Lecture 16 — The cause of the generation of rivers.

Let us explain the nature of winds, and all windy vapors, also of rivers and of the sea. But here, too, we must first discuss the difficulties involved: for, as in other matters, so in this no theory has been handed down to us that the most ordinary man could not have thought of.

Some say that what is called air, when it is in motion and flows, is wind, and that this same air when it condenses again becomes cloud and water, implying that the 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 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 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 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 from a great reservoir, 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 reservoir 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.

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. 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 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 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 drip, as it were, out of the earth and then unite. This is proved by facts. When men construct an aqueduct they collect the water in pipes and trenches, as if the earth in the higher ground were 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 neighborhood of mountains and of 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 whether a spongy receptacle is concave and turned up or convex and turned down: in either case it will contain the same volume of matter) and, they also cool the vapor 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 itineraries: what is recorded in them consists 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 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 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 northeast, 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 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 thought 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) 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 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 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 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 have 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 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. second, the fact that rivers rise at the foot of mountains proves that a place transmits the water it contains by gradual percolation 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. To think that they are is just as absurd as if one were to 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 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 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. 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 the lake at the foot of the Caucasus, which the inhabitants of these parts call a sea, is considerable. 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 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.

123. After determining concerning things generated on high from the moist exhalation, the Philosopher here determines about things generated on earth from the same material—namely, about springs and rivers. It is divided into two parts:

in the first he determines concerning the cause of the generation of rivers;

in the second concerning their duration, at the same parts of the earth are not always (351a19).

As to the first, he does three things:

first, he states his intention and says that the discussion is about winds and everything caused by winds, and about rivers and the sea. These will be treated in the following order: we shall propose problems of our own; then we shall declare the true answer to the problems to ourselves and not others: since on these matters we have received no opinions from others that anybody could not have conceived, unlike that which was the case in regard to the other matters.

124. second at some say that (349a16), he presents the opinions of certain others about winds. And he says that some have declared that the body called "air," when it flows along and is in motion, is wind; but when it halts and condenses, it is cloud and water—as though the natures of water, air and wind were identical, and wind were nothing more than air and water. And because the whole of air is one, some, wishing to speak very wisely, have asserted that there is but one wind, and that if winds seem to differ, it is only because of the different places whence they move. This is like saying that all rivers are one river, and that all water is one: which is plainly false. Hence the generality of mankind, who speak as do the uneducated and without philosophic enquiry about the winds, speak with more truth than these, who in such an inquiry erred. For if it were true that all rivers flowed from one source, then it could also be somehow true that all winds were from one source: but it is plain that, for winds as well as for rivers, what they said is frivolously and deceitfully said. It is appropriate to discuss in a tract proper to it what wind is, and how it is generated, and what moves it, and from what do winds derive their source, and whether we must consider the wind as though flowing from some receptacle, and continuing to flow until it is emptied, as though wind were squeezed out of a wineskin as Homer's fable pretended; or whether it comes, not from one, but from many sources, as the painters depict the various winds puffing from out of themselves the source of breezes.

125. third at we find analogous views about the origin (349b2), he presents like opinions about the generation of rivers: this was the reason he introduced what he said concerning winds. About this he does three things:

first, he presents the false opinion of certain ones;

second, he rejects it;

third, he rejects a certain argument of their, at however, there is nothing impossible about the existence of such places (350b30).

He says therefore first, that to some, the same things seem to be true of the generation of rivers as was said of the generation of winds. For they say that when water is raised aloft through evaporation and then re-descends, it collects under the earth and thus flows on to generate springs and rivers. It is as if they were understood to emerge from some great womb (349b2), i.e., from some large depth where a great amount of water is gathered. It makes no difference to their theory whether all rivers flow from one such source or various rivers from various sources of this sort. According to this theory water is not newly generated under the earth to cause the flow of springs and rivers: what happens is that the water previously collected in the aforesaid receptacles is the source of the amount of waters and rivers. And they said that a sign of this is the fact that in winter there is more river-flow than in summer. From this they assign the cause why some rivers are unceasing and some not. For when, because of the size of the depth, a sufficient amount of water gathers under the earth to assure continuity of flow, in such a way that the flowing water does not run out before returning again the next winter, then the river is made perpetual to the end. But if the reservoir is small, then, because of the smallness of the amount of water, the river's source runs dry (as an emptied vessel) before water again flows from the sky. That is why such a river does not flow perennially.

126. Then when he says, **but if any one will picture to himself **(349b15), he rejects this position for four reasons.

First, he says that if one were to take into consideration the vast amount of water that continually flows through rivers in the entire world, the subterranean reservoir, or reservoirs, feeding these rivers, would have to be larger, or at least almost as large, as the earth. This would require that the whole interior of the earth be hollow, in order to hold such a vast amount of water; and even that would not be a sufficiently large container. Now this is plainly false: for, since earth is by nature in the middle [center], and its parts naturally tend to the middle, it cannot be said that the earth is hollow enough within to hold the water—although it is not inadmissible that there be certain receptacles of water in many parts of the earth.

127. second at yet it is unreasonable (349b21), he gives a second argument and says that it is inconsistent for one not to suppose that water comes to be within the earth from evaporated air, for the same reason that it is produced above the earth in the air. Hence, if evaporated air is condensed into water in the air above the earth on account of coldness, one must believe that the same is produced by reason of the coldness of the earth. And so, not only water existing separated in the earth as in a reservoir, flows through rivers, but water is also being continually generated within the earth by the refrigeration of vapors, and this will flow out through rivers.

128. But because someone could say that a certain water is generated within the earth from vapors, but that the whole is collected together into certain reservoirs, whence rivers flow, which theory would be similar to, and practically the same as, the previous position, therefore, third, he rejects this by means of a sign. And he says that it is even not to be supposed that water indeed is generated within the earth but remains there from day to day while rivers flow, as though there were certain pools of water under the earth, as some say. But one must understand that things take place in the same way within the earth, as above the earth. For above the earth, when vapor is first being condensed, small drops form and coalesce with others and in this way the flowing water easily descends in a certain quantity. The same thing takes place within the earth: for first small drops are generated; and thus the sources of rivers are springs gushing little by little below the earth. An indication of this is what is done—for people who wish to bring out water, e.g., who dig wells or the like, collect the water in low and excavated places, as though the earth perspired water from elevated areas to lower areas. From which it appears that water flows drop by drop from the earth to produce rivers and springs, rather than that in the earth are places acting as pools of actually existing water.

129. Fourth, at hence, too, the head-waters of rivers are found to flow (350a2), he supports the same view with another sign, based on the natural flow of waters: for the previous sign was based on a [human] work. And he says that for the same reason, the discharge, i.e., the flow of rivers, seems to be from the mountains, and the largest rivers flow from the largest mountains; moreover, most springs are close to mountains and high places, whereas on the plains there are few springs apart from rivers. The reason is that mountainous and high places are like certain thick sponges (because the rocks are hard) as to ejecting water; and they are suspended [elevated], so that the water can flow: consequently, they produce water in many places and also collect water that comes down from above as rain. But this occurs in small sections of the mountains and not in the sense that within the mountains there are depths in which water is gathered. That is why he says that they collect water, for they receive a great abundance of water coming as rainfall. The shape of mountains is well-adapted for this, for a round shape is the most capacious of shapes. So far as holding a great amount of water is concerned, it makes no difference whether the circumference is disposed above in a concave way, or according to a convex swelling—for in either case the capacity is equal. Hence, although mountains are not concave, but more convex, nevertheless they can receive a vast amount of water. And they not only hold a great amount of water as received from elsewhere, because of their shape, but also bring it forth as water born within because of the cold: for the vapor of earth, drawn out of the earth, and rising because of its inherent warmth, is coagulated within the earth by the coldness of the earth, so that it is again condensed into water. And that is why, as was said, the largest rivers flow from the largest mountains.

And this is plain, if one examines a traveler's description of the earth: for those who so described the earth have either themselves seen the rivers and regions, or learned of them from others. He therefore presents first the example of Parnassus which is in Asia at the winter rising [i.e., the southeast], and of the Caucasus, at the summer rising [north east]: since these are very large mountains, many and very large rivers rise from them. In Europe he gives the example of the Pyrenees located in the equinoctial setting [equatorial west], as well as of certain other mountains, which lie toward the north in Scythia: large rivers rise from them too. Then in Africa or in Libya he gives the example of certain other large mountains from which other great rivers flow. And he says the same is true of other mountains and rivers, and that, whatever other rivers flow from marshes, these marshes are located near mountains; and thus is the same thing true.

From these examples, therefore, he concludes to his proposition and repeats what was said above: thus the text is plain.

130. Then when he says, however, there is nothing impossible about the existence of such places (350b30) he refutes the argument of those who posit the opinion under discussion. And he says that it is not inadmissible that places should be found actually containing a vast amount of water after the manner of lakes: but this is not sufficient to account for the flow of rivers. For we can no more say that the waters which may exist collected under the earth or in the mountains, contain all the water of rivers, than we can say that the springs, which plainly appear outside the earth, actually contain all the water of rivers: for most rivers flow from springs (which he says, because some flow from marshes, as has been said). Hence to think that those subterranean collections of water contain the entire body of waters that flow through rivers is like supposing that those collections of water found outside the earth in springs contain all the water found in rivers. Hence, since it is plainly false to suppose this of the springs we can see, then it can be known by analogy that it is also false to suppose it of the collections of water under the earth. But that such collections of water do exist under the earth can be made manifest by the fact that many streams are absorbed by the earth. This he explains with many examples that are plain in the text.