The Complete Works of Aristotle

by Barnes, Jonathan, Aristotle

  The course of winds is oblique; for though the exhalation rises straight up from the earth, they blow round it because all the surrounding air follows the motion of [25] the heavens. Hence the question might be asked whether winds originate from above or from below. The motion comes from above: before24 we feel the wind blowing the air betrays its presence even if there are clouds or a mist; for they show that the wind has begun to blow before it has actually reached us; and this implies [30] that the source of winds is above. But since wind is a quantity of dry exhalation from the earth moving round the earth, it is clear that while the origin of the motion is from above, the matter and the generation of wind come from below. For the direction of flow of the rising exhalation is caused from above; for the motion of the heavens determines the processes that are at a distance from the earth, and the [35] motion from below is vertical and every cause is more active where it is nearest to the effect; but in its generation and origin wind plainly derives from the earth.

  [361b1] The facts bear out the view that winds are formed by the gradual union of many exhalations just as rivers derive their sources from the water that oozes from the earth. Every wind is weakest in the spot from which it blows; as they proceed [5] and leave their source at a distance they gather strength. (Again, the winter in the north is windless and calm: that is, in the north itself; but the breeze that blows from there so gently as to escape observation becomes a great wind as it passes on.

  We have explained the nature and origin of wind, the occurrence of drought [10] and rains, the reason why winds rise and fall after rain, the prevalence of north and south winds and also why wind moves in the way it does.

  5 · The sun both checks the formation of winds and stimulates it. When the [15] exhalation is small in amount and faint the sun wastes it and25 dissipates by its greater heat the lesser heat contained in the exhalation. It also dries up the earth before the outflow has appeared in bulk: just as, when you throw a little fuel into a [20] great fire, it is often burnt up before giving off any smoke. In these ways the sun checks winds and prevents them from rising at all: it checks them by wasting, and prevents their rising by drying up the earth quickly. Hence calm is very apt to prevail about the rising of Orion and lasts until the coming of the etesian winds and their forerunners.

  [25] Calm is due to two causes. Either cold quenches the exhalation, for instance a sharp frost; or excessive heat wastes it. In the intermediate periods, too, the causes are generally either that the exhalation has not had time to develop or that it has passed away and there is none as yet to replace it.

  [30] Both the setting and the rising of Orion are considered to be treacherous and stormy, because they take place at a change of season (namely of summer or winter; and the size of the constellation makes its rise last over many days) and a state of change is always indefinite and therefore liable to disturbance.

  The etesian winds blow after the summer solstice and the rising of the dog-star: [362a1] not at the time when the sun is closest nor when it is distant; and they blow by day and cease at night. The reason is that when the sun is near it dries up the earth before exhalation has taken place, but when it has receded a little its heat and the exhalation are present in the right proportion; so the ice melts and the earth, dried [5] by its own heat and that of the sun, smokes and vapours. They abate at night because the cold of the nights checks the melting of the ice. What is frozen gives off no evaporation, nor does that which contains no dryness at all: it is only where something dry contains moisture that it gives off evaporation under the influence of [10] heat.

  The question is sometimes asked: why do the north winds which we call etesian blow continuously after the summer solstice, when there are no corresponding south winds after the winter solstice? The facts are reasonable enough: for the so-called ‘white south winds’ do blow at the corresponding season, though they are not [15] equally continuous and so escape observation and give rise to this inquiry. The reason for this is that the north wind blows from the arctic regions which are full of water and much snow. The sun thaws them and so the etesian winds blow after rather than at the summer solstice. (For the greatest heat is developed not when the [20] sun is nearest to the north, but when its heat has been felt for a considerable period and it has not yet receded far. The ‘bird winds’ blow in the same way after the winter solstice. They, too, are weak etesian winds, but they blow less and later than the etesians. They begin to blow only on the seventieth day because the sun is [25] distant and therefore weaker. They do not blow so continuously because only things on the surface of the earth and offering little resistance evaporate then, the thoroughly frozen parts requiring greater heat to melt them. So they blow intermittently till the true etesians come on again at the summer solstice; for from that time onwards the wind has an especial tendency to blow continuously.) But the [30] south wind blows from the tropic of Cancer and not from the antarctic region.

  There are two inhabitable sections of the earth: one near our upper, or northern pole, the other near the other or southern pole; and their shape is like that of a drum. If you draw lines from the centre of the earth they cut out a drum-shaped [362b1] figure. The lines form two cones; the base of the one is the tropic, of the other the ever visible circle, their vertex is at the centre of the earth. Two other cones towards the south pole give corresponding segments of the earth. These sections alone are [5] habitable. Beyond the tropics no one can live; for there the shade would not fall to the north, whereas the earth is known to be uninhabitable before the shadow disappears or is thrown to the south; and the regions below the Bear are uninhabitable because of the cold.

  [The Crown, too, moves over this region; for it is in the zenith when it is on our [10] meridian].26

  So we see that the way in which they now draw maps of the earth is ridiculous. They depict the inhabited earth as round, but both observation and reason show this to be impossible. For reason proves that the inhabited region is limited in breadth, [15] while the climate admits of its extending all round the earth. For we meet with no excessive heat or cold in the direction of its length but only in that of its breadth; so that there is nothing to prevent our travelling round the earth unless the extent of the sea presents an obstacle anywhere. The observations made on journeys by sea [20] and land bear this out. They make the length far greater than the breadth. If we compute these voyages and journeys the distance from the Pillars of Heracles to India exceeds that from Aethiopia to Maeotis and the northernmost Scythians by a [25] ratio of more than 5 to 3, as far as such matters admit of accurate statement. Yet we know the whole breadth of the region we dwell in up to the uninhabited parts: in one direction no one lives because of the cold, in the other because of the heat.

  But it is the sea which divides as it seems the parts beyond India from those beyond the Pillars of Heracles and prevents the earth from being inhabited all round.

  [30] Now since there must be a region bearing the same relation to the southern pole as the place we live in bears to our pole, it will clearly correspond in the ordering of its winds as well as in other things. So just as we have a north wind, here they must have a corresponding wind from their pole. This wind cannot reach us [363a1] since our own north wind is like a land breeze and does not even reach27 the limits of the region we live in. The prevalence of north winds here is due to our lying near the [5] north. Yet even here they give out and fail to penetrate far: in the southern sea beyond Libya east and west winds are always blowing alternately, like north and south winds with us. So it is clear that the south wind is not the wind that blows from the south pole. It is neither that nor the wind from the winter tropic. For symmetry [10] would require another wind blowing from the summer tropic, which there is not, since we know that only one wind blows from that quarter. So the south wind clearly blows from the torrid region. Now the sun is so near to that region that it has no [15] water, or snow28 which might melt29 and cause etesian winds. But because that place is far more extensive and open the s
outh wind is greater and stronger and warmer than the north and penetrates farther to the north than the north wind does to the south.

  The cause of these winds and their relation to one another has now been [20] explained.

  6 · Let us now explain the position of the winds, their oppositions, which can blow simultaneously with which, and which cannot, their names and number, and any other of their affections that have not been treated among the particular [25] problems. What we say about their position must be followed with the help of the diagram. For clearness’ sake we have drawn the circle of the horizon—that is why it is round—and it30 must be taken to represent the zone in which we live; for the other [30] section too can be divided in the same way. Let us begin by laying down that those things are locally contrary which are locally most distant from one another just as things specifically most remote from one another are specific contraries. Now things that face one another from opposite ends of a diameter are locally most distant from one another.

  Let A be the point where the sun sets at the equinox and B, the point opposite, the place where it rises at the equinox. Let there be another diameter cutting this at [363b1] right angles, and let the point G on it be the north and its diametrical opposite H the south. Let F be the rising of the sun at the summer solstice and E its setting at the summer solstice; D its rising at the winter solstice, and C its setting at the winter [5] solstice. Draw a diameter from F to C and from D to E. Then since those things are locally contrary which are locally most distant from one another, and points diametrically opposite are most distant from one another, those winds must necessarily be contrary to one another that blow from opposite ends of a diameter. [10]

  The names of the winds according to their position are these. Zephyrus is the wind that blows from A, this being the point where the sun sets at the equinox. Its contrary is Apeliotes blowing from B the point where the sun rises at the equinox. The wind blowing from G, the north, is Boreas or Aparctias; while Notus blowing [15] from H is its contrary; for this point is the south and H is contrary to G, being diametrically opposite to it. Caecias blows from F, where the sun rises at the summer solstice. Its contrary is not the wind blowing from E but Lips blowing from C. For Lips blows from the point where the sun sets at the winter solstice and is diametrically opposite to Caecias: so it is its contrary. Eurus blows from D, coming [20] from the point where the sun rises at the winter solstice. It borders on Notus, and so we often find that people speak of Euro-Noti. Its contrary is not Lips blowing from C but the wind that blows from E which some call Argestes, some Olympias, and some Sciron. This blows from the point where the sun sets at the summer solstice, [25] and is the only wind that is diametrically opposite to Eurus. These are the winds that are diametrically opposite to one another and which have contraries.

  There are other winds which have no contraries. The wind they call Thrascias, which lies between Argestes and Aparctias, blows from I; the wind called Meses, which lies between Caecias and Aparctias, from K. (The diameter IK nearly [30] coincides with the ever visible circle, but not quite.) These winds have no contraries. Meses has not, or else there would be a wind blowing from the point M which is diametrically opposite. Thraskias corresponding to the point I has not, for then [364a1] there would be a wind blowing from N, the point which is diametrically opposite. (But perhaps a local wind which the inhabitants of those parts call Phoenicias blows from that point.)

  These are the most important and definite winds and these their places. [5]

  There are more winds from the north than from the south. The reason for this is that the region in which we live lies nearer to the north. Also, much more water and snow is pushed aside into this quarter because the other lies under the sun and its course. When this thaws and soaks into the earth and is exposed to the heat of the [10] sun and the earth it necessarily causes exhalation to rise in greater quantities and over a greater space.

  Of the winds we have described Aparctias is the north wind in the strict sense. Thracias and Meses are north winds too. (Caecias is half north and half east). [15] South are that which blows from due south and Lips. East, the wind from the rising of the sun at the equinox and Eurus. Phoenicias is half south and half east. West, the wind from the true west and that called Argestes. More generally these winds [20] are classified as northerly or southerly. The west winds are counted as northerly, for they blow from the place of sunset and are therefore colder; the east winds as southerly, for they are warmer because they blow from the place of sunrise. So the distinction of cold and hot or warm is the basis for the division of the winds into northerly and southerly. East winds are warmer than west winds because the sun [25] shines on the east longer, whereas it leaves the west sooner and reaches it later.

  Since this is the distribution of the winds it is clear that contrary winds cannot blow simultaneously. They are diametrically opposite to one another and one of the two must be overpowered and cease. Winds that are not diametrically opposite to [30] one another may blow simultaneously: for instance the winds from F and from D. Hence it sometimes happens that both of them, though different winds and blowing from different quarters, are favourable to sailors making for the same point.

  Contrary winds commonly blow at opposite seasons. Thus Caecias and in [364b1] general the winds north of the summer solstice blow about the time of the spring equinox, but about the autumn equinox Lips; and Zephyrus about the summer solstice, but about the winter Solstice Eurus.

  Aparctias, Thracias, and Argestes are the winds that fall on others most and [5] stop them. Their source is so close to us that they are greater and stronger than other winds. They bring fair weather most of all winds for the same reason; for, blowing as they do, from close at hand, they overpower the other winds and stop them; they also blow away the clouds that are forming and leave a clear sky—unless [10] they happen to be very cold. Then they do not bring fair weather, but being colder than they are strong they condense the clouds before driving them away.

  Caecias does not bring fair weather because it returns upon itself. Hence the saying: ‘Bringing it on himself as Caecias does clouds’.

  When they cease, winds are succeeded by their neighbours in the direction of [15] the movement of the sun. For an effect is most apt to be produced in the neighbourhood of its source, and the source of winds moves with the sun.

  Contrary winds have either the same or contrary effects. Thus Lips and Caecias, sometimes called Hellespontias, are both rainy.31 Argestes and Eurus are [20] dry: the latter being dry at first and rainy afterwards. Meses and Aparctias are coldest and bring most snow. Aparctias, Thrascias, and Argestes bring hail. Notus, Zephyrus, and Eurus are hot. Caecias covers the sky with heavy clouds, Lips with [25] lighter ones. Caecias does this because it returns upon itself and combines the qualities of Boreas and Eurus. By being cold it condenses and gathers the vaporous air, and because it is easterly it carries with it and drives before it a great quantity of such vaporous matter. Aparctias, Thrascias, and Argestes bring fair weather for the [30] reason we have explained before. These winds and Meses are most commonly accompanied by lightning. They are cold because they blow from nearby, and lightning is due to cold, being ejected when the clouds contract. Some of these same [365a1] winds bring hail with them for the same reason; namely, that they cause a sudden condensation.

  Hurricanes are commonest in autumn, and next in spring: Aparctias, Thrascias, and Argestes give rise to them most. This is because hurricanes are generally formed when some winds are blowing and others fall on them; and these are the winds which are most apt to fall on others that are blowing; the reason for which, [5] too, we have explained before.

  The etesian winds veer round, for dwellers in the west from Aparctias to Thrascias, Argestes, and Zephyrus, beginning from the north and ending far away;32 for dwellers in the east they veer round as far as Apeliotes. [10]

  So much for the winds, their origin and nature and the properties common to them all or peculiar to each.

&n
bsp; 7 · We must go on to discuss earthquakes and tremors of the earth next, for their cause is akin to our last subject. [15]

  The theories that have been put forward up to the present date are three, and their authors three men, Anaxagoras of Clazomenae, and before him Anaximenes of Miletus, and later Democritus of Abdera.

  Anaxagoras says that the ether, which naturally moves upwards, is caught in [20] hollows below the earth and so shakes it; for though the earth is really all of it equally porous, its surface is clogged up by rain. This implies that part of the whole sphere is above and part below: above being the part on which we live, below the other.

  This theory is perhaps too primitive to require refutation. It is absurd to think [25] of up and down otherwise than as meaning that heavy bodies move to the earth from every quarter, and light ones, such as fire, upwards; especially as we see that, as far as our knowledge of the earth goes, the horizon always changes with a change in our [30] position, which proves that the earth is convex and spherical. It is absurd, too, to maintain that the earth rests on the air because of its size, and then to say that impact upwards from below shakes it right through. Besides he gives no account of the circumstances attendant on earthquakes; for not every country or every season [35] is subject to them.