| 62 This View of Life |
Natural History 5/97 |
|
be found at all for this anomalous motion, the analogy between microcosm
and macrocosm offers the only reasonable hope for a solution:
Clearly it would seem that the whole surface of the oceans, when
not affected by the tempest, is equally distant from the center of the
earth, and that the tops of the mountains are as much farther removed
from this center as they rise above the surface of the sea. Unless therefore
the body of the earth resembled that of man, it would not be possible
that the water of the sea, being so much lower than the mountains, should
have power in its nature to rise to the summit of the mountains. We
must therefore believe that the same cause that keeps the blood at the
top of a man's head keeps water at the summit of the mountains.
(From the A Manuscript in the Institut de France.)
But to state a need is not to find a mechanism. Throughout the Codex
Leicester, Leonardo struggles to discover a physically workable way
to raise water within the earth. He tries and rejects several explanations,
as Martin Kemp documents in "The Body of the Earth," an article
written for the catalog to the recent display of the Codex Leicester.
Perhaps, Leonardo first argues, the heat of the sun draws water up through
the veins (internal streams) that run through mountains. (Leonardo, in
his strongest image of a living earth, had written in the Codex Leicester:
"The body of the earth, like the bodies of animals, is interwoven
with a network of veins, which are all joined together, and formed for
the nutrition and vivification of the earth and of its creatures".)
But he then decides that this explanation cannot work for two reasons--first,
because, on the highest mountaintops closest to the heating sun, water
remains cold and even icy; and, second, because this mechanism should
operate best in summer during maximal solar heat, but mountain streams
often flow with lowest output at this time.
In a second try, Leonardo turns to the earth's internal heat and a process
of distillation: perhaps the interior fires boil water in internal caverns,
and this water rises as vapor through mountain interiors, where it reverts
to liquid form and bursts through as a high spring. But this proposal
won't work either because such extensive distillation would require that
the roofs of internal caverns be wet with the rising steam--but they
are often bone dry. Leonardo then made a feebler third attempt: perhaps,
by analogy to a sponge, mountains somehow suck up water to a point of
saturation and subsequent oozing from the top. But Leonardo realizes that
he cannot cash out this analogy in mechanical terms:
If you should say that the earth's action is like that of a sponge
which, when part of it is placed in water, sucks up the water so that
it passes up to the top of the sponge, the answer is that even if the
water itself rises to the top of the sponge, it cannot then pour away
any part of itself down from this top, unless it is squeezed by something
else, whereas with the summits of the mountains one sees it is just
the opposite, for there the water always flows away of its own accord
without being squeezed by anything.
But if Leonardo, to his great disappointment, never solved the problem
of rising waters, he did (to his satisfaction) crack the equally knotty
problem of a general mechanism for the elevation of earth--a combination
of his views on gravity and his concept of erosion. (I struggled with
Leonardo's complex mix of idea for many days--a mélange of
Scholastic theories of gravity and the earth, mainly vouchsafed to Leonardo
by Jean Buridan through the books of Albert of Saxony, and of Leonardo's
conjectures on composition of the earth's interior combined with observations
on our planet's surface--but I am now confident that I grasp the argument
and can present a crisp epitome.)
Our planet has a geometric center, called by Leonardo the "center
of the world"--or sometimes the "center of the universe,"
for Leonardo accepts the Ptolemaic system of a central earth and a revolving
sun. The realm of liquid water must arrange itself as a perfect sphere
about this center, with the surface of the ocean equidistant at all points
from the center of the world. If the solid earth were homogeneous and
equally distributed, it would also form a smooth sphere with a surface
equidistant at all points from the center of the world.
But the heavy earth is far from homogeneous. The interior of our planet
is a complexly marbled mass of solid earth, liquid water running through
veins in the rocks, and even air, where water has hollowed out caverns
in the rocks. Therefore, as a result of this unequal distribution of earth,
one hemisphere will always be heavier than the other.
N ow, the planet also has a center of mass (called by
Leonard, in a terminology that we would not use today, a "center
of gravity"). On a homogeneous planet, this center of gravity
will coincide with the geometric center of the world. But on our actual
planet
with one hemisphere heavier than the other, the "center of gravity"
will lie below the geometric center and within the heavier hemisphere.
Yet the planet must strive, as a living body seeking balance, to bring the
center of gravity closer to the geometric center. The earth pursues this
goal in a manner known from time immemorial to all riders on seesaws (the
Codex Leicester contains a picture of such a seesaw, albeit for a
different purpose). To balance a seesaw, the heavier person must move toward
the fulcrum at the center, while the lighter person must move away. In exactly
the same manner, the solid masses of the heavier hemisphere must sink toward
the center of the world, while the rocks of the lighter hemisphere must
rise. The emergence of mountains from the seas, and the consequent placement
of marine fossils on high hills, records this rising of land in the earth's
lighter hemisphere.
Leonardo succinctly describes the general |