Geology

Perrault

THE SOURCE OF WATER IN SPRINGS AND RIVERS

Translated from De l’origine des fontaines, pp. 196–207, Paris, 1674.

If the springs and rivers are engendered, as Aristotle says, from air condensed and resolved into water in the caverns of the earth, that is to say, as Lydiat explains it, from vapor that exhales its humidity when it is heated: and if this humidity comes from rain which it absorbs, as Aristotle says in another place, there must then be sufficient rain to give the earth enough moisture to make the vapor which can supply water to the springs and rivers throughout the entire year. In this case, according to his opinion, rain water must not only equal the greatness of the earth but greatly surpass it, since it is true that that water is subjected to the great waste which we have observed.

Concerning the idea expressed in all ancient and modern philosophy, I believe there is greater probability in attributing the source of springs and rivers to rain and snow water than there is in attributing it solely to internal distillation in the earth; and that common sense will never prefer a medium as obscure as this distillation, whose consequences seem rather feeble, to a medium as apparent as the rains, whose consequences are so great and so well-known. But as these reasons only make for the destruction of the contrary opinion, we must try to give other reason s which can establish what I maintain and show that rain water is sufficient to make springs and rivers run throughout the entire year.

Although it is no more necessary for me to prove this affirmation than for those who make objection to prove their negative, one being as difficult as the other, I shall nevertheless attempt to do so, by making rough estimates of the quantity of rain relative to the basins of rivers. . . .

. . . It is necessary for the success of our plan, to measure or estimate the water of some river as it flows from its source to the place where some stream enters it and to see if the rain water which falls around its course, being put in a reservoir, as Aristotle said, would be sufficient to make it run a whole year. I have chosen the Seine River and have examined it with sufficient precision in its course from its source to Aynay le Duc, where a stream enters and enlarges it: that is why I shall take it as the subject for the examination which I wish to make.

The course of this river from its source to Aynay le Duc is about three leagues, and the lands along its course extend to right and left about two leagues on each side, where there are other streams which go elsewhere; granting that those streams must have rain water for their subsistance as well as the Seine, I want to consider only half of this region of slopes and to say that the basin that the Seine occupies from its source to Aynay le Duc is three leagues long and two leagues wide, and consequently I make the following deductions.

If a reservoir of this length and breadth were made, it would be six square leagues in area, which reduced to toises,^* following the measurements previously accepted, would be 31,245,144 square toises of surface.

In this reservoir one must imagine that during a year there has fallen from the rain a height of

poulces, which is the height of an ordinary year, as we have observed. This height of poulces gives 224,899,942 hogsheads of water or thereabouts, following the measure on which we have agreed.

All this water thus collected must serve to keep this river running during a year, from its source to the place we have indicated, and must serve also to provide for all that it must lose for the nourishment of trees, plants, and herbs, and for evaporation, and for useless drainage into the river, which only enlarges it for a time and while it rains, and for wandering streams which may take another course than to this river because of irregular and contrary slopes, and other such wastes, losses, and impairments.

It would be difficult to ascertain exactly the measurement or estimation of the water of this small river, and to tell what quantity it contains. Nevertheless, as far as I can judge, it cannot have more than 1000 or 1200 poulces of water constantly running, balancing the least that it has at its source with the most that it has at Aynay le Duc. . . .

All these things thus supposed, I say that following the measures on which we have agreed, 1200 poulces of water, on the basis of 83 hogsheads of water for a poulce, amounts in twenty-four hours to 899,600 hogsheads of water; during a year, which is 366 times more, this will total 36,453,600 hogsheads. The quantity of water flowing between the banks of this river from its source to Aynay le Duc during a year amounts only to that quantity of 36,453,600 hogsheads. But if I draw this quantity of water from 224,899,942 hogsheads, which are in this reservoir we have just imagined, there will remain 188,446,342 hogsheads, which amounts almost to five times more, and which serves to provide for the losses, diminution, and wastes which we have noticed. Therefore, only about the sixth part of the rain and snow water is necessary to make this river run continually throughout the year.

I well know that this deduction has no accuracy; but who could give one which would be precise? I believe, however, that it ought to be more satisfactory than a simple negative like Aristotle’s or the premise of those who hold, without knowing why, that it does not rain enough to furnish the flow of rivers. At any rate, until someone makes more exact computations, by which he proves the contrary of what I have advanced, I shall remain convinced and shall content myself with this feeble light which gives me the observation that I have made, not having anything greater.

If then this water suffices for one river, it will suffice for all the other rivers of the world in proportion, considering especially the large margin for waste and the small area for the basin and course of the river which I allow. This is only a league on each side, for rivers are not usually closer to each other than two leagues. There is therefore some reason for saying that rain and snow water are adequate for all the rivers in the world.

I might say that there are regions where it rains very rarely, and others where it does not rain at all, which nevertheless contain fairly large streams. But the streams of these countries, where it rains only occasionally, are not continual; they are large only in winter and they dry up almost entirely in the summer. Because they are near high mountains, from which they come, abundant snow which fails on these mountains and which afterwards melts there can fill their beds as long as it lasts. When the supply is exhausted, they are left to the dryness of summer.

There are hardly any countries in the world where it never rains. The torrid zone, where this is more nearly true than anywhere else, is watered abundantly twice a year, possibly more than France is in the summer, and at least with greater abundance at certain times. But when we speak of regions where it never rains, we do not deny the possibility that there are large rivers there which may have their sources in other regions where it does rain, as, for example, the Nile, which flows through Egypt where it does not rain. There are countries in the world which do not produce wine, where not much of it could be produced, and business and commerce bring it from afar; similarly the great rivers make a kind of commerce of their waters to irrigate the provinces not watered ordinarily from heaven.

^* A toise is about six feet.—EDITORS.