The same thing happens in reverse when you squirt whipped cream out of a can. The air that comes out in the cream has expanded suddenly and pushed on its surroundings, so it has given away energy and cooled down. The nozzle of the squirty cream canister feels cold to the touch for this reason—the gas that’s coming through it is giving away its energy as it reaches the free atmosphere. Less energy is left behind, so the can feels cool.
Air pressure is just a measure of how hard all those tiny molecules are hammering on a surface. Normally we don’t notice it much because the hammering is the same from every direction—if I hold up a piece of paper, it doesn’t move because it’s getting pushed equally from both sides. Each one of us is getting pushed by air all the time, and we hardly feel it at all. So it took people a long time to work out how hard that push actually is, and when it came along, the answer was a bit of a shock. The magnitude of the discovery was easy to appreciate because the demonstration was unusually memorable. It’s not often that an important scientific experiment is also set up to be a theatrical spectacle, but this one had all the proper ingredients: horses, suspense, an astonishing outcome, and the Holy Roman Emperor looking on.
The difficulty was that to work out how hard air is pushing on something, you really need to take away all the air on the other side of it, leaving behind a vacuum. In the fourth century BC, Aristotle had declared that “nature abhors a vacuum,” and that was still the prevailing view nearly a thousand years later. Creating a vacuum seemed out of the question. But some time around 1650, Otto von Guericke invented the first vacuum pump. Instead of writing a technical paper about it and disappearing into obscurity, he chose spectacle to make his point.? It probably helped that he was a well-known politician and diplomat, and was on good terms with the rulers of his day.
On May 8, 1654, Ferdinand III, the Holy Roman Emperor and overlord of a large part of Europe, joined his courtiers outside the Reichstag in Bavaria. Otto brought out a hollow sphere, 20 inches in diameter and made of thick copper. It was split into two separate halves with a smooth, flat surface where they touched. Each half had a loop attached to the outside, so that two ropes could be tied on and used to pull the halves apart. He greased the flat surfaces, pushed the two sides together, and used his new vacuum pump to remove the air from the inside of the sphere. There was nothing to hold it together, but after the air had been removed, the two halves behaved as though they were glued to each other. Otto had realized that the vacuum pump gave him a way to see how strongly the atmosphere could push. Billions of minuscule air molecules were hammering all over the outside of the sphere, pushing the halves together. But there was nothing inside to push back.? You could only pull the two hemispheres apart if you could pull harder than the air could push.
Then the horses were assembled. A team was hitched to each side of the sphere, pulling in opposite directions in a giant tug of war. As the Emperor and his retinue looked on, the horses strained against the invisible air. The only thing holding the sphere together was the force of air molecules hitting something the size of a large beach ball. But the strength of thirty horses could not pull the sphere apart. When the tug of war had finished, Otto opened the valve to let air into the sphere, and the two halves just fell open. There was no question about the winner. Air pressure was far stronger than anyone had suspected. If you take all the air out of a sphere that size and hang it vertically, the upward push of the air could theoretically support 4,400 pounds, the weight of a large adult rhino. That means that if you draw a circle 20 inches in diameter on the floor, the push of the air on just that bit of floor is also equal to the weight of a 4,400-pound rhino. Those tiny invisible molecules are hitting us very hard indeed. Otto did this demonstration many times for different audiences, and the sphere became known as a Magdeburg sphere, named after his home town.
Otto’s experiments became famous partly because others wrote about them. His ideas first reached the scientific mainstream in a book by Gaspar Schott, published in 1657. It was reading about Otto’s vacuum pump that inspired Robert Boyle and Robert Hooke to carry out their experiments on gas pressure.
You can try a version of this for yourself, without the need for either horses or emperors. Find a square of thick, flat cardboard that’s large enough to cover the mouth of a glass. It’s best to try this over a sink, just in case. Fill up the glass with water right to the rim and put the cardboard on top. Push it flat against the rim of the glass so there’s no air left between the surface of the water and the cardboard. Then turn the glass over—and remove your hand. The cardboard, supporting the entire weight of the water, will stay put. It stays there because air molecules are hitting it from the underside, pushing the cardboard upward. That push is easily enough to hold the water up.
The battering of air molecules isn’t just useful for keeping things in place. It can also be used to move things around, and humans weren’t the first ones to take advantage of that. Let’s meet an elephant, one of the Earth’s most impressive experts at manipulating its environment with air.
An African bush elephant is a majestic giant, usually found ambling peacefully across dusty dry savannah. Elephant family life is based around groups of females. An elder stateswoman, the matriarch, leads each group as they roam in search of food and water, relying on her memory of the landscape to make decisions. But these animals don’t just depend on their heft to survive. Each elephant may have a heavy lumbering body, but to make up for it, it’s got one of the most delicate and sensitive tools in the animal kingdom: a trunk. As a family group moves around, they’re constantly exploring the world with this odd appendage, signaling, sniffing, eating, and snorting.