by Aristotle
[15] Some think that the cause and origin of hail is this. The cloud is thrust up into the upper region, which is colder because the reflection of the sun’s rays from the earth ceases there, and upon its arrival there the water freezes. They think that this explains why hailstorms are commoner in summer and in warm countries; the heat [20] is greater and it thrusts the clouds further up from the earth. But the fact is that hail does not occur at all at a great height: yet it ought to do so, on their theory, just as we see that snow falls most on high mountains. Again, clouds have often been observed moving with a great noise close to the earth, terrifying those who heard [25] and saw them as portents of some catastrophe. Sometimes, too, when such clouds have been seen, without any noise, there follows a violent hailstorm, and the stones are of incredible size, and angular in shape. This shows that they have not been falling for long and that they were frozen near to the earth, and not as that theory [30] would have it. Moreover, where the hailstones are large, the cause of their freezing must be present in the highest degree; for hail is ice, as every one can see. Now those hailstones are large which are angular 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 fall and [35] become round and smaller in size.
It clearly follows that the freezing does not take place because the cloud is thrust up into the cold upper region. [348b1]
Now we see that warm and cold react upon one another. Hence in warm weather the lower parts of the earth are cold and in a frost they are warm. The same thing, we must suppose, happens in the upper region, so that in the warmer seasons [5] the cold is concentrated by the surrounding heat and causes the cloud to go over into water suddenly. (For this reason rain-drops are much larger on warm days than in winter, and showers more violent. A shower is said to be more violent in proportion [10] as the water comes down in a body, and this happens when the condensation takes place quickly—though this is just the opposite of what Anaxagoras says. He says that this happens when the cloud has risen into the cold air; whereas we say that it happens when the cloud has descended into the warm air, and that the more the further the cloud has descended). But when the cold has been concentrated within [15] still more by the outer heat, it freezes the water it has formed and there is hail. We get hail when the process of freezing is quicker than the descent of the water. For if the water falls in a certain time and the cold is sufficient to freeze it in less, there is [20] no difficulty about its having frozen in the air, provided that the freezing takes place in a shorter time than its fall. The nearer to the earth, and the more intensely, this process takes place, the more violent is the rain that results and the larger the raindrops and the hailstones because of the shortness of their fall. For the same reason large raindrops do not fall thickly. Hail is rarer in summer than in spring and [25] autumn, though commoner than in winter, because the air is drier in summer, whereas in spring it is still moist, and in autumn it is beginning to grow moist. It is for the same reason that hailstorms sometimes occur in the late summer too, as we [30] have said.
The fact that the water has previously been warmed contributes to its freezing quickly; for so it cools sooner. (Hence many people, when they want to cool water quickly, begin by putting it in the sun. So the inhabitants of Pontus when they encamp on the ice to fish (they cut a hole in the ice and then fish) pour warm water round their rods that it may freeze the quicker; for they use the ice like lead to fix [349a1 the rods.) Now it is in hot countries and seasons that the water which forms soon grows warm.
It is for the same reason that rain falls in summer and not in winter in Arabia [5] and Ethiopia too, and that in torrents and repeatedly on the same day. For the concentration due to the extreme heat of the country cools the clouds quickly.
So much for an account of the nature and causes of rain, dew, snow, hoar-frost, [10] and hail.
13 · Let us explain the nature of winds, and all windy vapours, also of rivers and of the sea. But here, too, we must first discuss for ourselves the difficulties [15] involved; for, as in other matters, so in this nothing has been handed down to us that anyone could not have thought of.
Some say that what is called air, when it is motion and flows, is wind, and that this same air when it condenses again becomes cloud and water, implying that the [20] nature of wind and water is the same. So they define wind as a motion of the air. Hence some, wishing to say a clever thing, assert that all the winds are one wind, because the air that moves is in fact all of it one and the same; they maintain that the winds appear to differ owing to the region from which the air may happen to [25] flow on each occasion, but really do not differ at all. This is just like thinking that all rivers are one and the same river, and the ordinary unscientific view is better than a scientific theory like this. If all rivers flow from one source, and the same is true in the case of the winds, there might be some truth in this theory; but if it is no more [30] true in the one case than in the other, this ingenious idea is plainly false. What requires investigation is this: the nature of wind and how it originates, its efficient cause and whence they derive their source; whether one ought to think of the wind as issuing from a sort of vessel and flowing until the vessel is empty, as if let out of a [349b1] wineskin, or, as painters represent the winds, as drawing their source from themselves.
We find analogous views about the origin of rivers. It is thought that the water is raised by the sun and descends in rain and gathers below the earth and so flows [5] from a great hollow, all the rivers from one, or each from a different one. No water at all is generated, but the volume of the rivers consists of the water that is gathered into such reservoirs in winter. Hence rivers are always fuller in winter than in summer, and some are perennial, others not. Rivers are perennial where the hollow [10] is large and so enough water has collected in it to last out and not be used up before the winter rain returns. Where the reservoirs are smaller there is less water in the rivers, and they are dried up and their vessel empty before the fresh rain comes on.
[15] But if any one will picture to himself a reservoir adequate to the water that is continuously flowing day by day, and consider the amount of the water, it is obvious that a receptacle that is to contain all the water that flows in the year would be larger than the earth, or, at any rate, not much smaller.
[20] Though it is evident that many reservoirs of this kind do exist in many parts of the earth, yet it is unreasonable for any one to refuse to admit that air becomes water in the earth for the same reason as it does above it. If the cold causes the vaporous air to condense into water above the earth we must suppose the cold in the [25] earth to produce this same effect, and recognize that there not only exists in it and flows out of it actually formed water, but that water is continually forming in it too.
Again, even in the case of the water that is not being formed from day to day but exists as such, we must not suppose as some do that rivers have their source in [30] definite subterranean lakes. On the contrary, just as above the earth small drops form and these join others, till finally the water descends in a body as rain, so too we must suppose that in the earth the water at first trickles together little by little, and that the sources of the rivers are formed where the earth gushes out, as it were, at a single point. This is proved by facts. When men construct irrigation works they collect the water in pipes and trenches, as if the earth in the higher ground were [350a1] sweating the water out. Hence, too, the head-waters of rivers are found to flow from mountains, and from the greatest mountains there flow the most numerous and greatest rivers. Again, most springs are in the neighbourhood of mountains and of [5] high ground, whereas if we except rivers, water rarely appears in the plains. For mountains and high ground, suspended over the country like a saturated sponge, make the water ooze out and trickle together in minute quantities but in many places. They receive a great deal of water falling as rain (for it makes no difference [10] whether a spongy receptacle is concave and turned up or convex and turne
d down: in either case it will contain the same volume of matter) and they also cool the vapour that rises and condense it back into water.
Hence, as we said, we find that the greatest rivers flow from the greatest mountains. This can be seen by looking at maps: what is recorded in them consists [15] either of things which the writer has seen himself or of such as he has compiled after inquiry from those who have seen them.
In Asia we find that the most numerous and greatest rivers flow from the mountain called Parnassus, admittedly the greatest of all mountains towards the [20] south-east. When you have crossed it you see the outer ocean, the further limit of which is unknown to the dwellers in our world. Besides other rivers there flow from it the Bactrus, the Choaspes, the Araxes: from the last a branch separates off and flows into lake Maeotis as the Tanais. From it, too, flows the Indus, the volume of [25] whose stream is greatest of all rivers. From the Caucasus flows the Phasis, and very many other great rivers besides. Now the Caucasus is the greatest of the mountains that lie to the north-east, both as regards its extent and its height. A proof of its height is the fact that it can be seen from the so-called ‘deeps’ and from the entrance [30] to the lake. Again, the sun shines on its peaks for a third part of the night before sunrise and again after sunset. Its extent is proved by the fact that though it contains many inhabitable regions which are occupied by many nations and in which there are said to be great lakes, yet they say that all these regions are visible up to the last peak. From Pyrene (this is a mountain towards the west in Celtice) [350b1] there flow the Istrus and the Tartessus. The latter flows outside the pillars, while the Istrus flows through all Europe into the Euxine. Most of the remaining rivers flow northwards from the Hercynian mountains, which are the greatest in height [5] and extent about that region. In the extreme north, beyond furthest Scythia, are the mountains called Rhipae. The stories about their size are altogether too fabulous: however, they say that the most and (after the Istrus) the greatest rivers flow from [10] them. So, too, in Libya there flow from the Aethiopian mountains the Aegon and the Nyses; and from the so-called Silver Mountain the two greatest of named rivers, the river called Chremetes that flows into the outer ocean, and the main source of the Nile. Of the rivers in the Greek world, the Achelous flows from Pindus, the [15] Inachus from the same mountain; the Strymon, the Nessus, 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 have [20] mentioned these as examples. Even where rivers flow from marshes, the marshes in almost every case are found to lie below mountains or gradually rising ground.
It is clear then that we must not suppose rivers to originate from definite reservoirs; for the whole earth, we might almost say, would not be sufficient (any [25] more than the region of the clouds would be) if we were to suppose that they were fed by actually existing water only and it were not the case that as some water passed out of existence some more came into existence, but rivers always drew their stream from an existing store. Secondly, the fact that rivers rise at the foot of mountains proves that a place transmits the water it contains by gradual percolation [30] of many drops, little by little, and that this is how the sources of rivers originate. However, there is nothing impossible about the existence of such places containing a quantity of water like lakes: only they cannot be big enough to produce the supposed effect, any more than one could suppose that rivers drew all their water from the sources we see (for most rivers do flow from springs). So it is no more reasonable to [35] suppose those lakes to contain the whole volume of water than these springs.
That there exist such chasms and cavities in the earth we are taught by the [351a1] rivers that are swallowed up. They are found in many parts of the earth: in the Peloponnesus, for instance, there are many such rivers in Arcadia. The reason is that Arcadia is mountainous and there are no channels from its valleys to the sea. [5] So these places get full of water, and this, having no outlet, under the pressure of the water that is added above, finds a way out for itself underground. In Greece this kind of thing happens on quite a small scale; but there is the lake at the foot of the [10] Caucasus, which the inhabitants of these parts call a sea. Many great rivers fall into it and it has no visible outlet but issues below the earth off the land of the Coraxi about the so-called deeps of Pontus. This is a place of unfathomable depth in the sea: at any rate no one has yet been able to find bottom there by sounding. At this [15] spot, about three hundred stadia from land, there comes up sweet water over a large area, not all of it together but in three places. And in Liguria a river equal in 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 they [20] change according as rivers come into existence and dry up. And so the relation of land to sea changes too and a place does not always remain land or sea throughout all time, but where there was dry land there comes to be sea, and where there is now [25] sea, there one day comes to be dry land. But we must suppose these changes to follow some order and cycle. The principle and cause of these changes is that the interior of the earth has its periods of maturity, like the bodies of plants and animals. Only in the case of these latter the process does not go on by parts, but each [30] of them necessarily grows or decays as a whole, whereas it does go on by parts in the case of the earth. Here the causes are cold and heat, which increase and diminish on account of the sun and its course. It is owing to them that the parts of the earth come to have a different character, so that some parts remain moist for a certain time, and then dry up and grow old, while other parts in their turn are filled with life and moisture. Now when places become drier the springs necessarily give out, and when this happens the rivers first decrease in size and then finally become dry; and when [351b1] rivers change and disappear in one part and come into existence correspondingly in another, the sea must needs be affected.
For where the sea is pushed out by rivers and encroaches upon the land, it [5] necessarily leaves that place dry when it recedes; and where it becomes dry, being silted up by the rivers when at their full, the time must come when this place will be flooded again.13
But the whole vital process of the earth takes place so gradually and in periods of time which are so immense compared with the length of our life, that these [10] changes are not observed, and before their course can be recorded from beginning to end whole nations perish and are destroyed. Of such destructions the most utter and sudden are due to wars; but pestilence or famine cause them too. Famines, again, are either severe or else gradual. In the latter case the disappearance of a nation is [15] not noticed because some leave the country while others remain; and this goes on until the land is unable to maintain any inhabitants at all. So a long period of time is likely to elapse from the first departure to the last, so that no one remembers and the [20] lapse of time destroys all record even before the last inhabitants have disappeared. In the same way a nation must be supposed to lose account of the time when it first settled in a land that was changing from a marshy and watery state and becoming dry. Here, too, the change is gradual and lasts a long time and men do not [25] remember who came first, or when, or what the land was like when they came. This has been the case with Egypt. Here it is obvious that the land is continually getting drier and that the whole country is a deposit of the river Nile. But because the [30] neighbouring peoples settled in the land gradually as the marshes dried, the lapse of time has hidden the beginning of the process. Thus, all the mouths of the Nile, with the single exception of that at Canopus, are obviously artificial and not natural. And Egypt was originally what is called Thebes, as Homer, too, shows, modern [35] though he is in relation to such changes. For Thebes is the place that he mentions; which implies that Memphis did not yet exist, or at any rate was not as important as [352a1 it is now. That this should be so is natural, since the lower land came to be inhabited later than that which lay higher. For t
he parts that lie nearer to the place where the river is depositing the silt are necessarily marshy for a longer time since the water always lies most in the newly formed land. But in time this land changes its [5] character, and in its turn enjoys a period of prosperity. For these places dry up and come to be in good condition while the places that were formerly well-tempered some day grow excessively dry and deteriorate. This happened to the land of Argos and Mycenae in Greece. In the time of the Trojan wars the Argive land was marshy [10] and could only support a small population, whereas the land of Mycenae was in good condition (and for this reason Mycenae was the superior). But now the opposite is the case, for the reason we have mentioned: the land of Mycenae has become completely dry and barren, while the Argive land that was formerly barren owing to the water has now become fruitful. Now the same process that has taken [15] place in this small district must be supposed to be going on over whole countries and on a large scale.
Men whose outlook is narrow suppose the cause of such events to be change in the universe, in the sense of a coming to be of the world as a whole. Hence they say [20] that the sea is being dried up and is growing less, because this is observed to have happened in more places now than formerly. But this is only partially true. It is true that many places are now dry, that formerly were covered with water. But the opposite is true too; for if they look they will find that there are many places where [25] the sea has invaded the land. But we must not suppose that the cause of this is that the world is in process of becoming. For it is absurd to make the universe to be in process because of small and trifling changes, when the bulk and size of the earth are surely as nothing in comparison with the whole world. Rather we must take the [30] cause of all these changes to be that, just as winter occurs in the seasons of the year, so in determined intervals in some great period of time there comes a great winter and with it excess of rain. But this excess does not always occur in the same place. The so-called flood in the time of Deucalion, for instance, took place chiefly in the Greek world and in it especially about ancient Hellas, the country about Dodona [352b1] and the Achelous, a river which has often changed its course. Here the Selli dwelt and those who were formerly called Graeci and now Hellenes. When, therefore, such an excess of rain occurs we must suppose that it suffices for a long time. Just as [5] some say that the size of the subterranean cavities is what makes some rivers perennial and others not, whereas we maintain that the size of the mountains is the cause, and their density and coldness (for they catch and keep and create most water: whereas if the mountains that overhang the sources of rivers are small or [10] porous and stony and clayey, these rivers run dry earlier); so too we must realize that where such abundance of rain falls it tends to make the moisture of those places almost everlasting. But as time goes on places of the latter type dry up more,14 while [15] those of the former, moist type, do so less:15 until at last the beginning of the same cycle returns.