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he himself conducted similar experiments and had pits dug in the earth on mountains, on
hillslopes, in bottom lands, in cultivated gardens, after long and heavy rains, but he never
found the earth moistened beyond a depth of 2 ft. Perrault next invokes the results from his
own sand column experiments described above (p. 175).
The second difficulty with this Common Opinion is, that I do not believe that the rains, which fall on the
high plains, suffice to maintain the springs, not because of their smallness...butbecause of the waste
& the loss of nearly everything which falls on these plains, without any of it benefiting the springs &
live fountains...Forbefore a certain quantity of water can traverse a certain quantity & thickness of
earth, all the particles of this earth must be moistened, each one in particular & with all their surfaces;
& this is a pure loss, for this water will only leave by evaporation, because of its adherent property,
which causes it to attach itself to everything it touches, and to stay there suspended without moving
downward, where its weight should normally attract it, as can be seen by our experiment.
With the Common Opinion disposed of, Perrault turns to the statement of Aristotle,
quoted earlier, that the volume of the yearly flows of the rivers is “...aslargeastheearth
in size, or at least not so much smaller.” Thus he will allow the reader to judge
. . . that these waters of the rivers will not equal the mass of the earth in one year, as he says, but even
in a thousand years.
Follows now Perrault's celebrated analysis of the comparison of the flow in the head-
waters of the Seine River in Burgundy with the rainfall on the upstream watershed. In
brief, he estimated the distance between the source of the river and Ainay le Duc (now
Aignay-le-Duc) as roughly 13.5 km (3 lieues) with an average distance to the divides on
either side of roughly 4.5 km (1 lieue); with an average annual precipitation estimated at
51.96 cm (19 pouces, 2.333 lignes), this made him conclude that the total annual volume
of precipitation over that area was of the order of 224 899 896 muids. (Units of length and
volume were not always standardized and they tended to vary in different periods and in
different regions; therefore it is not easy to check Perrault's calculations. However, since 1
muid equals 8 ft
3
, adopting the conversion that 1 ft is equivalent to 32.484 cm, one finds that
this volume is equivalent to roughly 6.167
10
7
m
3
; to obtain this volume with the 51.96
cm of precipitation requires the magnitude of the lieue (i.e. the league) in this calculation
to be about 4447.7 m. This result is remarkably accurate and shows that Perrault used the
“lieue de terre” (land league), which according to the
Petit Larousse
(1964) has a formal
length of 4445 m or 1
/
25 of a degree on a great circle.) He did not have any discharge
measurements for the Seine at Ainay le Duc, but by comparing the flow situation to that of
the Gobbelins River near Versailles, he guesses it to be about 36 453 600 muids per year,
which is roughly equivalent with 1.0
×
10
7
×
m
3
per year or 8.42 cm of annual rainfall.
This allows Perrault to conclude that
. . . only one sixth of the water which falls as rain and snow on the upstream catchment is needed to
make this river run continuously for an entire year...
and the remaining five sixths will serve to supply the losses, diminutions and wastes which
one observes, as nourishment of vegetation, evaporation and useless outflows. The case of
this one river also suggests that rain and snow should suffice for all the other rivers of the
world as well, provided one takes the wastes into account.
After thus having shown that the Common Opinion cannot possibly be correct, also
that the river flows are not as large as Aristotle had supposed and that the rains are more
than adequate to feed the rivers, Perrault (p. 207 ff.) is ready to formulate his views on the
origin of springs, the central topic of his treatise. In brief, water cannot penetrate the Earth
directly to any appreciable depth. As a result, most of the rain and snow waters, which fall
