|
We
are pleased to present the following
excerpt from the book
An Ocean of Air:
Why the Wind Blows and Other Mysteries
of the Atmosphere
by Gabrielle Walker
Harcourt - August
2007
Chapter 1
The
Ocean Above Us
Nearly four hundred years ago, in a
patchwork of individual fiefdoms that we
now call Italy, a revolution of ideas was
struggling to take place. The traditional
way to understand the workings of the
world -- through a combination of divine
revelation and abstract reasoning -- had
begun to come under attack from a new
breed. These people called themselves
"natural philosophers," because the word
"scientist" had not yet been invented. To
find out the way the world worked, they
didn't sit around and talk about it. They
went out and looked. This was not an
approach that was likely to find favor
with the Church, home of received wisdom,
or with its instruments -- the whispering
Inquisitors, with their hotline back to
Rome. Now, a certain natural philosopher
had fallen very foul of those Inquisitors
and been forced to stop his investigations
into the structure of the heavens. His
name was Galileo Galilei, and our story
begins with him.
Convent of Minerva, Rome
June 22, 1633
I, Galileo Galilei, son of the late
Vincenzo Galilei, Florentine, aged seventy
years, arraigned personally before this
tribunal, and kneeling before you, most
Eminent and Reverend Lord Cardinals,
Inquisitors general against heretical
depravity throughout the whole Christian
Republic . . . have been pronounced by the
Holy Office to be vehemently suspected of
heresy, that is to say, of having held and
believed that the sun is the center of the
world and immovable, and that the earth is
not the center and moves:
Therefore, desiring to remove from
the minds of your Eminences, and of all
faithful Christians, this strong
suspicion, reasonably conceived against
me, with sincere heart and unfeigned faith
I abjure, curse, and detest the aforesaid
errors and heresies . . . and I swear that
in the future I will never again say or
assert, verbally or in writing, anything
that might furnish occasion for a similar
suspicion regarding me.
As the great Galileo rose from his
knees at the end of this infamous, and
forced, recantation, he is said to have
muttered "Eppur si muove!" ("And
yet it moves!"). He knew in his heart that
Earth moves around the sun, in spite of
what the Inquisitors had made him say.
Still, devoutly religious as he was, he
had no taste for defying his own church.
Nor had he any desire to share the fate of
the unfortunate monk Giordano Bruno, who a
few decades earlier had been publicly
burned for holding similar views. Galileo
may have been the most famous philosopher
in all Italy, but he knew that in itself
wouldn't save him from the fire.
And though he was now seventy years
old, frail, and steadily losing his sight,
he was not yet ready to die. He had
damaged his eyes by staring through a
telescope at wonders he himself had
discovered: blemishes that appeared
periodically on the surface of the sun;
craters on the moon; distant but distinct
moons circling the planet Jupiter (who
would have thought that other planets
could have moons of their own?), and stars
that nobody knew existed. Now, before the
cataracts and glaucoma finally clouded his
sight, in secret, if necessary, he had one
last task to complete. Galileo had seen
this "trial" coming; he'd known for some
time that he couldn't continue his study
of the heavens. So for some years he had
been discreetly changing tack, turning his
attention inwards to Earth itself. And,
failing eyesight notwithstanding, he was
about to change the way we see the most
apparently ordinary substance in the
world: air.
The Inquisitors knew nothing of this.
They were satisfied with his recantation,
and decided, graciously, to spare his
life. He would be allowed to return to his
villa at Arcetri in Florence, though he
should understand that he was still
considered dangerous and would therefore
be held under house arrest. There would be
no visitors, save those given prior
permission by the Church. Meanwhile,
Galileo himself was to spend his time
reciting the holy psalms as penance, and
praying for his immortal soul.
Galileo returned to his villa as
instructed and performed his penance
diligently. But the Inquisitors had also
obliged him to swear never again to
publish work that might offend the Holy
Office, and he had no intention of
complying. For with him to Arcetri he had
taken a certain manuscript that was
already nearly finished.
He had started the experiments it
described while awaiting his summons to
Rome. Having turned away from his
telescope, Galileo had become fascinated
instead by the different ways that objects
move through the air. The result was to
become his masterpiece. The manuscript
already recounted findings that would
become just as famous as the moons of
Jupiter. For instance, Galileo had made
the surprising discovery that Earth's
gravity doesn't care in the least how much
something weighs. Drop a cannonball and a
pebble from a high tower, and both will
reach the ground at exactly the same
moment.
But within its pages was another
discovery that would prove to be less
famous yet no less significant. Galileo
had measured the weight of air.
This might seem like a bizarre notion.
How can something so insubstantial as the
air weigh anything at all? In fact our
planet's air is constantly pushing down on
us with great force. We don't notice this
because we're used to it, like lobsters
sauntering along on the seafloor, unaware
of the crushing weight of the ocean of
water above them. We give our own
overlying air-ocean so little respect that
we even describe anything that's full of
air as being "empty."
Back in Galileo's time, notions about
air were similarly hazy. Most people
accepted the idea put forward by Aristotle
in the fourth century b.c. that everything
in the world was made up of four elements:
earth, air, fire, and water. Earth and
water were obviously pulled downward by
gravity. Fire was obviously weightless.
But air was the problem child. Was it
heavy enough to be dragged to the ground,
light enough to rise like flames do, or
did it simply ignore Earth's gravitational
tug and hover?
Galileo believed that air is heavy and
had set about testing his idea. The
experiments he performed were typically
ingenious. First, he took a large glass
bottle with a narrow neck and a tight
leather stopper. Into this stopper he
inserted a syringe attached to a bellows
and by working vigorously managed to
squeeze two or three times more air into
the bottle than it had previously
contained. Next, he weighed the glass
bottle most precisely, adding and
subtracting the finest of sand to his
scales until he was satisfied with the
answer. Then, he opened a valve in the
lid. Immediately, the compressed air
rushed out of its confinement, and the
bottle was suddenly a handful of grains
lighter. The air that had escaped must
account for the missing weight.
This showed that air is not the
insubstantial body we usually take it for.
But now Galileo wanted to know how much
air corresponded to how many grains of
sand. For that he would somehow need to
measure both the weight of the escaping
air and its volume.
This time, he took the same glass
bottle with its long, narrow neck.
However, instead of pumping it full of
extra air, he forced in some water. When
the bottle was three-quarters full of
water, its original air was squeezed
uncomfortably into a quarter of its
original space. Galileo weighed the bottle
accurately, opened the valve, allowed this
pressurized air to escape, and then
weighed the bottle again to find out how
much air he had lost. As for the volume,
Galileo reasoned that the portion of air
that had been forced to leave the bottle
had been pushed aside by the water he had
squeezed in, so the volume of air that had
fled must be exactly the same as the
volume of water that remained. All he had
to do was pour out the water and measure
its volume and voilà, he had found
the weight for a given volume of air.
The value Galileo came up with was
surprisingly large: Air seemed to weigh as
much as one four-hundredth the weight of
an equivalent amount of water. If that
doesn't sound like much, consider this.
Picture a particular volume of air for a
moment -- such as the "empty" space inside
Carnegie Hall in New York. How heavy would
you expect that amount of air to be? Would
it weigh ten pounds? Or a hundred? Or
maybe even five hundred?
The answer is somewhere in the region
of seventy thousand pounds.
The weight of air is so extreme that
even Galileo didn't see the whole story.
He never considered the question of how we
can shoulder such a crushing, overwhelming
burden, for the simple reason that he
didn't realize the air above us is still
heavy. He had measured the weight of air
in his bottle, but he was convinced that
the moment this air was released back into
its natural element, the sky, it
immediately ceased to weigh anything at
all.
Galileo believed that our atmosphere as
a whole is incapable of pushing. It was
one of the few occasions when the great
man was wrong.
In spite of the Church's opposition
Galileo finished his manuscript -- and
published it. After fruitless efforts to
convince publishers in Florence, Rome, and
Venice to defy the Inquisitors, Galileo
finally smuggled the manuscript out to a
printer in the Netherlands. Four years
later, as he approached the end of his
life, a few copies began filtering back to
Italy. Each bore a disingenuous disclaimer
by Galileo himself, who wrote how
astonished he was that his words had
somehow found their way to a printer's in
spite of his obedience to the Papal
diktat.
And although Galileo was wrong about
the way our air behaves aloft, the
experiments his great work contained would
influence two very different people to
discover the truth.
Copyright
2007 by Gabrielle Walker. Published with
permission.
Gabrielle
Walker is an award-winning science writer
who has a Ph.D. in chemistry from
Cambridge and has presented many programs
for BBC radio. She has served as a
climate-change editor at Nature,
features editor at New Scientist,
and visiting professor at Princeton
University. She lives in London. For more
information, please visit www.GabrielleWalker.com.
Read
Dr. Dolhenty's Review of this Book
Order
at Amazon Books
Order
at Powell's Books
|
