies of plantsand animals. Only in the case of these latter the process does notgo on by parts, but each of them necessarily grows or decays as awhole, whereas it does go on by parts in the case of the earth. Herethe causes are cold and heat, which increase and diminish on accountof the sun and its course. It is owing to them that the parts of theearth come to have a different character, that some parts remain moistfor a certain time, and then dry up and grow old, while other parts intheir turn are filled with life
and moisture. Now when places becomedrier the springs necessarily give out, and when this happens therivers first decrease in size and then finally become dry; and whenrivers change and disappear in one part and come into existencecorrespondingly in another, the sea must needs be affected.
If the sea was once pushed out by rivers and encroached upon theland anywhere, it necessarily leaves that place dry when it recedes;again, if the dry land has encroached on the sea at all by a processof silting set up by the rivers when at their full, the time must comewhen this place will be flooded again.
But the whole vital process of the earth takes place so graduallyand in periods of time which are so immense compared with the lengthof our life, that these changes are not observed, and before theircourse can be recorded from beginning to end whole nations perishand are destroyed. Of such destructions the most utter and suddenare due to wars; but pestilence or famine cause them too. Famines,again, are either sudden and severe or else gradual. In the lattercase the disappearance of a nation is not noticed because some leavethe country while others remain; and this goes on until the land isunable to maintain any inhabitants at all. So a long period of time islikely to elapse from the first departure to the last, and no oneremembers and the lapse of time destroys all record even before thelast inhabitants have disappeared. In the same
way a nation must besupposed to lose account of the time when it first settled in a landthat was changing from a marshy and watery state and becoming dry.Here, too, the change is gradual and lasts a long time and men donot remember who came first, or when, or what the land was like whenthey came. This has been the case with Egypt. Here it is obviousthat the land is continually getting drier and that the wholecountry is a deposit of the river Nile. But because the neighbouringpeoples settled in the land gradually as the marshes dried, thelapse of time has hidden the beginning of the process. However, allthe mouths of the Nile, with the single exception of that atCanopus, are obviously artificial and not natural. And Egypt wasnothing more than what is called Thebes, as Homer, too, shows,modern though he is in relation to such changes. For Thebes is theplace that he mentions; which implies that Memphis did not yetexist, or at any rate was not as important as it is now. That thisshould be so is natural, since the lower land came to be inhabitedlater than that which lay higher. For the parts that lie nearer to theplace where the river is depositing the silt are necessarily marshyfor a longer time since the water always lies most in the newly formedland. But in time this land changes its character, and in its turnenjoys a period of prosperity. For these places dry up and come tobe in good condition while the places that were formerly well-temperedsome day grow excessively dry and deteriorate. This happened to theland of Argos and Mycenae in Greece. In the time of the Trojan warsthe Argive land was marshy and could only support a smallpopulation, whereas the land of Mycenae was in good condition (and forthis reason Mycenae was the superior). But now the opposite is thecase, for the reason we have mentioned: the land of Mycenae has becomecompletely dry and barren, while the Argive land that was formerlybarren owing to the water has now become fruitful. Now the sameprocess that has taken
place in this small district must be supposedto be going on over whole countries and on a large scale.
Men whose outlook is narrow suppose the cause of such events to bechange in the universe, in the sense of a coming to be of the world asa whole. Hence they say that the sea being dried up and is growingless, because this is observed to have happened in more places nowthan formerly. But this is only partially true. It is true that manyplaces are now dry, that formerly were covered with water. But theopposite is true too: for if they look they will find that there aremany places where the sea has invaded the land. But we must notsuppose that the cause of this is that the world is in process ofbecoming. For it is absurd to make the universe to be in processbecause of small and trifling changes, when the bulk and size of theearth are surely as nothing in comparison with the whole world. Ratherwe must take the cause of all these changes to be that, just as winteroccurs in the seasons of the year, so in determined periods therecomes a great winter of a great year and with it excess of rain. Butthis excess does not always occur in the same place. The deluge in thetime of Deucalion, for instance, took place chiefly in the Greek worldand in it especially about ancient Hellas, the country about Dodonaand the Achelous, a river which has often changed its course. Here theSelli dwelt and those who were formerly called Graeci and nowHellenes. When, therefore, such an excess of rain occurs we mustsuppose that it suffices for a long time. We have seen that some saythat the size of the subterranean cavities is what makes some riversperennial and others not, whereas we maintain that the size of themountains is the cause, and their density and coldness; for great,dense, and cold mountains catch and keep and create most water:whereas if the mountains that overhang the sources of rivers are smallor porous and stony and clayey, these rivers run dry earlier. Wemust recognize the same kind of thing in this case too. Where suchabundance of rain falls in the great winter
it tends to make themoisture of those places almost everlasting. But as time goes onplaces of the latter type dry up more, while those of the former,moist type, do so less: until at last the beginning of the samecycle returns.
Since there is necessarily some change in the whole world, but notin the way of coming into existence or perishing (for the universeis permanent), it must be, as we say, that the same places are not forever moist through the presence of sea and rivers, nor for ever dry.And the facts prove this. The whole land of the Egyptians, whom wetake to be the most ancient of men, has evidently gradually comeinto existence and been produced by the river. This is clear from anobservation of the country, and the facts about the Red Sea suffice toprove it too. One of their kings tried to make a canal to it (for itwould have been of no little advantage to them for the whole region tohave become navigable; Sesostris is said to have been the first of theancient kings to try), but he found that the sea was higher than theland. So he first, and Darius afterwards, stopped making the canal,lest the sea should mix with the river water and spoil it. So it isclear that all this part was once unbroken sea. For the same reasonLibya-the country of Ammon-is, strangely enough, lower and hollowerthan the land to the seaward of it. For it is clear that a barrierof silt was formed and after it lakes and dry land, but in course oftime the water that was left behind in the lakes dried up and is nowall gone. Again the silting up of the lake Maeotis by the rivers hasadvanced so much that the limit to the size of the ships which can nowsail into it to trade is much lower than it was sixty years ago. Henceit is easy to infer that
it, too, like most lakes, was originallyproduced by the rivers and that it must end by drying up entirely.
Again, this process of silting up causes a continuous currentthrough the Bosporus; and in this case we can directly observe thenature of the process. Whenever the current from the Asiatic shorethrew up a sandbank, there first formed a small lake behind it.Later it dried up and a second sandbank formed in front of the firstand a second lake. This process went on uniformly and withoutinterruption. Now when this has been repeated often enough, in thecourse of time the strait must become like a river, and in the end theriver itself must dry up.
So it is clear, since there will be no end to time and the worldis eternal, that neither the Tanais nor the Nile has always beenflowing, but that the region whence they flow was once dry: fortheir effect may be fulfilled, but time cannot. And this will beequally true of all other rivers. But if rivers come into existenceand perish and the same parts of the earth were not always moist,the sea must needs change correspondingly. And if the sea is alwaysadvancing in one place and receding in another it is clear that thesame parts of the whole earth are not always either sea or land, butthat all this changes in course of time.
So we have explained that the same parts of the earth are not alwaysland or sea and why that is so: and also why some rivers are perennialand others not.
1
LET us explain the nature of the sea and the reason why such a largemass of water is salt and the way in which it originally came to be.
The old writers who invented theogonies say that the sea hassprings, for they want earth and sea to have foundations and rootsof their own. Presumably they
thought that this view was grander andmore impressive as implying that our earth was an important part ofthe universe. For they believed that the whole world had been built upround our earth and for its sake, and that the earth was the mostimportant and primary part of it. Others, wiser in human knowledge,give an account of its origin. At first, they say, the earth wassurrounded by moisture. Then the sun began to dry it up, part of itevaporated and is the cause of winds and the turnings back of thesun and the moon, while the remainder forms the sea. So the sea isbeing dried up and is growing less, and will end by being some dayentirely dried up. Others say that the sea is a kind of sweat exudedby the earth when the sun heats it, and that this explains itssaltness: for all sweat is salt. Others say that the saltness is dueto the earth. Just as water strained through ashes becomes salt, sothe sea owes its saltness to the admixture of earth with similarproperties.
We must now consider the facts which prove that the sea cannotpossibly have springs. The waters we find on the earth either flowor are stationary. All flowing water has springs. (By a spring, aswe have explained above, we must not understand a source from whichwaters are ladled as it were from a vessel, but a first point at whichthe water which is continually forming and percolating gathers.)Stationary water is either that which has
collected and has beenleft standing, marshy pools, for instance, and lakes, which differmerely in size, or else it comes from springs. In this case it isalways artificial, I mean as in the case of wells, otherwise thespring would have to be above the outlet. Hence the water fromfountains and rivers flows of itself, whereas wells need to beworked artificially. All the waters that exist belong to one orother of these classes.
On the basis of this division we can sec that the sea cannot havesprings. For it falls under neither of the two classes; it does notflow and it is not artificial; whereas all water from springs mustbelong to one or other of them. Natural standing water from springs isnever found on such a large scale.
Again, there are several seas that have no communication with oneanother at all. The Red Sea, for instance, communicates but slightlywith the ocean outside the straits, and the Hyrcanian and Caspian seasare distinct from this ocean and people dwell all round them. Hence,if these seas had had any springs anywhere they must have beendiscovered.
It is true that in straits, where the land on either sidecontracts an open sea into a small space, the sea appears to flow. Butthis is because it is swinging to and fro. In the open sea this motionis not observed, but where the land narrows and contracts the seathe motion that was imperceptible in the open necessarily strikesthe attention.
The whole of the Mediterranean does actually flow. The directionof this flow is determined by the depth of the basins and by thenumber of rivers. Maeotis flows
into Pontus and Pontus into theAegean. After that the flow of the remaining seas is not so easy toobserve. The current of Maeotis and Pontus is due to the number ofrivers (more rivers flow into the Euxine and Maeotis than into thewhole Mediterranean with its much larger basin), and to their ownshallowness. For we find the sea getting deeper and deeper. Pontusis deeper than Maeotis, the Aegean than Pontus, the Sicilian seathan the Aegean; the Sardinian and Tyrrhenic being the deepest of all.(Outside the pillars of Heracles the sea is shallow owing to themud, but calm, for it lies in a hollow.) We see, then, that just assingle rivers flow from mountains, so it is with the earth as a whole:the greatest volume of water flows from the higher regions in thenorth. Their alluvium makes the northern seas shallow, while the outerseas are deeper. Some further evidence of the height of the northernregions of the earth is afforded by the view of many of the ancientmeteorologists. They believed that the sun did not pass below theearth, but round its northern part, and that it was the height of thiswhich obscured the sun and caused night.
So much to prove that there cannot be sources of the sea and toexplain its observed flow.
2
We must now discuss the origin of the sea, if it has an origin,and the cause of its salt and bitter taste.
What made earlier writers consider the sea to be the original andmain body of water is this. It seems reasonable to suppose that tobe the case on the analogy of the other elements. Each of them has amain bulk which by reason of its mass is the origin of that element,and any parts which change and mix with the other elements come fromit. Thus the main body of fire is in
the upper region; that of airoccupies the place next inside the region of fire; while the mass ofthe earth is that round which the rest of the elements are seen tolie. So we must clearly look for something analogous in the case ofwater. But here we can find no such single mass, as in the case of theother elements, except the sea. River water is not a unity, nor isit stable, but is seen to be in a continuous process of becomingfrom day to day. It was this difficulty which made people regard thesea as the origin and source of moisture and of all water. And so wefind it maintained that rivers not only flow into the sea butoriginate from it, the salt water becoming sweet by filtration.
But this view involves another difficulty. If this body of wateris the origin and source of all water, why is it salt and not sweet?The reason for this, besides answering this question, will ensureour having a right first conception of the nature of the sea.
The earth is surrounded by water, just as that is by the sphere ofair, and that again by the sphere called that of fire (which is theoutermost both on the common view and on ours). Now the sun, moving asit does, sets up processes of change and becoming and decay, and byits agency the finest and sweetest water is every day carried up andis dissolved into vapour and rises to the upper region, where it iscondensed again by the cold and so returns to the earth. This, as wehave said before, is the regular course of nature.
Hence all my predecessors who supposed that the sun was nourished bymoisture are absurdly mistaken. Some go on to say that the solsticesare due to this, the reason being that the same places cannot alwayssupply the sun with nourishment and that without it he must perish.For the fire we are familiar with lives as long as it is fed, and
the only food for fire is moisture. As if the moisture that israised could reach the sun! or this ascent were really like thatperformed by flame as it comes into being, and to which theysupposed the case of the sun to be analogous! Really there is nosimilarity. A flame is a process of becoming, involving a constantinterchange of moist and dry. It cannot be said to be nourishedsince it scarcely persists as one and the same for a moment. Thiscannot be true of the sun; for if it were nourished like that, as theysay it is, we should obviously not only have a new sun every day, asHeraclitus says, but a new sun every moment. Again, when the suncauses the moisture to rise, this is like fire heating water. So, asthe fire is not fed by the water above it, it is absurd to supposethat the sun feeds on that moisture, even if its heat made all thewater in the world evaporate. Again, it is absurd, considering thenumber and size of the stars, that these thinkers should considerthe sun only and overlook the question how the rest of the heavenlybodies subsist. Again, they are met by the same difficulty as thosewho say that at first the earth itself was moist and the world roundthe earth was warmed by the sun, and so air was generated and thewhole firmament grew, and the air caused winds and solstices. Theobjection is that we always plainly see the water that has beencarried up coming down again. Even if the same amount does not comeback in a year or in a given country, yet in a certain period all thathas been carried up is returned. This implies that the celestialbodies do not feed on it, and that we cannot distinguish betweensome air which preserves its character once it is generated and someother which is generated but becomes
water again and so perishes; onthe contrary, all the moisture alike is dissolved and all of itcondensed back into water.
The drinkable, sweet water, then, is light and is all of it drawnup: the salt water is heavy and remains behind, but not in its naturalplace. For this is a question which has been sufficiently discussed (Imean about the natural place that water, like the other elements, mustin reason have), and the answer is this. The place which we see thesea filling is not its natural place but that of water. It seems tobelong to the sea because the weight of the salt water makes it remainthere, while the sweet, drinkable water which is light is carriedup. The same thing happens in animal bodies. Here, too, the foodwhen it enters the body is sweet, yet the residuum and dregs of liquidfood are found to be bitter and salt. This is because the sweet anddrinkable part of it has been drawn away by the natural animal heatand has passed into the flesh and the other parts of the bodyaccording to their several natures. Now just as here it would be wrongfor any one to refuse to call the belly the place of liquid foodbecause that disappears from it soon, and to call it the place ofthe residuum because this is seen to remain, so in the case of ourpresent subject. This place, we say, is the place of water. Henceall rivers and all the water that is generated flow into it: for waterflows into the deepest place, and the deepest part of the earth isfilled by the sea. Only all the light and sweet part of it isquickly carried off by the sun, while herest remains for the reason wehave explained. It is quite natural that some people should havebeen puzzled by the old question why such a mass of water leaves notrace anywhere (for the sea does not increase though innumerable andvast rivers are flowing into it every day.) But if one considers thematter the solution is easy. The same amount of water does not take aslong to dry up when it is spread out as when it is gathered in a body,and indeed the difference is so great that in the one case it mightpersist the whole day long while in the other it might all disappearin a moment-as for instance if one were to
spread out a cup of waterover a large table. This is the case with the rivers: all the timethey are flowing their water forms a compact mass, but when it arrivesat a vast wide place it quickly and imperceptibly evaporates.
But the theory of the Phaedo about rivers and the sea is impossible.There it is said that the earth is pierced by intercommunicatingchannels and that the original head and source of all waters is whatis called Tartarus-a mass of water about the centre, from which allwaters, flowing and standing, are derived. This primary and originalwater is always surging to and fro, and so it causes the rivers toflow on this side of the earth’s centre and on that; for it has nofixed seat but is always oscillating about the centre. Its motion upand down is what fills rivers. Many of these form lakes in variousplaces (our sea is an instance of one of these), but all of themcome round again in a circle to the original source of their flow,many at the same point, but some at a point opposite to that fromwhich they issued; for instance, if they started from the other sideof the earth’s centre, they might return from this side of it. Theydescend only as far as the centre, for after that all motion isupwards. Water gets its tastes and colours from the kind of earththe rivers happened to flow through.
But on this theory rivers do not always flow in the same sense.For since they flow to the centre from which they issue forth theywill not be flowing down any more than up, but in whatever directionthe surging of Tartarus inclines to. But at this rate we shall get theproverbial rivers flowing upwards, which is impossible. Again, whereis the water that is generated and what goes up again as vapour tocome from? For this must all of it simply be ignored, since thequantity of water is always the same and all the water that flowsout from the original source flows back to it again. This itself isnot true, since all rivers are seen to end in the sea except where oneflows into another. Not one of them ends in the earth, but even whenone is swallowed up it comes to the surface again. And those
riversare large which flow for a long distance through a lowying country,for by their situation and length they cut off the course of manyothers and swallow them up. This is why the Istrus and the Nile arethe greatest of the rivers which flow into our sea. Indeed, so manyrivers fall into them that there is disagreement as to the sourcesof them both. All of which is plainly impossible on the theory, andthe more so as it derives the sea from Tartarus.
Enough has been said to prove that this is the natural place ofwater and not of the sea, and to explain why sweet water is only foundin rivers, while salt water is stationary, and to show that the sea isthe end rather than the source of water, analogous to the residualmatter of all food, and especially liquid food, in animal bodies.
3
We must now explain why the sea is salt, and ask whether iteternally exists as identically the same body, or whether it did notexist at all once and some day will exist no longer, but will dry upas some people think.
Every one admits this, that if the whole world originated the seadid too; for they make them come into being at the same time. Itfollows that if the universe is eternal the same must be true of thesea. Any one who thinks like Democritus that the sea is diminishingand will disappear in the end reminds us of Aesop’s tales. His storywas that Charybdis had twice sucked in the sea: the first time shemade the mountains visible; the second time the islands; and whenshe sucks it in for the last time she will dry it up entirely. Sucha tale is appropriate enough to Aesop in a rage with the ferryman, butnot to serious inquirers. Whatever made the sea remain at first,whether it was its weight, as some even of those who hold theseviews say (for it is easy to see the cause here), or some otherreason-clearly the same thing must make it persist for ever. They musteither deny that the water raised by the sun will return at all, or,if it does, they
must admit that the sea persists for ever or aslong as this process goes on, and again, that for the same period oftime that sweet water must have been carried up beforehand. So the seawill never dry up: for before that can happen the water that hasgone up beforehand will return to it: for if you say that this happensonce you must admit its recurrence. If you stop the sun’s course thereis no drying agency. If you let it go on it will draw up the sweetwater as we have said whenever it approaches, and let it descend againwhen it recedes. This notion about the sea is derived from the factthat many places are found to be drier now than they once were. Whythis is so we have explained. The phenomenon is due to temporaryexcess of rain and not to any process of becoming in which theuniverse or its parts are involved. Some day the opposite will takeplace and after that the earth will grow dry once again. We mustrecognize that this process always goes on thus in a cycle, for thatis more satisfactory than to suppose a change in the whole world inorder to explain these facts. But we have dwelt longer on this pointthan it deserves.
To return to the saltness of the sea: those who create the seaonce for all, or indeed generate it at all, cannot account for itssaltness. It makes no difference whether the sea is the residue of allthe moisture that is about the earth and has been drawn up by the sun,or whether all the flavour existing in the whole mass of sweet wateris due to the admixture of a certain kind of earth. Since the totalvolume of the sea is the same once the water that evaporated hasreturned, it follows that it must either have been salt at firsttoo, or, if not at first, then not now either. If it was salt from thevery beginning, then we want to know why that was so; and why, if saltwater was drawn up then, that is not the case now.
Again, if it is maintained that an admixture of earth makes thesea salt (for they say that earth has many flavours and is washed downby the rivers and so makes the sea salt by its admixture), it isstrange that rivers
should not be salt too. How can the admixture ofthis earth have such a striking effect in a great quantity of waterand not in each river singly? For the sea, differing in nothing fromrivers but in being salt, is evidently simply the totality of riverwater, and the rivers are the vehicle in which that earth is carriedto their common destination.
It is equally absurd to suppose that anything has been explainedby calling the sea ’the sweat of the earth’, like Empedicles.Metaphors are poetical and so that expression of his may satisfy therequirements of a poem, but as a scientific theory it isunsatisfactory. Even in the case of the body it is a question howthe sweet liquid drunk becomes salt sweat whether it is merely bythe departure of some element in it which is sweetest, or by theadmixture of something, as when water is strained through ashes.Actually the saltness seems to be due to the same cause as in the caseof the residual liquid that gathers in the bladder. That, too, becomesbitter and salt though the liquid we drink and that contained in ourfood is sweet. If then the bitterness is due in these cases (as withthe water strained through lye) to the presence of a certain sort ofstuff that is carried along by the urine (as indeed we actually find asalt deposit settling in chamber-pots) and is secreted from theflesh in sweat (as if the departing moisture were washing the stuffout of the body), then no doubt the admixture of something earthy withthe water is what makes the sea salt.
Now in the body stuff of this kind, viz. the sediment of food, isdue to failure to digest: but how there came to be any such thing inthe earth requires explanation. Besides, how can the drying andwarming of the earth cause the secretion such a great quantity ofwater; especially as that must be a mere fragment of what is left inthe earth? Again, waiving the question of quantity, why does not theearth
sweat now when it happens to be in process of drying? If itdid so then, it ought to do so now. But it does not: on thecontrary, when it is dry it graws moist, but when it is moist itdoes not secrete anything at all. How then was it possible for theearth at the beginning when it was moist to sweat as it grew dry?Indeed, the theory that maintains that most of the moisture departedand was drawn up by the sun and that what was left over is the seais more reasonable; but for the earth to sweat when it is moist isimpossible.
Since all the attempts to account for the saltness of the sea seemunsuccessful let us explain it by the help of the principle we haveused already.
Since we recognize two kinds of evaporation, one moist, the otherdry, it is clear that the latter must be recognized as the source ofphenomena like those we are concerned with.
But there is a question which we must discuss first. Does the seaalways remain numerically one and consisting of the same parts, oris it, too, one in form and volume while its parts are in continualchange, like air and sweet water and fire? All of these are in aconstant state of change, but the form and the quantity of each ofthem are fixed, just as they are in the case of a flowing river or aburning flame. The answer is clear, and there is no doubt that thesame account holds good of all these things alike. They differ in thatsome of them change more rapidly or more slowly than others; andthey all are involved in a process of perishing and becoming which yetaffects them all in a regular course.
This being so we must go on to try to explain why the sea is salt.There are many facts which make it clear that this taste is due to theadmixture of something. First, in animal bodies what is leastdigested, the residue of liquid food, is salt and bitter, as we saidbefore. All animal excreta are undigested, but especially that whichgathers in the bladder (its extreme lightness proves this; foreverything that is digested is condensed), and also sweat; in thesethen is excreted (along with other matter) an identical substance towhich this flavour is due. The case of things burnt is analogous. Whatheat fails to assimilate becomes the excrementary residue in animalbodies, and, in things burnt, ashes. That is why some people saythat it was burnt earth that made the sea salt. To say that it wasburnt earth is absurd; but to say that it was something like burntearth is true. We must suppose that just as in the cases we havedescribed, so in the world as a whole, everything that grows and isnaturally generated always leaves an undigested residue, like thatof things burnt, consisting of this sort of earth. All the earthystuff in the dry exhalation is of this nature, and it is the dryexhalation which accounts for its great quantity. Now since, as wehave said, the moist and the dry evaporations are mixed, some quantityof this stuff must always be included in the clouds and the water thatare formed by condensation, and must redescend to the earth in rain.This process must always go on with such regularity as the sublunaryworld admits of. and it is the answer to the question how the seacomes to be salt.
It also explains why rain that comes from the south, and the firstrains of autumn, are brackish. The south is the warmest of winds andit blows from dry and hot regions. Hence it carries little moistvapour and that is why it is hot. (It makes no difference even if thisis not
its true character and it is originally a cold wind, for itbecomes warm on its way by incorporating with itself a greatquantity of dry evaporation from the places it passes over.) The northwind, on the other hand, comb ing from moist regions, is full ofvapour and therefore cold. It is dry in our part of the worldbecause it drives the clouds away before it, but in the south it israiny; just as the south is a dry wind in Libya. So the south windcharges the rain that falls with a great quantity of this stuff.Autumn rain is brackish because the heaviest water must fall first; sothat that which contains the greatest quantity of this kind of earthdescends quickest.
This, too, is why the sea is warm. Everything that has beenexposed to fire contains heat potentially, as we see in the case oflye and ashes and the dry and liquid excreta of animals. Indeedthose animals which are hottest in the belly have the hottest excreta.
The action of this cause is continually making the sea more salt,but some part of its saltness is always being drawn up with thesweet water. This is less than the sweet water in the same ratio inwhich the salt and brackish element in rain is less than the sweet,and so the saltness of the sea remains constant on the whole. Saltwater when it turns into vapour becomes sweet, and the vapour does notform salt water when it condenses again. This I know by experiment.The same thing is true in every case of the kind: wine and allfluids that evaporate and condense back into a liquid state becomewater. They all are water modified by a certain admixture, thenature of which determines their flavour. But this subject must beconsidered on another more suitable occasion.
For the present let us say this. The sea is there and some of itis continually being drawn up and becoming sweet; this returns fromabove with the rain. But it is now different from what it was whenit was drawn up, and its weight makes it sink below the sweet water.This process
prevents the sea, as it does rivers, from drying upexcept from local causes (this must happen to sea and rivers alike).On the other hand the parts neither of the earth nor of the sea remainconstant but only their whole bulk. For the same thing is true ofthe earth as of the sea: some of it is carried up and some comesdown with the rain, and both that which remains on the surface andthat which comes down again change their situations.
There is more evidence to prove that saltness is due to theadmixture of some substance, besides that which we have adduced.Make a vessel of wax and put it in the sea, fastening its mouth insuch a way as to prevent any water getting in. Then the water thatpercolates through the wax sides of the vessel is sweet, the earthystuff, the admixture of which makes the water salt, being separatedoff as it were by a filter. It is this stuff which make salt waterheavy (it weighs more than fresh water) and thick. The difference inconsistency is such that ships with the same cargo very nearly sink ina river when they are quite fit to navigate in the sea. Thiscircumstance has before now caused loss to shippers freighting theirships in a river. That the thicker consistency is due to anadmixture of something is proved by the fact that if you make strongbrine by the admixture of salt, eggs, even when they are full, floatin it. It almost becomes like mud; such a quantity of earthy matter isthere in the sea. The same thing is done in salting fish.
Again if, as is fabled, there is a lake in Palestine, such that ifyou bind a man or beast and throw it in it floats and does not sink,this would bear out what we have said. They say that this lake is sobitter and salt that no fish live in it and that if you soak clothesin it and shake them it cleans them. The following facts all of themsupport our theory that it is some earthy stuff in the water whichmakes it salt. In Chaonia there is a spring of brackish water thatflows into a neighbouring river which is sweet but contains no fish.The local story is that when Heracles came from Erytheia driving
theoxen and gave the inhabitants the choice, they chose salt inpreference to fish. They get the salt from the spring. They boil offsome of the water and let the rest stand; when it has cooled and theheat and moisture have evaporated together it gives them salt, notin lumps but loose and light like snow. It is weaker than ordinarysalt and added freely gives a sweet taste, and it is not as white assalt generally is. Another instance of this is found in Umbria.There is a place there where reeds and rushes grow. They burn someof these, put the ashes into water and boil it off. When a littlewater is left and has cooled it gives a quantity of salt.
Most salt rivers and springs must once have been hot. Then theoriginal fire in them was extinguished but the earth through whichthey percolate preserves the character of lye or ashes. Springs andrivers with all kinds of flavours are found in many places. Theseflavours must in every case be due to the fire that is or was in them,for if you expose earth to different degrees of heat it assumesvarious kinds and shades of flavour. It becomes full of alum and lyeand other things of the kind, and the fresh water percolates throughthese and changes its character. Sometimes it becomes acid as inSicania, a part of Sicily. There they get a salt and acid waterwhich they use as vinegar to season some of their dishes. In theneighbourhood of Lyncus, too, there is a spring of acid water, andin Scythia a bitter spring. The water from this makes the whole of theriver into which it flows bitter. These differences are explained by aknowledge of the particular mixtures that determine different savours.But these have been explained in another treatise.
We have now given an account of waters and the sea,
why theypersist, how they change, what their nature is, and have explainedmost of their natural operations and affections.
4
Let us proceed to the theory of winds. Its basis is a distinction wehave already made. We recognize two kinds of evaporation, one moist,the other dry. The former is called vapour: for the other there isno general name but we must call it a sort of smoke, applying to thewhole of it a word that is proper to one of its forms. The moistcannot exist without the dry nor the dry without the moist: wheneverwe speak of either we mean that it predominates. Now when the sun inits circular course approaches, it draws up by its heat the moistevaporation: when it recedes the cold makes the vapour that had beenraised condense back into water which falls and is distributed throughthe earth. (This explains why there is more rain in winter and more bynight than by day: though the fact is not recognized because rain bynight is more apt to escape observation than by day.) But there is agreat quantity of fire and heat in the earth, and the sun not onlydraws up the moisture that lies on the surface of it, but warms anddries the earth itself. Consequently, since there are two kinds ofevaporation, as we have said, one like vapour, the other like smoke,both of them are necessarily generated. That in which moisturepredominates is the source of rain, as we explained before, whilethe dry evaporation is the source and substance of all winds. Thatthings must necessarily take this course is clear from the resultingphenomena themselves, for the evaporation that is to produce them mustnecessarily differ; and the sun and the warmth in the earth not onlycan but must produce these evaporations.
Since the two evaporations are specifically distinct, wind andrain obviously differ and their substance is not the same, as thosesay who maintain that one and the same air when in motion is wind, butwhen it condenses again is water.
Air, as we have explained in anearlier book, is made up of these as constituents. Vapour is moistand cold (for its fluidity is due to its moistness, and because itderives from water it is naturally cold, like water that has notbeen warmed): whereas the smoky evaporation is hot and dry. Hence eachcontributes a part, and air is moist and hot. It is absurd that thisair that surrounds us should become wind when in motion, whatever bethe source of its motion on the contrary the case of winds is likethat of rivers. We do not call water that flows anyhow a river, evenif there is a great quantity of it, but only if the flow comes froma spring. So too with the winds; a great quantity of air might bemoved by the fall of some large object without flowing from any sourceor spring.
The facts bear out our theory. It is because the evaporation takesplace uninterruptedly but differs in degree and quantity that cloudsand winds appear in their natural proportion according to theseason; and it is because there is now a great excess of the vaporous,now of the dry and smoky exhalation, that some years are rainy andwet, others windy and dry. Sometimes there is much drought or rain,and it prevails over a great and continuous stretch of country. Atother times it is local; the surrounding country often gettingseasonable or even excessive rains while there is drought in a certainpart; or, contrariwise, all the surrounding country gets little oreven no rain while a certain part gets rain in abundance. The reasonfor all this is that while the same affection is generally apt toprevail over a considerable district because adjacent places (unlessthere is something special to differentiate them) stand in the samerelation to the sun, yet on occasion the dry evaporation willprevail in one part and the moist in another, or conversely. Again thereason for this latter is
that each evaporation goes over to that ofthe neighbouring district: for instance, the dry evaporationcirculates in its own place while the moist migrates to the nextdistrict or is even driven by winds to some distant place: or else themoist evaporation remains and the dry moves away. Just as in thecase of the body when the stomach is dry the lower belly is often inthe contrary state, and when it is dry the stomach is moist andcold, so it often happens that the evaporations reciprocally takeone another’s place and interchange.
Further, after rain wind generally rises in those places where therain fell, and when rain has come on the wind ceases. These arenecessary effects of the principles we have explained. After rainthe earth is being dried by its own heat and that from above and givesoff the evaporation which we saw to be the material cause of. wind.Again, suppose this secretion is present and wind prevails; the heatis continually being thrown off, rising to the upper region, and sothe wind ceases; then the fall in temperature makes vapour form andcondense into water. Water also forms and cools the dry evaporationwhen the clouds are driven together and the cold concentrated in them.These are the causes that make wind cease on the advent of rain, andrain fall on the cessation of wind.
The cause of the predominance of winds from the north and from thesouth is the same. (Most winds, as a matter of fact, are north windsor south winds.) These are the only regions which the sun does notvisit: it approaches them and recedes from them, but its course isalways over the-west and the east. Hence clouds collect on eitherside, and when the sun approaches it provokes the
moist evaporation,and when it recedes to the opposite side there are storms and rain. Sosummer and winter are due to the sun’s motion to and from thesolstices, and water ascends and falls again for the same reason.Now since most rain falls in those regions towards which and fromwhich the sun turns and these are the north and the south, and sincemost evaporation must take place where there is the greatest rainfall,just as green wood gives most smoke, and since this evaporation iswind, it is natural that the most and most important winds should comefrom these quarters. (The winds from the north are called Boreae,those from the south Noti.)
The course of winds is oblique: for though the evaporation risesstraight up from the earth, they blow round it because all thesurrounding air follows the motion of the heavens. Hence thequestion might be asked whether winds originate from above or frombelow. The motion comes from above: before we feel the wind blowingthe air betrays its presence if there are clouds or a mist, fortheir motion shows that the wind has begun to blow before it hasactually reached us; and this implies that the source of winds isabove. But since wind is defined as ’a quantity of dry evaporationfrom the earth moving round the earth’, it is clear that while theorigin of the motion is from above, the matter and the generation ofwind come from below. The oblique movement of the rising evaporationis caused from above: for the motion of the heavens determines theprocesses that are at a distance from the earth, and the motion frombelow is vertical and every cause is more active where it is nearestto the effect; but in its generation and origin wind plainly derivesfrom the earth.
The facts bear out the view that winds are formed by the gradualunion of many evaporations just