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14.2.2 Concerning Water on Earth
The first lesson concerns general features and dimensions of the globe, reviewing
how the human race, in moving beyond a geocentric vision of the universe, progressively discovered new lands and oceans and determined how to represent them on
maps. Secchi points out how the continents are disproportionately smaller than the
oceans (“sproporzione della terra coi mari”) and speculates on their shapes, such as
the “pointed edges” in their southern parts.
One can find in this lesson several examples of his surprisingly good intuition.
For example, relying only on contemporary information about bathymetry and
almost a century before the theory of plate tectonics, he perceived the importance of
the mid-Atlantic ridge (the underwater mountain range that divides the Atlantic
Ocean, emerging only occasionally such as at the Cape Verde archipelago). The
same can be seen in his discussion of the deep crustal roots of mountain chains,
which he compared to obelisks standing above high pedestals, long before modern
theories of isostatic compensation. “The cause is unknown,” he concludes, adding
with sharp insight: “the explanation of these facts lies in the history of those events
that for centuries led up to the state of current affairs, and which need further study.”
4
As we know, only in the mid-twentieth century did geology find a solution to those
“puzzles” by gradually reconstructing the long geological evolution of continents
and using modern technologies to explore the ocean bottom.
The next four lessons concern terrestrial water. In the second lecture, Secchi
describes the geomorphological action of rainwater on the Earth’s surface (Fig. 14.2),
namely how water in its different physical forms destroys continents and, driven by
gravity, transports and accumulates material to the ocean bottom.
His method of describing this action is another instance of Secchi’s creative
mind and his capacity to go further than the ordinary concepts of his time and cultural environment. In order to explain the lithogenic cycle (or at least, the part of the
cycle that science at that time could understand), Secchi chose a practical example:
a journey along the Val d’Ossola, in the Piedmont sector of Western Alps. Describing
runoff into a catchment basin, he notes how erosion of Alpine granites brings pebbles and sand downstream to Lake Maggiore and then, carried along the Po River,
the sediments eventually reach the Adriatic and finally the Mediterranean Sea. In
this way the ultimate fate of mountains is sealed: “all the great mountain ranges will
be leveled and will disappear into the sea as so much mud” ("tutte le grandi catene
montuose saranno livellate e spariranno in tanta fanghiglia in seno al mare"). The
lesson ends with a discussion of our dynamic Earth, a system changing continuously through geological eras: “This gives us an impressive idea of nature’s action:
slow, but constant and continuous (...) which, just as the precession of the equinoxes
changes the face of the heavens over the centuries, so does the face of the Earth
4 La causa è ignota … la spiegazione di questi fatti risiede nella storia di quelli eventi che da secoli
prepararono lo stato delle cose attuali, e che dobbiamo investigare.
A. Argentieri and M. Parotto
14.2.2 Concerning Water on Earth
The first lesson concerns general features and dimensions of the globe, reviewing
how the human race, in moving beyond a geocentric vision of the universe, progressively discovered new lands and oceans and determined how to represent them on
maps. Secchi points out how the continents are disproportionately smaller than the
oceans (“sproporzione della terra coi mari”) and speculates on their shapes, such as
the “pointed edges” in their southern parts.
One can find in this lesson several examples of his surprisingly good intuition.
For example, relying only on contemporary information about bathymetry and
almost a century before the theory of plate tectonics, he perceived the importance of
the mid-Atlantic ridge (the underwater mountain range that divides the Atlantic
Ocean, emerging only occasionally such as at the Cape Verde archipelago). The
same can be seen in his discussion of the deep crustal roots of mountain chains,
which he compared to obelisks standing above high pedestals, long before modern
theories of isostatic compensation. “The cause is unknown,” he concludes, adding
with sharp insight: “the explanation of these facts lies in the history of those events
that for centuries led up to the state of current affairs, and which need further study.”
4
As we know, only in the mid-twentieth century did geology find a solution to those
“puzzles” by gradually reconstructing the long geological evolution of continents
and using modern technologies to explore the ocean bottom.
The next four lessons concern terrestrial water. In the second lecture, Secchi
describes the geomorphological action of rainwater on the Earth’s surface (Fig. 14.2),
namely how water in its different physical forms destroys continents and, driven by
gravity, transports and accumulates material to the ocean bottom.
His method of describing this action is another instance of Secchi’s creative
mind and his capacity to go further than the ordinary concepts of his time and cultural environment. In order to explain the lithogenic cycle (or at least, the part of the
cycle that science at that time could understand), Secchi chose a practical example:
a journey along the Val d’Ossola, in the Piedmont sector of Western Alps. Describing
runoff into a catchment basin, he notes how erosion of Alpine granites brings pebbles and sand downstream to Lake Maggiore and then, carried along the Po River,
the sediments eventually reach the Adriatic and finally the Mediterranean Sea. In
this way the ultimate fate of mountains is sealed: “all the great mountain ranges will
be leveled and will disappear into the sea as so much mud” ("tutte le grandi catene
montuose saranno livellate e spariranno in tanta fanghiglia in seno al mare"). The
lesson ends with a discussion of our dynamic Earth, a system changing continuously through geological eras: “This gives us an impressive idea of nature’s action:
slow, but constant and continuous (...) which, just as the precession of the equinoxes
changes the face of the heavens over the centuries, so does the face of the Earth
4 La causa è ignota … la spiegazione di questi fatti risiede nella storia di quelli eventi che da secoli
prepararono lo stato delle cose attuali, e che dobbiamo investigare.
A. Argentieri and M. Parotto