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