From mythology to science: the development of scientific hydrological concepts in the Greek antiquity and its relevance to modern hydrology

Whilst hydrology is a Greek term, it has not been in use in the Classical literature but much later, during the Renaissance, in its Latin version, hydrologia. On the other hand, Greek natural philosophers created robust knowledge in related scientific areas, to which they gave names such as meteorology, climate and hydraulics. These terms are now in common use internationally. Within these areas, Greek natural philosophers laid the foundation of hydrological concepts and 10 the hydrological cycle in its entirety. Knowledge development was brought about by search for technological solutions to practical problems, as well as by scientific curiosity to explain natural phenomena. While initial explanations belong to the sphere of mythology, the rise of philosophy was accompanied by attempts to provide scientific descriptions of the phenomena. It appears that the first geophysical problem formulated in scientific terms was the explanation of the flood regime of the Nile, then regarded as a paradox because of the spectacular difference from the river flow regime in Greece and other Mediterranean 15 regions, i.e., the fact that the Nile flooding occurs in summer when in most of the Mediterranean the rainfall is very low. While some of the early attempts to explain it were influenced by Homer’s mythical view (archaic period), eventually, Aristotle was able to formulate a correct hypothesis, which he tested through what it appears to be the first in history scientific expedition, in the turn from the Classical to Hellenistic period. This confirms the fact that the hydrological cycle was well understood during the Classical period yet it poses the question why Aristotle’s correct explanation had not been accepted and, instead, 20 ancient and modern mythical views had been preferred up to the 18 century. ὁ βίος βραχὺς, ἡ δὲ τέχνη μακρὴ, ὁ δὲ καιρὸς ὀξὺς, ἡ δὲ πεῖρα σφαλερὴ, ἡ δὲ κρίσις χαλεπή (Life is short and Art long; the times sharp, experience perilous and judgment difficult.) Hippocrates, Aphorismi, 1.1. 25 歸根得旨 隨照失宗 (To return to the root is to find the meaning) Sengcan, Hsin Hsin Ming (Verses on the Faith Mind, translated by R.B. Clarke; original from https://www.sacred-texts.com/bud/zen/fm/fm.htm) https://doi.org/10.5194/hess-2021-7 Preprint. Discussion started: 12 January 2021 c © Author(s) 2021. CC BY 4.0 License.


Introduction -Ancient wisdom and its modern perception
In all ancient civilizations, the causes of natural processes, particularly the geophysical and hydrological, were attributed to 30 supernatural powers, usually deities. Mythological explanations have been very influential in triggering social behaviours but also in developing human skills, such as imagination and symbolism. In this respect, the rich Ancient Greek mythology has been inspiring in the arts and continues to be even in modern times. This is illustrated in Figure 1, depicting the mythological battle of Hercules, the well-known hero, against Achelous, a deity personifying the most important river of Greece. The three panels in the figure represent different arts, different aesthetic styles and different periods: 6 th century BC, 19 th century and 35 20 th century but with influences from the byzantine tradition.
able to teach that men have more teeth than women, when simple observation would have dispelled this idea. From a hydrological viewpoint, however, he had a more serious misconceptionhe believed that rainfall alone was inadequate to sustain the flow of rivers.
It is true that Plato (Figure 1) advocated the search for truth by reasoning as he regarded reasoning an important element 80 distinguishing what is and what is not science (see below)-and we do not have any hesitation to support this view of Plato.
However, all other information contained in this extract is mythology. In particular portraying Aristotle (Figure 3) as hating observation is absolutely absurd. A careful search in the literature reveals that this absurd idea about Aristotle, including the joke about women's teeth is not Price's (1989) but Bertrand Russell's (1952): Observation versus Authority: To modern educated people, it seems obvious that matters of fact are to be ascertained 90 by observation, not by consulting ancient authorities. But this is an entirely modern conception, which hardly existed before the seventeenth century. Aristotle maintained that women have fewer teeth than men; although he was twice married, it never occurred to him to verify this statement by examining his wives' mouths.
https://doi.org/10.5194/hess-2021-7 Preprint. Discussion started: 12 January 2021 c Author(s) 2021. CC BY 4.0 License. Now, what Aristotle has actually written is this: Males have more teeth than females in the case of men, sheep, goats, and swine; in the case of other animals observations have not yet been made […] The last teeth to come in man are molars called 'wisdom-teeth', which come at the age of about twenty years, in the case of both men and women. Cases  Which Authority is right, Aristotle or Russell? Perhaps both-but they have different perceptions of nature. Russell seems to have a purely deterministic view, in which a rule, norm or formula (in this case the formula of 32 teeth per person) holds universally † . Aristotle, who is not a determinist (cf. his theory on potentiality and actuality; see section 3), as clearly seen in 105 the above excerpt, trusts empirical Observation more-as evident in the extract. But what do we mean by Observation? Does information from school teachers, professors, books, TV, internet, model outputs, etc., classify as observation? In our view * Ἔχουσι δὲ πλείους οἱ ἄρρενες τῶν θηλειῶν ὀδόντας καὶ ἐν ἀνθρώποις καὶ ἐπὶ προβάτων καὶ αἰγῶν καὶ ὑῶν· ἐπὶ δὲ τῶν ἄλλων οὐ τεθεώρηταί πω.
[…] Φύονται δ' οἱ τελευταῖοι τοῖς ἀνθρώποις γόμφιοι, οὓς καλοῦσι κραντῆρας, περὶ τὰ εἴκοσιν ἔτη καὶ ἀνδράσι καὶ γυναιξίν. Ἤδη δέ τισι γυναιξὶ καὶ ὀγδοήκοντα ἐτῶν οὔσαις ἔφυσαν γόμφιοι ἐν τοῖς ἐσχάτοις. (Ἀριστοτέλης, Τῶν περὶ τὰ ζῶα ἱστοριῶν, 2.3.2 -2.4.1) † Russell does not provide information on how he knew whether or not Aristotle examined his two wives' teeth, nor whether or not he himself examined his four wives' teeth. By the way, we did not find it polite to examine our own wives' teeth, but this would be irrelevant. We know, of course, that each of the two of us has fewer than 32 teeth, while in the past one of us had 33, but again this does not enable any type of induction-for the latter we would need a large sample of observations. not-and real observation can hardly confirm the universal validity of a formula referring to the real world. Some modern studies that could support the idea that, what Aristotle wrote in the above excerpt is a result of observation, is contained in Appendix A. 110 After this necessary parenthesis on odontology, which has some epistemological interest, we return to hydrology, presenting another useful extract from Price (1989): The first person to make a forthright and unequivocal statement that rivers and springs originate entirely from rainfall appears to have been a Frenchman called Bernard Palissy, who put forward this proposition in 1580. Despite this, in the early 17 th century many workers were still in essence following the Greeks in believing that sea water was drawn 115 into vast caverns in the interior of the Earth, and raised up to the level of the mountains by fanciful processes usually involving evaporation and condensation. The water was then released through crevices in the rocks to flow into the rivers and so back to the sea.
A similar extract from Todd and Mays (2005) is this: As late as the seventeenth century it was generally assumed that water emerging from springs could not be derived 120 from rainfall, for it was believed that the quantity was inadequate and the earth too impervious to permit penetration of rainwater far below the surface. Thus, early Greek philosophers such as Homer, Thales, and Plato hypothesized that springs were formed by seawater conducted through subterranean channels below the mountains, then purified and raised to the surface. Aristotle suggested that air enters cold dark caverns under the mountains where it condenses into water and contributes to springs. 125 Finally, a recent text on the history of hydrology by Rosbjerg and Rodda (2019) contains the following: It was, however, not before the beginning of the 1500s that a scientific approach to hydrology started to take off, albeit with a very slow starting speed. Leonardo da Vinci undertook physical experiments, e.g. measuring stream velocity, to support his advanced thoughts about hydrology […]. In 1575, Bernard Palissy, based on observations in nature, claimed that springs originated from rain, and 100 years later, in 1674, Pierre Perrault measured the rainfall, runoff 130 and drainage area of the Seine River and concluded that rainfall was enough to support springs and rivers. The pathways, however, were not correctly described. In  At the general assembly of the IUGG in Rome in 1922, a delegate proposed a motion to form an additional section within the union to deal with the scientific problems in hydrology, such as "river-gauging, lake phenomena including seiches, run-off and evaporation, transport of material in suspension and in solution, glacier movement, etc." A https://doi.org/10.5194/hess-2021-7 Preprint. Discussion started: 12 January 2021 c Author(s) 2021. CC BY 4.0 License. committee was set up to give its opinion on the desirability of such a new activity. The committee gave favourable 140 advice and proposed that the new organism should be named Section of Scientific Hydrology. The adjective "scientific" was added to distinguish the section's participants from the 'charlatans and simpletons', who with the help of all sorts of rods tried to find water, calling themselves hydrologists, and also to make clear that the branch would not deal with the commercial exploitation of mineral waters.
In the following sections we will see that all above extracts contain useful information but also serious misinformation 145 about the history of hydrology. Our method, already illustrated above, it to retrieve the ancient documents in their original version and quote relevant extracts, rather than resort to what modern scholars have said about them. We will see that not only was the notion of the hydrological cycle known to ancient Greek scholars, but hydrology appeared in the cradle of science.
The first geophysical problem posed was hydrological: the explanation of the flooding of the Nile. The problem plagued scientists for almost three centuries before it was resolved by Aristotle. We will also trace the links of the developments in the 150 early modern period (after the Renaissance) with the ancient thinkers, including Aristotle and Hippocrates; it is the strong link with the latter and the health aspects of water that dictated the adjective "scientific" in hydrology in the beginning of the 20 th century. In other words, the need to distinguish it from the 'charlatans and simpletons' (Rosbjerg and Rodda, 2019) does not correspond to reality-unless one characterizes medical doctors as such, which hopefully is not the case.
But before we proceed to the ancient and early modern developments in hydrology, is useful to find the origin for the 155 misunderstanding of what ancient Greek science actually was. After some search in classical Greek texts, we suspect that the culprit is Plato and the misunderstanding stems from the following passage from his Dialogue Phaedo: Interestingly, in this excerpt Zeus is responsible for the rainfall process, the most complex and most difficult to understand.
All other transformations of water throughout the hydrological cycle are natural. As we will see in next sections, others have completely expelled Zeus and other gods from the entire hydrological cycle. 175 The critics of Plato with respect to his scientific views should be aware that he was the author of the first work in history about epistemology, i.e., his Dialogue Theaetetus, and the first who tried to define science (ἐπιστήμη) per se therein:

2
Hydrology at the birth of science Natural philosophy and science start with Thales of Miletus (Figure 4), one of the Seven Sages of Greece and the father of the 185 Ionian philosophical school. (Ionia was located at the western coast of Asia Minor by the Aegean Sea, which was inhabited by Greeks from ancient times till 1922 AD). As a philosopher is famous for the foremost importance he gave to water as a natural element, as well as for several apothegms. § As a scientist he is known for his contribution in several areas, i.e.: • Mathematics. He introduced deduction through theorems; he proved several theorems in geometry, including those bearing his name: the Thales' angle theorem and interception theorem. 190 • Astronomy. He predicted the solar eclipse in 28 May 585 BC.
• Physics. He studied static electricity by experimenting on amber (in Greek ήλεκτρον-electron) as well as magnetism.
• Surveying engineering. He measured the heights of pyramids and the distance of ships from the shore.
• Hydraulic engineering. He made a diversion of the river Halys for military purposes.
His contribution to hydrology is less known but it is important as he formulated for the first time in history a hydrological 195 behaviour as a scientific problem, thus highlighting the importance of hydrology in the cradle of science. The problem is the so-called paradox of the Nile and, as we will see in section 4, the solution he gave is clearly wrong. Yet the important development is that he formulated the problem in scientific terms, expelling the divine element from natural processes. Anaximander (c. 610 -c. 546 BC), who succeeded Thales in Miletus, is the first to dare write a book «Περί Φύσεως» ("On Nature"; lost), rejecting mythological and religious views. He understood the relationship of rainfall and evaporation: the winds arise when the air becomes partially condensed and is lifted up; and when it comes together and more condensed, clouds are generated, and thus a change is made into water. And hail is produced when the water precipitating from the clouds freezes; and snow is generated when these clouds, being more moist, acquire congelation; and lightning is caused when the clouds are parted by force of the winds; […]. And a rainbow is produced from solar 210 rays falling on condensed air. * (Hippolytus, Refutation of All Heresies, Ι, 6).
The entire hydrological cycle was described by Xenophanes (c. 570 -478 BC), another Ionian philosopher, who supported his theory by the discovery of fossilized marine organisms at three island locations. Hippolitus (c. 170-235 AD; Christian theologian) attributes to him a theory of alternating periods of flood and drought. Xenophanes expressed his philosophy in poetic form (hexameters, elegies, iambics), as in the following fragment: 215 The sea is the source of water and the source of wind; for neither in the clouds <would there be nor any blasts of wind blowing forth> from within, without the mighty sea, nor river flows nor rain water from the sky. The mighty sea is father of clouds and of winds and of river. † (Fragment B 30, recovered from Geneva Scholia on Homer.) Hydrology is the science of change and randomness; Heraclitus (Figure 5) described the nature of each in just a few words, using the metaphor of flow in the first case and of dice in the second case: 220 Πάντα ῥεῖ (Everything flows) (Heraclitus; quoted in Plato's Cratylus, 339-340) Time is a child playing, throwing dice ‡ (Heraclitus; Fragment 52)  Interestingly, the former aphorism has become the emblem of the current hydrological decade (Montanari et al., 2013). The latter symbol, the dice, has been used by other famous aphorisms such as by Julius Cesar and by Einstein. Einstein expressed (in a less poetic manner) exactly the opposite view; however, the recent developments in physics seem to vindicate Heraclitus.
Anaxagoras of Clazomenae ( Figure 6) was another Ionian philosopher who proved to be very influential in history. As he moved to Athens and taught there for about 30 years, he transplanted the ideas of Ionian philosophers to Athenians, having 230 prominent students such as Pericles, Euripides, Sophocles, and Herodotus. He proposed a theory of "everything-ineverything," and was the first to give a correct explanation of eclipses. While his scientific theories were mostly related to astronomy, including the claims that the sun is a mass of red-hot metal and the moon is earthy, they also include hydrology: The rivers receive their contents from the rains and from the waters in the earth; for the earth is hollow and has water in its hollow portions. * (Hippolytus, Refutation of All Heresies, Ι, 7). 235 This may seem as an historical paradox because it is a dry and infertile place. The paradox have been explained by the Athenian Thucydides (Figure 7), father of scientific history, who observed that infertility has also a good side and scarcity may be preferable to abundance: The richest soils were always most subject to this change of masters; such as the district now called Thessaly, Boeotia, most of the Peloponnese, Arcadia excepted, and the most fertile parts of the rest of Hellas. The goodness of the land 245 favoured the aggrandizement of particular individuals, and thus created faction which proved a fertile source of ruin.
It also invited invasion. Accordingly Attica, from the poverty of its soil enjoying from a very remote period freedom from faction, never changed its inhabitants. And here is no inconsiderable exemplification of my assertion that the migrations were the cause of there being no correspondent growth in other parts. The most powerful victims of war or faction from the rest of Hellas took refuge with the Athenians as a safe retreat; and at an early period, becoming 250 naturalized, swelled the already large population of the city to such a height that Attica became at last too small to hold them, and they had to send out colonies to Ionia. * (Thucydides, The Peloponnesian War, 1.2.3-6.)

255
Among the philosophers who lived and taught in Athens, Aristotle has been the most influential in subsequent developments of philosophy and science, including hydrology; therefore, we devote to him the entire section 3. Among those who lived in other places of Greece we should mention Hippocrates ( Figure 8) who lived in the island of Kos. He is often referred to as the father of medicine, but, as we will see, his contribution to the ancient and modern hydrology through his treatise On Airs, Waters, Places is not negligible. From this treatise we quote the following passage, in which he describes the 260 hydrological cycle: * μάλιστα δὲ τῆς γῆς ἡ ἀρίστη αἰεὶ τὰς μεταβολὰς τῶν οἰκητόρων εἶχεν, ἥ τε νῦν Θεσσαλία καλουμένη καὶ Βοιωτία Πελοποννήσου τε τὰ πολλὰ πλὴν Ἀρκαδίας, τῆς τε ἄλλης ὅσα ἦν κράτιστα. διὰ γὰρ ἀρετὴν γῆς αἵ τε δυνάμεις τισὶ μείζους ἐγγιγνόμεναι στάσεις ἐνεποίουν ἐξ ὧν ἐφθείροντο, καὶ ἅμα ὑπὸ ἀλλοφύλων μᾶλλον ἐπεβουλεύοντο. τὴν γοῦν Ἀττικὴν ἐκ τοῦ ἐπὶ πλεῖστον διὰ τὸ λεπτόγεων ἀστασίαστον οὖσαν ἄνθρωποι ᾤκουν οἱ αὐτοὶ αἰεί. καὶ παράδειγμα τόδε τοῦ λόγου οὐκ ἐλάχιστόν ἐστι διὰ τὰς μετοικίας ἐς τὰ ἄλλα μὴ ὁμοίως αὐξηθῆναι· ἐκ γὰρ τῆς ἄλλης Ἑλλάδος οἱ πολέμῳ ἢ στάσει ἐκπίπτοντες παρ᾽ Ἀθηναίους οἱ δυνατώτατοι ὡς βέβαιον ὂν ἀνεχώρουν, καὶ πολῖται γιγνόμενοι εὐθὺς ἀπὸ παλαιοῦ μείζω ἔτι ἐποίησαν πλήθει ἀνθρώπων τὴν πόλιν, ὥστε καὶ ἐς Ἰωνίαν ὕστερον ὡς οὐχ ἱκανῆς οὔσης τῆς Ἀττικῆς ἀποικίας ἐξέπεμψαν. (Θουκυδίδης, Ἱστορία τοῦ Πελοποννησιακοῦ Πολέμου, 1.2.3-6.) https://doi.org/10.5194/hess-2021-7 Preprint. Discussion started: 12 January 2021 c Author(s) 2021. CC BY 4.0 License. Rain waters, then, are the lightest, the sweetest, the thinnest, and the clearest; for originally the sun raises and attracts 265 the thinnest and lightest part of the water, as is obvious from the nature of salts; for the saltish part is left behind owing to its thickness and weight, and forms salts; but the sun attracts the thinnest part, owing to its lightness, and he abstracts this not only from the lakes, but also from the sea, and from all things which contain humidity, and there is humidity in everything; and from man himself the sun draws off the thinnest and lightest part of the juices. In another passage, he expresses (in addition to the link of water and wine) the relationship of spring water temperature and depth of its origin: The best [waters] are those that flow from high places and earthy hills. By themselves they are fresh and clear, and the wine they can stand is but little. In winter they are warm, in summer cold. They would naturally be so, coming from very deep springs. Apparently, the reference to "warm" and "cold" should be read relative to the environmental temperature as Hippocrates did not have an instrument to measure temperature in objective terms. Today we measure temperature to infer the depth.
Compared to modern knowledge, that contained in the above extracts of the ancient philosophers is incomplete and sometimes erroneous. This is normal as scientific knowledge is a result of endless and torturous process. It is not revelation knowledge like in religion. 285

Aristotle
Aristotle was student of Plato, but his theories were influenced by Ionian philosophers. Instead of continuing in Plato's Academy, he founded his own school, known as the Lycaeum or the Peripatetic school (Περιπατητική, meaning "by walking about"). His theories expand to all aspects of knowledge and are relevant not only in his period but throughout the entire history of science, including the recent period. Science and the Scientific Method owes him basic notions on research and laws on 290 inference, sometimes referred to as Aristotelian Logic, exposed in his six books that are collectively known as the Organon, as well in his book Metaphysics. These includes the laws of identity (Prior Analytics † ), excluded middle and noncontradiction (Metaphysics ‡ ) and the distinction of deduction (παραγωγὴ, ἀπόδειξις) and induction (ἐπαγωγὴ). Furthermore, the principle of parsimony (also known as Ockham's razor) is expressed in at least three Aristotle's books (Posterior Analytics, On the Heavens, Nicomachean Ethics) § . 295 Another concept introduced by Aristotle that has acquired great importance in modern science, particularly in physics and stochastics, is his dipole potentiality (δύναμις, Latin potentia) vs actuality (ενέργεια, Latin actualitas), formulated in his books Physics, Metaphysics, Nicomachean Ethics and De Anima. The first who utilized the dipole in modern science, namely in quantum physics, has been Heisenberg (1962) The most important of these [features of the interpretation by Bohr, Kramers and Slater] was the introduction of the 300 probability as a new kind of "objective" physical reality, the "potentia" of the ancients such as Aristotle; it is, to a certain extent, a transformation of the old "potentia" concept from a qualitative to a quantitative idea. This Heisenberg's idea was quoted by Popper (1982), who fully incorporated it in his philosophical system, further extending it to claim, for example, that "Both classical physics and quantum physics are indeterministic". More recently this Aristotelian dipole has been proposed by several scientists and philosophers, independently of Popper, as a simpler, more comprehensible 305 and more effective interpretation (Jaeger 2017(Jaeger , 2018Kastner et al. 2018;Driessen 2019;Sanders 2018). In particular, Kastner et al. (2018), building on Heisenberg's (1962) idea, propose an ontological dualism of actualities (res extensa) and potentia (res potentia), with the latter not bounded by spacetime constraints and being transformed to the former by an acausal process.

Now coming to Aristotle's proposals that focus on hydrological processes, we should first mention his treatise
Meteorologica which offers a great contribution to the explanation of hydrometeorogical phenomena. As we know, the entire 310 hydrological cycle is based on the phase change of water, which Aristotle understood in this way: We maintain that fire, air, water and earth are transformable one into another, and that each one potentially exists in the others, as all have a single common underlying substratum, in which are ultimately resolved. * (Meteorologica, I.1, The sun causes the moisture to rise; this is similar to what happens when water is heated by fire. † (ibid., II.2, 355a 15.) 315 The vapour that is cooled, for lack of heat in the area where it lies, condenses and turns from air into water; and after the water has formed in this way it falls down again to the earth; the exhalation of water is vapour; air condensing into water is cloud ‡ (ibid., I.9, 346b 30.) In addition, he recognized the principle of mass conservation within the hydrological cycle: Thus, [the sea] will never dry up; for [the water] that has gone up beforehand will return to it § (ibid., II.3, 356b 26.) 320 Even if the same amount does not come back every year or in a given place, yet in a certain period all quantity that has been abstracted is returned ** (ibid., II.2, 355a 26.) Furthermore, Aristotle penetrated into the concept of change. He was fully aware that the Earth changes through the ages and that rivers are formed and disappear in the course of time: * φαμὲν δὴ πῦρ καὶ ἀέρα καὶ ὕδωρ καὶ γῆν γίγνεσθαι ἐξ ἀλλήλων, καὶ ἕκαστον ἐν ἑκάστῳ ὑπάρχειν τούτων δυνάμει, ὥσπερ καὶ τῶν ἄλλων οἷς ἕν τι καὶ ταὐτὸν ὑπόκειται, εἰς ὃ δὴ ἀναλύονται ἔσχατον. (Μετεωρολογικά, Α1, 339a,b.) † ἔτι δ' ἡ ὑπὸ τοῦ ἡλίου ἀναγωγὴ τοῦ ὑγροῦ ὁμοία τοῖς θερμαινομένοις ἐστὶν ὕδασιν ὑπὸ πυρός. (αὐτόθι, Β2, 355a 15.) ‡ συνίσταται πάλιν ἡ ἀτμὶς ψυχομένη διά τε τὴν ἀπόλειψιν τοῦ θερμοῦ καὶ τὸν τόπον, καὶ γίγνεται ὕδωρ ἐξ ἀέρος· γενόμενον δὲ πάλιν φέρεται πρὸς τὴν γῆν. ἔστι δ' ἡ μὲν ἐξ ὕδατος ἀναθυμίασις ἀτμίς, ἡ δ' ἐξ ἀέρος εἰς ὕδωρ νέφος. But if rivers are formed and disappear and the same places were not always covered by water, the sea must change 325 correspondingly. And if the sea is receding in one place and advancing in another it is clear that the same parts of the whole earth are not always either sea or land, but that all changes in course of time. * (ibid., I.14, 353a 16.) In the Introduction we stressed the importance given by Aristotle on observation. He also conducted experimentation.
In the following passage he explains that he found by experiment that the salt contained in water is not evaporated:

Salt water when it turns into vapour becomes drinkable [freshwater] and the vapour does not form salt water when it 330
condenses again; this I know by experiment. † (ibid., II.3, 358b.) This has certainly found technological application in desalination (removal of salt from sea water), useful in a country with scarcity of fresh water and many shores and islands. Thus, we learn from a commentary on Aristotle's Meteorologica II, written by Olympiodorus the Peripatetic (a 5 th -century philosopher), that:

Sailors, when they labour under a scarcity of fresh water at sea, boil the sea-water, and suspend large sponges from 335 the mouth of a brazen vessel, to imbibe what is evaporated, and in drawing this off from the sponges, they find it to be
The Nile paradox and its solution by Aristotle As already mentioned in the Introduction, the flooding of the Nile has been the first geophysical problem posed in scientific terms. The problem plagued scientists for almost three centuries before it was resolved by Aristotle but it took much more 340 before this correct explanation was generally accepted by the scientific community. What was regarded as a paradox was the different hydrological regime compared to other Mediterranean rivers: Nile floods occur in summer. Figure 9 illustrates the reasons why it was regarded a paradox using modern data of the Nile flows on monthly scale, along with monthly precipitation data at stations in the wider area.

360
Herodotus's spirit to seek physical explanations for natural phenomena, which reflects the more general trend developed in Greece after Thales, is contrasted here with the Egyptian people's attitude (including their priests) who seem to have been uninterested for physics. Subsequently, Herodotus describes three explanations given by Greeks, without mentioning their names, but only their ambition to achieve reputation for wisdom: Some of the prominent Greeks, however, wishing to get a reputation for wisdom, have offered explanations of the 365 phenomena of the river, for which they have accounted in three different ways. Two of these I do not think it worth while to speak of, further than simply to mention what they are. * (ibid. 2, 20.) The first explanation is this: One says that the Etesian [i.e. monsoon] winds cause the rise of the river by preventing the Nile-water from running off into the sea. But in the first place it has often happened, when the Etesian winds did not blow, that the Nile has risen 370 according to its usual wont; and further, if the Etesian winds produced the effect, the other rivers which flow in a direction opposite to those winds ought to present the same phenomena as the Nile, and the more so as they are all * ἀλλὰ Ἑλλῄνων μὲν τινὲς ἐπίσημοι βουλόμενοι γενέσθαι σοφίην ἔλεξαν περὶ τοῦ ὕδατος τούτου τριφασίας ὁδούς· τῶν τὰς μὲν δύο τῶν ὁδῶν οὐδ᾽ ἀξιῶ μνησθῆναι εἰ μὴ ὅσον σημῆναι βουλόμενος μοῦνον. (αὐτόθι, 2, 20.) https://doi.org/10.5194/hess-2021-7 Preprint. Discussion started: 12 January 2021 c Author(s) 2021. CC BY 4.0 License. smaller streams, and have a weaker current. But these rivers, of which there are many both in Syria and Libya, are entirely unlike the Nile in this respect. * (ibid. 2, 20.)

He continues: 375
The second opinion is even more unscientific than the one just mentioned, and also, if I may so say, more marvellous.
It is that the Nile acts so strangely, because it flows from the ocean, and that the ocean flows all round the earth. [ Information about who supported each of the three explanations has later been given by later authors, e.g., Aetius, the 1 st -or 2 nd -century AD doxographer and Eclectic philosopher. Interestingly, the first explanation is attributed to Thales, which 395 verifies our claim about the strong link of hydrology with science (or natural philosophy), at the dawn of the latter: Thales thinks that the Etesian winds [monsoons], blowing straight on to Egypt, raise up the mass of the Nile's water through cutting off the outflow by the swelling of the sea coming against it. * (Aetius IV, 1, 1).
The second view was supported by Euthymenes of Massalia (Εὐθυμένης ὁ Μασσαλιώτης; fl. early 6 th century BC), a Greek explorer from Massilia (Marseille), who explored the coast of West Africa. The third seems to have been supported by 400 Anaxagoras and in another version by Democritus (460-370 BC).
Herodotus does not accept any of the three explanations and proceeds to give his own: Perhaps, after censuring all the opinions that have been put forward on this obscure subject, one ought to propose some theory of one's own. I will therefore proceed to explain what I think to be the reason of the Nile's swelling in the summer time. During the winter, the sun is driven out of his usual course by the storms, and removes to the upper parts 405 of Libya. This is the whole secret in the fewest possible words; for it stands to reason that the country to which the Sungod approaches the nearest, and which he passes most directly over, will be scantest of water, and that there the streams which feed the rivers will shrink the most. † (Herodotus, The Histories, 2, 24).
Apparently, all explanations are wrong. Yet two of them, the first and the third, are scientific, while the second is mythical and Herodotus's one contains mythical elements and a belief of a flat Earth. 410 Modern knowledge of the hydrological regime of Nile's basin, illustrated in Figure 11 by means of graphs of monthly flow and precipitation at several sites, clearly shows that the origin of floods are the high precipitation rates in the Blue Nile in Ethiopea, driven by monsoons and peaking in July and August.
Was any ancient philosopher able to find a correct explanation? In particular, what was the opinion of Aristotle, who lived a century after Herodotus? Here comes another puzzle, which seems to have been resolved very recently. The reason for 415 such delay is the fact that most of the Greek texts, which certainly contained relevant information, have been lost. Alexandria's library was accidentally burned by Romans at least twice (by Julius Cesar and Aurelian) and perhaps redestroyed by Arabs (Caliph Omar). The Imperial Library of Constantinople was destroyed in 1204 by the knights of the Fourth Crusade and again in 1453 the Fall of Constantinople, invaded by Ottoman Turks, was accompanied by destruction of the city's libraries.
Among the manuscripts that were saved, one Patriarch Photius's (c. 810/820 -893) Myriobiblon or Biblioteheca, 420 composed of 279 reviews of books which he had read. This book, perhaps the first in history collection of book-reviews, written in Greek, was printed in 1611with Latin translation (Figure 12). One of the books reviewed is a lost one entitled Life of Pythagoras by an anonymous author. The book contained important information about Aristotle's decisive contribution in solving the Nile paradox, which Photius summarizes as follows:    It is reminded that Alexander (Figure 13) was student of Aristotle and was exchanging letters with him (and his mother Olympias), addressing his as professor (καθηγητὴν) during his campaign to Asia and Africa. Therefore, the information contained in the latter extract is not implausible. In our view this is very important information as it describes the first scientific expedition in history in order to confirm a scientific hypothesis. Confirmation of the truth of the story is provided by other ancient authors, such as Proclus (Πρόκλος, 412 -485 AD; Neoplatonist philosopher), John the Lydian (Ἰωάννης Λαυρέντιος ὁ Λυδός; a 6 th -century Byzantine administrator) and Cleomedes (Κλεομήδης, astronomer who lived sometime between the mid-1st century BC and 400 AD): Topography.
In addition to these references in Greek, there has been a treatise in Latin entitled Liber Aristotelis de Inundacione Nili, (in short De Nilo) which is presumably a Latin translation of a lost text in Greek by Aristotle, whose Greek tile should be Περὶ τῆς τοῦ Νείλου ἀναβάσεως: * The treatise was left out of Corpus Aristotelicum and received little scholarly attention. However, Rose (1886) published the full Latin script of De Nilo, while an improved transcription thereof has been recently published by 485 Beullens (2014). There have been also translations of the work in two modern languages, French (Bonneau, 1971) and Dutch (Beullens, 2011). Some recent developments support the case that it is a translation of a genuine text by Aristotle or at least contains some portions of an original work by the philosopher (Beullens, 2014). The new evidence is: (a) the publication of a papyrus (P. Oxy. 4458), which was shown to contain a short quotation from the original Greek text of De Nilo (Jakobi and Luppe, 2000)

and (b) the observation that the quotation by Anonymus Florentinus almost literally follows the wording of De 490
Nilo, if it be back translated to Greek (Beullens, 2014).
De Nilo has the form of an Aristotelian problem, starting with the question to be solved:

How can it be explained that while other rivers swell in winter and become much smaller in summer, the Nile as the
only river that flows into the sea, in the summer expands over a vast area and become so wide that only the villages stand out as islands? † (Liber Aristotelis de Inundacione Nili, 1, translation by authors based on Google translation of 495 the Dutch text by Beullens, 2011.) The text continues with what we would call today literature review, enumerating the explanations already given by other authors about the phenomenon (including those referred to by Herodotus) and then rejecting them one by one with logical arguments, until it remains one, Aristotle's own theory, as precisely quoted by Anonymus Florentinus.
Overall, there is overwhelming evidence that Aristotle had resolved the paradox in scientific terms. However, it is 500 relevant to ask the question: How long did it take for the scientific (and wider) community to assimilate and completely accept this scientific truth? The surprising answer to this question is: 21 centuries. Thrasyalces the Thasian (one of the early physicists), and Thrasyalces from someone else, and he from Homer, who calls the Nile "heaven-fed": "And back again to the land of Aegyptus, heaven-fed river." (Strabo, Geography, 17.1.5, translated by H.L. Jones). * Here we may remark that by attributing the explanation to Thrasyalces (an old natural philosopher, probably of the 5th century 510 BC, from the island of Thasos), Srtabo devalues Aristotle's contribution, hiding the fact that, even if Thrasyalces had indeed made the same conjecture, there is a big difference as Aristotle verified the hypothesis by observation (ὄψει-by sight) through Alexander's expedition. Furthermore, Strabo seems to equate all explanations, eventually matching the Aristotle's scientific one with Homer's mythological.
And indeed, the mythological views are more charming and, hence, they continued to be popular during the Roman 515 times. The Roman epicurean philosopher Lucretius (c. 99c. 55 BC) and the stoic philosopher Seneca (4 BC -65 AD), both of whom wrote about Nile, did not rely on Aristotle's scientific explanation. Rather, they were fascinated by the Nile for its mystery, not its demystification. An excellent summary of the reasons is contained in the following quotation by Merrills (2017): The metaphysical qualities of the Nile-a river that replicated each year the origins of the world, and which overspilled 520 its banks even into the bathhouses and taverns of Pompeii-were essential to its resonance in the Roman world.

The reference to Pompei encapsulates the archaeological evidence of sacred objects and iconographies for Nile and its waters.
And what about modern times? Were the mythical views abandoned after the first quantification of the hydrological cycle in the 17 th century (section 6)? This question is studied in detail in Appendix 5. In brief, the surprising answer is that a new mythology was developed around a "theory" of the "nitre" which was a mythical element that presumably caused the 525 flooding of the Nile, while rainfall in Ethiopia had a minor role, if any. It took the visit to the origins of the Blue Nile of the Scottish traveller James Bruce and the publication of his book (Bruce, 1813) for the modern mythical theory to cease.

Prominent scientists of the Hellenistic period with relevance to geosciences and hydrology
The Hellenistic period, which starts with the death of Alexander in 323 BC and ends with the emergence of the Roman Empire in 31 BC, is marked by the wide dissemination of the Greek civilization and the flourishing of science. During this period 530 several important scientific developments and breakthroughs had occurred, some of which were not accepted as consensus theories for centuries. The reluctance to Aristotle's theory on Nile is repeated in several other cases. *  Aristarchus of Samos (Ἀρίσταρχος ὁ Σάμιος; c. 310c. 230 BC; mathematician and astronomer), introduced the heliocentric model for the solar system 1800 years before Copernicus. He also said that the stars were distant suns and made calculations on the relative sizes of the Sun, Earth and Moon. Notably, before him also the Pythagorean philosopher Philolaus 535 (c. 470c. 385 BC) had moved the Earth from the center of the cosmos and made it a planet, but in Philolaus's system Earth does not orbit the Sun but rather a central fire. Interestingly, Copernicus in the manuscript of his book De revolutionibus included a citation to Philolaus and Aristarchus but he crossed it out before publication (Figure 14). The point that was crossed out, translated in English (Gingerich, 1973(Gingerich, , 1985, reads:

And if we should admit that the motion of the Sun and Moon could be demonstrated even if the Earth is fixed, then with 540 respect to the other wandering bodies there is less agreement. It is credible that for these and similar causes (and not because of the reasons that Aristotle mentions and rejects), Philolaus believed in the mobility of the Earth and some even say that Aristarchus of Samos was of that opinion. But since such things could not be comprehended except by a keen intellect and continuing diligence, Plato does not conceal the fact that there were very few philosophers in that
time who mastered the study of celestial motions. 545 While Aristarchus's ideas were contrary to "consensus theory" for 1800 years, it is important to notice that they were 550 adopted by Archimedes (c. 287c. 212 BC), the leading scientist (mathematician, physicist, engineer, inventor and astronomer) of the Hellenistic world, who is regarded to be perhaps the greatest mathematician of all time * . In fact, as his treatise The Sand Reckoner provides the most precious information about Aristarchus's ideas. Specifically, Archimedes writes: * This is illustrated by the fact that the Fields Medal (regarded as the highest honour for mathematicians) depicts Archimedes. The reader may be find as food for thought about the history of civilization the fact that the head of Archimedes in the medal is synthesized by the imagination of the artist (Tropp, 1976), as there is original sign about it, neither in sculpture nor in coins (that is the reason we do not included any illustration about him in this paper). It has been speculated (Vardi, 1997) that Archimedes chose, among different cosmological theories, Aristarchus's for the single reason that it was the one yielding largest size of the universe-as he wanted that size as large as possible for his 560 calculations of big numbers. However, we believe that a mind of the calibre of Archimedes would not choose a theory on this basis and certainly would not consider it if he thought it was erroneous.
It is well known that Archimedes offered several important contributions in mathematics, including the concept of infinitesimals and a first version of integral calculus. From the hydrological perspective, important is the principle named after him and the foundation of hydrostatics. From his inventions most relevant to hydrology is Archimedes' screw, which is still 565 in wide use for pumping.
While some early Greek philosophers believed that the Earth is flat, Pythagoras and later Aristotle provided arguments that it is round. Now,Eratosthenes (Ἐρατοσθένης,c. 276c. 195/194 BC;a mathematician,geographer,poet,astronomer, and music theorist; head of the Library at Alexandria), among other achievements, calculated with remarkable accuracy (<2.5%) the Earth's circumference by measuring, at the noon of the day of summer solstice, the shadow cast by a gnomon at 570 Alexandria and the distance between and Alexandria and Syene, where the latter is situated close to the Tropic of Cancer.
Eratosthenes also calculated, in following the windings of the Nile, the distances between several points on the Nile up to Meroe (Strabo, Geography, 17.1.2;Rawlins, 1982). Perhaps because of this, he has often been credited by several authors (including Koutsoyiannis, 2014) for solving the paradox of the Nile. However, in view of the information provided here (section 4), his achievement seems to be no more than a further verification of Aristotle's theory. He also seems to have been 575 aware of the earlier expedition to the Nile sources for the purpose of proving Aristotle's theory (Burstein, 1976). Despite the advancements in geography during the Hellenistic period, the achieved geographical representation of the Earth was rather poor (Figure 15).
He studied the notion of pressure and pneumatics and invented a steam machine. He introduced the term hydraulic (organ) for a musical instrument operated by hydraulics (ὑδραυλικὸν ὄργανον), which he describes in his book Pneumatica (Πνευματικὰ; 595 Schmidt, 1899, p. 192, "Ὑδραυλικοῦ ὀργάνου κατασκευὴ";Woodcroft, 1851, p. 105). His contribution to hydrology is that he introduced the concept of discharge and its measurement. Here is the relevant passage from his book Dioptra (Διόπτρα): Given a spring, to determine its flow, that is, the quantity of water which it delivers. One must, however, note that the flow does not always remain the same. Thus, when there are rains the flow is increased, for the water on the hills being in excess is more violently squeezed out. But in times of dryness the flow subsides because no additional supply of water 600 comes to the spring. In the case of the best springs, however, the amount of flow does not contract very much. Now it is necessary to block in all the water of the spring so that none of it runs of at any point, and to construct a lead pipe of rectangular cross section. Care should be taken to make the dimensions of the pipe considerably greater than those of the stream of water. The pipe should then be inserted at a place such that the water in the spring will flow out through it. That is, the pipe should be placed at a point below the spring so that it will receive the entire low of water. Such a 605 place below the spring will be determined by means of the dioptra. Now the water that flows through the pipe will cover a portion of the cross-section of the pipe at its mouth. Let this portion be, for example, 2 digits [in height]. Now suppose that the width of the opening of the pipe is 6 digits. 6 × 2=12. Thus, the flow of the spring is 12 [square] digits. It is to be noted that in order to know how much water the spring supplies it does not suffice to find the arca of the cross section of the flow which in this case we say is 12 square digits. It is necessary also to find the speed of flow, for the swifter is 610 the flow, the more water the spring supplies, and the slower it is, the less. One should therefore dig a reservoir under the stream and note with the help of a sundial how much water flows into the reservoir in a given time, and thus calculate how much will flow in a day. It is therefore unnecessary to measure the arca of the cross section of the stream.
Interestingly, he published his book anonymously in 1674 in French, as well as an extended abstract in English (Anonymous, 1675), but a few years later the book was republished with his name (Perrault, 1678), while more recently a full translation in English appeared (Perrault, 1967). In its first part, constituting about half of the book, he critically reviews other philosophers, ancient Greek (Plato, Aristotle, Epicurus), Roman (Vitruvius, Seneca, Pliny), medieval (Thomas Aquinas) and early modern 640 (Scaliger, Cardano, Agricola, Dobrzenski, van Helmont, Lydiat, Davity, Descartes, Gassendi, François the Jesuit, Palissy and others). In particular he appears to disagree with Vitruvius, Gassendi, François and Palissy, whose ideas he refers to as the Common Opinion (l'Opinion Commune). In the second part he presents his measurements, calculations and theories. Referring to the River of Seine, his final result is this: So that there needs but the sixth part of the Rain and Snow-water that falls in a year, to run continually through the

Continuation of the Author's opinion. 650
After having rejected the Common Opinion, after having shown that the water which flows in the Rivers for a year is not so considerable as Aristotle and those who followed him imagined, and that the rains can provide sufficient water to maintain their course for a year, it only remains for me to show how the waters of the rain and the snow that have fallen in the Rivers, can come out through the top of the mountains to make springs. * (Perrault, 1678, p. 207, translation by authors based on Google Translate.) 655 This is puzzling as in fact Aristotle's theory on the Nile was exactly this, i.e., that rainfall in another area (Ethiopia) was provided the water to sustain the flow (actually flood) of the Nile.
One interesting observation is that none of the celebrated publications of all these pioneers, namely da Vinci (1828) (1687) and Dalton (1802), contains the term hydrology. This raises the question, how and when did this term appear? The question is studied in full detail in Appendix C 660 and the findings can be summarized in the following points: • The term hydrology is Greek (ὑδρολογία from ὕδωρ = water and λόγος = reason) but not ancient Greek.
• Most probably it appeared for first time in its French variant, hydrologie, in 1614 in a book of medical and philosophical orientation (Landrey, 1614), following the Hippocratic approach on the relationship of water and health. 665 • It further appeared in other books of the 17 th and 18 th centuries mostly in Latin but also in modern languages and mostly with medical and philosophical orientation, but also chemical, mineralogical and physical.
• In the end of the 18 th century and during the 19 th century, the domain covered by the term hydrology is expanded to include natural sciences (physics, meteorology, climatology), geography and hydraulics.
• In the end of the 19 th century, an international congress of hydrology and climatology was held in Biarritz, France, 670 in which hydrology was divided in two branches, medical hydrology and scientific hydrology; key persons of that congress were medical doctors but there was also one explorer and geographer, and one meteorologist.
IASH and its domain are described in the following extract from Lyons (1922) Later, at the XV IUGG General Assembly in Moscow in 1971, the Association replaced the term scientific hydrology in its name with the unfortunate term hydrological sciences (in plural).
On the other hand, the branch of medical hydrology continued to exist but with a declining activity. Today there still 685 exist university departments (e.g. the Department of Medical Hydrology of the Complutense University of Madrid, 1912today) * , as well as national and international organizations (e.g. the International Society of Medical Hydrology and Climatology † each year convening in World Congresses ‡ , yet no so populated and rich in activity as their "scientific" hydrological counterparts).
In the meantime, specifically in the 1960s, hydrology (without an adjective) acquired a clear definition as a science 690 (UNESCO, 1963(UNESCO, , 1964: This definition complemented an earlier one by the US Ad Hoc Panel on Hydrology (1962), adding an essential element, the 695 interaction of water with human activity. This definition, however, does not explicitly recognize the link of hydrology with hydraulic engineering (despite the fact that it was this very link that advanced it as a modern quantitative scientific discipline; Koutsoyiannis, 2014) nor with health issues (despite the facts exhibited above). It is probable that in the future such links would be reinstated, particularly after the importance given recently on health issues. However, those colleagues who may propose new sciences linking water with engineering or with health, should be aware that such links are as old as Thales and 700 Hippocrates. * In the University of Athens there existed a Chair of Clinical Hydrology andClimatotherapy (1938-1953), while the Greek Rheumatology Society had been earlier named Greek Society of Rheumatology and Hydrology (Ελληνική Εταιρεία Ρευματολογίας και Υδρολογίας) † http://www.ismh-direct.net/info.aspx?sp=1 ‡ http://www.ismh-direct.net/hirek.aspx?s=0&archiv=1 https://doi.org/10.5194/hess-2021-7 Preprint. Discussion started: 12 January 2021 c Author(s) 2021. CC BY 4.0 License.

Epilogue
Scientific theories are mostly wrong. It is a matter of time for any theory to be replaced by a better one. Naturally, most of the theories developed in the dawn of science (2600 years ago) have been replaced. This does not make them unscientific.
It is a good practice to study the history of science, recognize the past contributions and give credit to those who made 705 them. This necessitates consulting original texts as citations by later authors, particularly to the works of the greatest minds, may distort the original meaning. And there is a lot of distortion, accompanied with remarkable arrogance, about the contribution of ancient scientists in geophysics-and hydrology in particular. Certainly, the ancient theories contain elements that are blatantly incorrect, according to modern knowledge, but these do not justify treating them with arrogance. Here we preferred to highlight the more correct elements, which justify our respect and admiration. 710 The study of the history of the development of scientific ideas is useful as it reveals the effectiveness of thought and logic, which were the basic tools of ancient philosophers, in compiling a sensible world vision with some admirable elements, even though other elements are inconsistent according to modern knowledge. As the information provided here shows, in addition to thought and logic, observation, experimentation and measurement were all used by ancient philosophers, particularly by Aristotle. 715 As evident from present day terminology (meteorology, climate, hydraulics), modern science is not independent from the ancient one. Advances of the Greek antiquity have been particularly seminal for the modern science after the Renaissance.
The above discourse may be useful to learn several lessons that are pretty relevant in our times. First, it is useful to have in mind that, in accordance to Plato's definition quoted in the Introduction, scientists are "lovers of the vision of truth". The importance of seeking the truth is also highlighted by Aristotle in the following quotations: 720 Socrates is dear [friend], but truth is dearest. * (Ammonius, Life of Aristotle.) Still perhaps it would appear desirable, and indeed it would seem to be obligatory, especially for a philosopher, to sacrifice even one's closest personal ties in defense of the truth. Both are dear to us, yet it is our duty to prefer the truth . † (Aristotle, Nicomachean Ethics 1096a11).
A second lesson, perhaps not obvious from our discourse, is that it takes courage to formulate scientific theories -now 725 as well as then. A relevant extract is the following, by Plutarch: The first man to put in writing, most clearly and most courageously of all, the explanation of the moon's illumination and darkness, was Anaxagoras. But he was no ancient authority, nor was his account in high repute. It was still under seal of secrecy, and made its way slowly among a few only, who received it with a certain caution rather than with confidence. For people did not tolerate the natural philosophers and stargazers, as they were then called, because they 730 reduced the divine agency down to unreasoning causes, blind forces, and necessary incidents. Even Protagoras was exiled, Anaxagoras was imprisoned and with difficulty rescued by Pericles, and Socrates, though he had nothing whatever to do with such matters, nevertheless lost his life because of philosophy. * (Plutarch, Nicias, 23; translation by I. Velikovsky in Anaxagoras. † ) Note that Anaxagoras was charged of impiety and he was sentenced to death by the Athenian court. He avoided this penalty 735 by leaving Athens, and he spent his remaining years in exile. From Plutarch's information we may infer that Anaxagoras enjoyed the gratitude of his student Pericles. Similar is the relationship of Aristotle and his student Alexander the Great. This, however, does not happen all the time in history. (A remarkable counterexample is the contribution of Kolmogorov, Alexandrov and other students of Egorov, to convict their mentor likely to death-an attempt which was prevented by intervention of Kapitsa and ultimately by a decision of Stalin; Graham and Kantor, 2009). 740 Courage is a necessary condition for formulating scientific theories but it does not suffice for the acceptance of the theories, even if they are correct. This is a third lesson, which also suggests that the dilemma posed by Russel, Observation vs Authority (see Introduction), is not principal-not even the more relevant dilemma Scientific Truth vs. Authority. Rather, the above discourse points to a more characteristic dilemma, Scientific Truth vs. Public Acceptance. This is both diachronic and also very modern. The case of Aristotle's correct theory on the Nile flooding, which was also confirmed by observation through 745 the first scientific expedition in history, is the most characteristic. Neither the fact that Aristotle was an Authority, nor the backing of the theory by Observation helped acceptance of the theory. Aristarchus's heliocentric model is another similar case.
Interestingly, even before Vansleb's trip and book, the French physician and philosopher Marin Cureau de la Chambre (1594 -1669) published a book citing other travellers to Egypt (e.g., the Venetian physician and botanist Prospero Alpini or 785 Prosper Alpinus; 1553 -1617), in which he adopts the cause of floods by this mysterious dew, but also introduces the nitre (also referred to as niter in some English books) "theory" (de la Chambre, 1665). His book was also presented in Britain with a summary published in the Philosophical Transactions (Oxford), from which we quote the following (Anonymous, 1665- He further proceeded to contend: The learned Cambraeus as cited by Gassendus [Petrus Gassendi;1592-1655 thinks this fermentation to be caused by Niter, wherewith the Country and especially the Channel of the River is acknowledged to abound, which being heated 820 by the Sun, thus dilates it self and makes the River to swell. From a systematic search in Philosophical Transactions, it turns out that there was at least one scholar who opposed those theories: the Dutch manuscript collector Isaacus Vossius (Isaac Voss;1618-1689. His book written in Latin (Vossius, 1666) was also presented in the Transactions (Anonymous, 1665-1666a,c), where from the latter we quote this: [Vossius] easily gives an account, why the Nile yearly overflows about the end of June: For, as at that time there falls 825 much rain in AEthiopia, it must needs be, that the Nile, whose source is in that Country, should then overflow, when those rains begin, and subside, when they cease.
Interestingly, Vossius's (1666) view is not far from Aristotle's and he quotes Greek authors to support it, namely, Cleomedes, Nonnosus and Cosmas Indicopleustes (see section 4).
However, Vossius's view remained unpopular. According to Garnier (1892), during the 18 th century: 830 The learned societies of France, England, and Germany recognised the nitrous salt in the fertilising essence of Nile water, dung, snow, rain water, and other real or imaginary manures; and the whole scientific world extolled in extravagant terms the virtues of a compound the true nature of which it had as yet failed to grasp. This is reflected in the popularity of the use of the word nitre in books written in the languages of these three countries. As shown in Figure B1 this name is not in use any more, but in the 18 th century its use had peaked, exceeding those of the word 835 Nile.
https://doi.org/10.5194/hess-2021-7 Preprint. Discussion started: 12 January 2021 c Author(s) 2021. CC BY 4.0 License. Complete dismissal of these imaginative theories would require additional modern evidence and thus it had to wait a 840 century, up to the end of the 18 th century. Then, James Bruce (1730 -1794), a Scottish traveller and travel writer who visited North Africa and Ethiopia, including the origins of the Blue Nile, wrote with a modest tone of irony (Bruce, 1813)

Appendix C: The appearance of the term hydrology
Several terms related to hydrology appear in ancient Greek literature, which are etymologized from the noun ὕδωρ (hydor, water). Specifically: • The conveyance of water or liquids is termed ὑδραγωγία (ἡ) and a person (or device) related to it ὑδραγωγός. 860 • The actions of drawing, fetching or distributing water are termed ὑδρεία, ὕδρευσις and ὑδροπαροχία; a person related to them is termed ὑδροπάροχος and a guard or inspector of aqueducts or irrigation works ὑδροφύλαξ.
• The action or art of seeking or discovering water is termed ὑδροσκοπία, ὑδροσκοπική or ὑδροφαντική (verb: ὑδροσκοπέω); a person related to it is ὑδρόσκοπος, ὑδρογνώμων or ὑδροφάντης and a related instrument is ὑδροσκόπιον. 865 These, however, have not been transplanted to the international scientific or technological vocabulary, where words of Latin origin (e.g. aqueduct) dominated. On the other hand, the following Greek terms have become global: • The modern term ὑδραυλικἡ (hydraulics) stems from ὑδραυλικὸν ὄργανον (hydraulic organon), first used by Hero for a musical instrument operated by hydraulics. Earlier, Ctesibius (Κτησίβιος; fl. 285-222 BC) invented the instrument called ὕδραυλις (ἡ) (hydraulis), which is played by a musician called ὑδραύλης (hydraules). Its etymology stems from 870 ὕδωρ (water) and αὐλός (aulos; pipe, flute, clarionet). Thus, the term hydraulics was not introduced by Robert Boyle (1627-1691), as commonly written (Biswas, 1970, p. 225), but almost two millennia earlier.
• The term climate stems from κλίμα (meaning the inclination angle of the incoming sunbeams; pl. κλίματα); a property pertaining to κλίμα is κλιματικός.
Hydrology is also a Greek word, i.e., ὑδρολογία (feminine noun transliterated in Latin as hydrologia), but it does not appear in the ancient Greek literature. * The closest match it contains is ὑδρολόγιον (hydrologion, a noun in neuter gender), 880 which however is a water-clock. Its plural, ὑδρολόγια, is transliterated in Latin as hydrologia, precisely the same as the transliteration of ὑδρολογία (notice that in Greek there is a difference in the location of the accent). Among the first books published after the invention by Gutenberg of mechanical printing press, was the Lexicon of Festus (in Latin), typically dated to the 2nd century, with original title De Verborum Significatione (On the Meaning of Words). This does not contain the term hydrologia, but commentaries on it published several years after do. Thus, this term appears in the book of Commentaries on 885 de Verborum Significatione by three famous interpreters (Alciatus, Brechaeus, Fornerius, 1589, p. 10) but from the context it becomes clear that it is plural of hydrologion (or hydrologium in Latin). It also appears with the same meaning in an encyclopaedic collection of mathematical curiosities by Bettinus (1642).
According to our own search in digital archives of old books, the first containing the term hydrology in its French version, hydrologie, is the book by Landrey (1614). Other books whose title (or subtitle) contains the term hydrology, published in the 890 17 th through 19 th century, are listed in Table C1 and illustrated in Figure C1 to Figure C12. It appears that the main orientation of those books was medical. At the end of the 19 th century an international congress of hydrology and climatology was held at Biarritz, France (in Bay of Biscay close to the Spanish borders) as reported by Symons (1887), in which a distinction was made between medical hydrology and scientific hydrology. The key persons of that congress, shown in Figure Table C1. Books published in the 17 th through 19 th century whose title (or subtitle) contains the term hydrology (or the equivalent term in another language).

doctor) (Landrey, 1614). From the title page it becomes clear that the book is about the virtue and power of medicinal waters (la vertu & puissance des eaues médicinales).
In the first pages the author declares that he follows the doctrine of the philosopher to begin with the genus and proceed to the species, while he quotes Pindar's verse ὕδωρ ἄριστον (l'eau tres bone; water is best; the exact quotation is ἄριστον μὲν ὕδωρ, Pindar, Olympian Odes, 1).     (Monnet, 1772). In addition to the quality of potable water, it examines the sea water and the natural salts (sels naturels). Notable is the spelling "hydraulogie" (likely influenced by hydraulics) in the first page, also used throughout the entire book, which is different from that in the book cover, "hydrologie".