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 momentsuch 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 manuscriptand 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.
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