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of these waters penetrate through underground conduits to beneath the mountains; from
there the heat, which is in the earth, raises them as vapor to the peaks, where they replenish
fountains and rivers. Seawater moving through sand becomes fresh because the salty parts,
which are larger, more rigid and interlaced, cannot follow the tortuous paths around the
sand grains as easily as the more slippery and smaller fresh water parts, and they are left
behind.
14.4 From philosophy to science by experimentation
In the course of the seventeenth century the general approach to science started to change,
and gradually experimentation became an essential part of it. Pierre Perrault (1608-1680)
and Edme Mariotte (1620-1684) were two central figures at this juncture of the history of
hydrology. Their main merit was that, in contrast to the earlier writers on the subject, both
relied on experiment and quantitative arguments. But to put their work in proper context,
it is necessary to bear in mind the various opinions on the causes and mechanisms of river
runoff, as they were then known to them.
14.4.1 The Common Opinion at the end of the seventeenth century
The topic
On the Origin of Springs
by Perrault (1674) can provide some insight in this;
the first half of it, covering 146 pages, is devoted to a thorough review of the better-known
theories and explanations of the day. The authors discussed by Perrault are Plato, Aristo-
tle, Epikouros, Vitruvius, Seneca, Pliny, Thomas de Aquino, Scaliger, Cardano, Agricola,
Dobrzenski, Van Helmont, Lydiat, Davity, Descartes, Papin, Gassendi, Du Hamel, Schot-
tus, Rohault, Francois and Palissy. For each of these authors Perrault first gives a brief
description of the main features of the propounded theory, followed by his own critique and
reasons for rejection. After completing the survey, he then singles out one of these theories
and further specifies (p. 148) how those, who support this particular view,
. . . believe that the waters of the rains & of the melted snows, which fall on the earth, penetrate it until
they encounter heavy (lit. greasy) soil or some other matter, which stops them; whereupon they flow
to some opening on the slope of a mountain...Theybelieve that the waters, which fall on the high
plains, are the cause of the springs, by means of this penetration, which they assume (to take place)....
They believe that the rains, which fall on the slope of hills, are lost & of no use for the springs, for the
reason that from there they fall into the rivers which carry them to the sea . . . They also believe that
it is the springs, which being joined together produce rivers, & that if there weren't any springs, there
wouldn't be any rivers.
This description of the sequence of processes, which is elaborated on further on pp. 151-
152, could have been written today, and it would not be out of place among the descriptions
reviewed in Chapter 11. It is remarkable, therefore, that in 1674 Perrault calls this the
“Opinion Commune” or “Common Opinion.” But even more remarkable is the fact that
he also points out that among his 22 “authors,” by which he means the learned men and
authorities on the subject, only four espoused this opinion, to wit Vitruvius, Gassendi,
Palissy and Francois. In other words, although only a small minority among the expert
natural philosophers held this view, he chooses to call it the Common Opinion. Could this
mean that toward the end of the seventeenth century, almost everyone else, that is the person
“in the street,” was already of the opinion that springs and rivers are produced by rainfall
percolation?
