Geoscience Reference
In-Depth Information
It is remarkable that, at the dawn of the nineteenth century, these are still essentially the
opinions which were being discussed among the Presocratics and Aristotle more than 2300
years earlier. Dalton proceeds then to show that the experimental disproof of the first opinion
by de La Hire (1703), was in fact invalid and unwarranted, and this leads him finally to
conclude (p. 371) as follows.
The origin of springs may still therefore be attributed to rain, till some more decisive experiments
appear to the contrary; and it becomes unnecessary to controvert the other two opinions respecting this
subject.
14.5
CLOSING COMMENTS
This previous quotation was probably the last time that any other “opinions” on the
origin of springs were brought up in the scientific literature. Still, although the debate
on the main issue was closed, details of this “rainfall percolation” continue to be the
subject of enquiry to this day, as seen in Chapter 11. In any event, Dalton's (1802a)
analysis was a sign that the time was ripe for the rapid developments in the nineteenth
century, that laid the foundations for the emergence of hydrologic science in its present
form. For instance, it was Dalton (1802b) who introduced next several of the principles
on which modern evaporation theory is based (see Brutsaert, 1982, p. 31). He proposed
the law of partial pressures in gases and he determined the saturation vapor pressure of
water as a function of temperature; he was then the first to express surface evaporation
as a mass transfer equation nearly in its current form, and in recognition of this the
mass transfer coefficient is still called the Dalton number. Fundamental developments
by others followed in rapid succession throughout the nineteenth century, and several of
the highlights are mentioned in the previous chapters of this topic. But most of this is
well-trodden terrain in the history of science, so there is no need to repeat the details.
Among the more striking facts of this historical sketch is that, while humans were
able to grasp the essence and the significance of the atmospheric phase of the water cycle
very early in prehistoric times, a full understanding of the origin of springs and streams
took much longer.
The perceptions and opinions of those who commented on the movement of water
in nature, were usually strongly affected by the specific hydrologic conditions in their
immediate environment. Some of the early civilizations developed in rather arid and
semi-arid climates, where rain, springs and streamflow were not always abundant, so
that the linkages of the terrestrial water cycle were not very obvious. A case in point
is the eastern Mediterranean region, where karst phenomena are ubiquitous and play a
pronounced role. In this perspective many of the early concepts, such as the underground
Tartaros or Abyss of Homer and Plato, and the caves of Anaxagoras and Aristotle, can
be explained and are not as far-fetched as a superficial review might suggest. Similarly,
to Thales or to the writer of Ecclesiastes, who must have known about underground
seawater intrusion near the coast or in the Nile delta, the seawater filtration mechanism
would not have been unreasonable.
The concept that finally survived, the rainfall percolation mechanism, is not a recent
invention. In recorded history it can be followed as a thread running through the works of
