Chapter Content
Okay, so, like, you wanna hear about some of the most, uh, disastrous scientific expeditions ever? You gotta hear about this one from 1735. The French Academy of Sciences sent this group to Peru, right? Lead by this hydrographer, Pierre Bouguer, and this military mathematician, Charles Marie de la Condamine. And, like, their whole mission was to measure the distance across the Andes using triangulation. You know, that thing where you use geometry? If you know, like, one side of a triangle and the angles, you can figure out the rest, right? It's kind of like, if you and I wanted to figure out how far away the moon is, you know? We'd stand a distance apart. You'd be in, say, Paris, and I'd be in Moscow. We'd both look at the moon, and bam, triangle! Measure the distance between us, get the angles to the moon, and, uh, boom, you got it.
Anyway, everybody at this time was, like, super into figuring out Earth, you know? Like, how old it was, how big it was, where it fit in the cosmos, all that. So, the Frenchies, they were supposed to measure one degree of latitude. That's, like, one 360th of the Earth's circumference. They had to go in a straight line, from near Quito, Ecuador, to a little past Cuenca, Ecuador. It's about 200 miles, and it would help nail down the planet's circumference.
But, like, things went south almost immediately. And, um, sometimes they went really, really south. In Quito, the locals got super mad at the French guys for some reason. They were actually chased out of town by a mob with rocks. Then, like, one of the doctors in the group got murdered because of a misunderstanding with a woman, yikes. The botanist went nuts. People died from fever, and some fell to their deaths. And the third in command, Jean Godin, he just, like, ran off with a 13-year-old girl. Couldn't talk him out of it.
At one point, the measurement team, like, completely shut down for eight months. While Condamine rode off to Lima to deal with some permit issues. Eventually, Bouguer and Condamine just stopped talking to each other, completely uncooperative. And everywhere they went, local officials were suspicious, like, "Why are these French dudes going halfway around the world just to measure the world?" You know? Didn't make sense then, and it almost still doesn't. Like, why not just measure France, right? Save yourself the trouble.
Well, partly it's because 18th-century scientists, especially the French, didn't do things the easy way, you know? But there was a real scientific reason, too. It went all the way back to this English astronomer, Edmund Halley.
Halley was this amazing guy, you know? In his career, he was a ship captain, mapmaker, professor, coin maker, Royal Astronomer. He invented the diving bell. He wrote about magnetism, tides, planets, even opium. He invented weather maps and actuarial tables. Came up with ways to calculate Earth's age and how far away the sun was. He even figured out how to keep fish fresh in the off-season, like, what? The only thing he didn't do was *discover* the comet that's named after him, you know? He just figured out that the one he saw in 1682 was the same one people had seen earlier. They didn't even call it Halley's Comet until like, 16 years after he died.
But, get this, all that being said, Halley's biggest contribution to knowledge might have been this scientific bet he got into, you know? A small bet with these two other big shots: Robert Hooke, who, like, figured out cells, and Sir Christopher Wren, the architect, who, yeah, he was actually an astronomer first, which most people forget.
So, these guys are having dinner one night in London, and they start talking about, like, the motion of the planets, right? People thought planets orbited in ellipses, those special oval shapes. Richard Feynman called it "a special and precise curve." But nobody knew why. Wren was like, "Okay, whoever figures this out gets 40 shillings from me!" Two weeks' wages.
Hooke was, like, always showing off, even if he wasn't right. He claimed he had the answer, but he wouldn't share. He said it would, like, ruin it for everyone else, that they'd have to find it themselves, so they'd know how to value it. But, yeah, no evidence he ever figured it out. Halley, though, he was super interested. He went to Cambridge to get help from Isaac Newton.
And Newton, what a character, right? Super smart, but also, like, a total recluse. He was moody, suspicious, and, like, had zero focus. He would, like, sit in bed for hours in the morning, just thinking. He set up his own lab, first at Cambridge, and did some weird stuff. He stuck a needle, like, a big sewing needle, into his eye socket and wiggled it around to see what would happen. And, uh, nothing much did. Another time, he stared at the sun, as long as he could, to see what it would do to his vision. He was okay, but he had to stay in a dark room for days afterwards.
But, these eccentricities, they don't even compare to how brilliant he was, you know? Even when he worked normally, he was special. As a student, he thought regular math was lame. So he invented a new kind, calculus. Didn't tell anyone for 27 years. He did the same with optics. Changed how we see light. But waited 30 years to share that.
And even with all that brainpower, real science was only part of what interested him. He spent half his life doing alchemy and being super religious. Like, not just dabbling, but going all in, you know? He secretly believed in Arianism, this heretical thing where they didn't believe in the Trinity, which is kinda ironic because he worked at Trinity College, Cambridge. He spent hours studying plans for Solomon's Temple, learning Hebrew so he could read the original texts, looking for math clues about the Second Coming and the end of the world. And alchemy? Forget about it. In 1936, the economist John Maynard Keynes bought a box of Newton's papers and found out most of it had nothing to do with planets or light. It was all about turning cheap metals into gold. And in the 1970s, they analyzed Newton's hair and found he had, like, 40 times the normal level of mercury, which only alchemists, hatters, and thermometer makers would be exposed to. No wonder he had trouble getting out of bed, right?
So, August 1684, Halley shows up unannounced at Newton's place. What he expected, we can only guess. But, luckily, Newton's friend, Abraham De Moivre, wrote down an account, so we have a record of one of the most important meetings in science.
Halley asked Newton, like, what kind of curve a planet would make if the sun's gravity got weaker with distance by the square. This is a math thing called the inverse square law. Halley was sure it was important, but he didn't get it.
Newton just goes, "An ellipse." Halley was so surprised, he asked how he knew. Newton goes, "I've calculated it." Then Halley asks for the calculations. Newton looks through his papers, can't find them.
I mean, can you imagine? Like saying you've cured cancer, but you can't remember where you put the prescription. Halley pressed Newton to redo the calculations. He promised, and he did way more than that. He locked himself away for two years, worked like crazy, and came out with his masterpiece, *Principia Mathematica*, or just *Principia*.
It's so rare in history that someone makes an observation so insightful and unexpected that you don't know what's more amazing: the fact or the idea. That's what *Principia* was.
It made Newton instantly famous. For the rest of his life, he was praised and honored, especially when he became the first scientist to be knighted. Even the German mathematician, Gottfried Leibniz, said Newton's contributions to math were better than everything that came before him, even though they fought for years over who invented calculus first. Halley wrote that "no mortal can approach nearer to the deity than Sir Isaac Newton." His contemporaries agreed, and a lot of people since then too.
*Principia* is called "one of the least readable books", because Newton made it hard on purpose to keep out what he called mathematical "smatterers". But, to those who could understand it, it was like a light bulb going off. It not only explained the orbits of planets mathematically, but it pointed out the force that made them move: gravity. Suddenly, everything in the universe made sense.
At the core of *Principia* are Newton's three laws of motion, you know, like, things move in the direction they're pushed. They keep going in a straight line unless some other force stops them or changes their direction. And every action has an equal and opposite reaction. Plus, his law of universal gravitation, which says every object attracts every other object. Doesn't seem likely, but you're doing it right now, attracting everything around you with your tiny gravitational field. The walls, the ceiling, your pet cat. And they're attracting you. Newton realized that the force of gravity between any two objects is, in Feynman's words, "proportional to the mass of each one and varies inversely as the square of the distance between them." In other words, double the distance, and gravity gets four times weaker. You can put it in an equation: F equals G times m times m prime over R squared.
Okay, it doesn't really help most of us, but, hey, it's elegant, right? Quick multiplication and division, and you know your gravitational status. It was the first real universal law of nature. That's why Newton's respected everywhere.
*Principia* almost didn't happen. When the work was almost done, Newton and Hooke got into a fight over who came up with the inverse square law, and Newton refused to publish the third volume, which was crucial. Halley had to do a lot of talking and negotiating to get the last volume out of the grumpy professor.
Halley's troubles weren't over. The Royal Society had promised to publish the book, but they backed out, claiming they were broke. They'd lost money on a costly book about fish the year before, and they figured a math book wouldn't sell. Halley paid for the book out of his own pocket, even though he wasn't rich. Newton, of course, didn't pay a dime. To make it worse, Halley had just taken a job at the Royal Society, and they told him they couldn't pay him the promised 50 pounds a year. They could only pay him in copies of the fish book.
Newton's laws explained a lot, like, the tides, the movement of planets, why cannonballs fly in a certain way, why we don't get flung into space even though Earth is spinning really fast. It takes a while to get the full meaning of those laws, but the facts that they revealed sparked controversy almost instantly.
Because it meant that the Earth wasn't perfectly round. Newton said that the centrifugal force of the Earth's spin made it a little flat at the poles and a little bulging at the equator. So, the planet's a bit squashed, and that meant that a degree of latitude wasn't the same length in Italy and Scotland. The closer you are to the poles, the longer it is. That's not good news for anyone who thought the Earth was a perfect sphere and measured it that way.
For half a century, people tried to figure out the size of the Earth using precise measurements. One of the first was this English mathematician, Richard Norwood. When he was young, he went to Bermuda with a diving bell he'd made after Halley's design to try to find pearls and get rich. That didn't work. There weren't any pearls, and Norwood's diving bell stunk. But Norwood wasn't alone in that experience. In the early 17th century, Bermuda was known to be hard for captains to locate. The ocean was too big, Bermuda was too small, and the tools to find it weren't good enough. They couldn't even agree on how long a nautical mile was. Even small errors added up, so ships often missed a target the size of Bermuda by a long way. Norwood loved trigonometry and triangles. He wanted to use math for navigation, so he decided to calculate the length of one degree of latitude.
Norwood started at the Tower of London and walked north for two years, 280 miles to York, constantly straightening and measuring a chain. He accounted for the ups and downs of the land, the curves in the road, and kept his data super precise. Then, on the same day and time a year later, he measured the angle of the sun in York, having done the first measurement in London. From those measurements, he figured he could get the length of one degree of latitude and figure out the whole circumference of the Earth. It was a huge undertaking. Mess up by a little bit, and the error would be massive. But, actually, Norwood claimed his calculation was really good, accurate to "scarcely considerable." Less than 550 meters. In metric terms, he said one degree of latitude was 110.72 kilometers.
In 1637, Norwood published *The Seaman's Practice*, and it was a hit. Seventeen editions! Still in print 25 years after he died. Norwood went back to Bermuda with his family, became a successful planter, and spent his free time doing his beloved trigonometry. He lived there for 38 years. You'd think he'd be happy and respected, right? Nope. On the voyage from England, his two young sons shared a cabin with a preacher, Nathaniel White. Somehow, this scarred the preacher, and he spent the rest of his life finding ways to harass Norwood.
Norwood's two daughters had bad marriages, which made things worse for him. One of his sons-in-law, probably egged on by the preacher, kept suing Norwood over small things, which made him mad. He had to go to court on the other side of Bermuda to defend himself. Finally, in the 1650s, Bermuda got into witch trials. Norwood spent his last years terrified that his trigonometry papers with mysterious symbols would be seen as talking to the devil, and he'd be sentenced to death. We don't know much about what happened to Norwood. He had an unpleasant old age, and he might have deserved it. But that's how it went down.
Meanwhile, the race to measure the Earth's circumference had reached France. There, an astronomer, Jean Picard, invented super-complex triangulation methods using quadrants, pendulum clocks, zenith sectors, and telescopes to see the movement of Saturn's moons. He spent two years measuring across France using triangulation, then announced an even more accurate measurement: 110.46 kilometers for one degree of latitude. The French were proud, but this was based on the idea that the Earth was a sphere, and now Newton was saying it wasn't.
To make things even messier, after Picard died, the father-son duo Giovanni and Jacques Cassini repeated Picard's experiments over a bigger area. They claimed that the Earth bulged at the poles, not the equator. Newton was wrong, they said. So, the Academy of Sciences sent Bouguer and Condamine to South America to measure again.
They chose the Andes because they needed to measure near the equator to see if there really was a difference in roundness. Plus, they figured the mountains would give them a clear view. Actually, the mountains were often covered in clouds, so the team would often wait weeks for an hour of clear weather to take measurements. Also, they chose almost the most difficult terrain on Earth. The Peruvians called it "singularly uncommon," which was definitely right. The French guys had to climb some of the world's most challenging mountains, ones their mules couldn't even get through. They had to cross rushing rivers, dense jungles, and miles of high-altitude pebble deserts, all far from any supplies. But Bouguer and Condamine were persistent. They stuck to it through sun and rain for nine and a half years. Near the end of the project, they heard that another French team, measuring in northern Scandinavia, had found that a degree of latitude *was* longer near the poles, just like Newton had said. The Earth was 43 kilometers thicker around the equator than from top to bottom.
So, Bouguer and Condamine spent ten years figuring out something they didn't want to find out. Plus, it wasn't even their discovery. They just finished up the measurements, proving the first French team was right. Then they quietly packed up, went to the coast, and sailed home separately.
Another thing Newton predicted in *Principia* was that a plumb line near a big mountain would be pulled slightly toward the mountain by its mass and Earth's gravitational pull. It was an interesting idea. If you measured that deflection, and figured out the mass of the mountain, you could calculate the universal constant of gravitation, the basic value of gravity, called G. And figure out the mass of the Earth.
Bouguer and Condamine tried this at Mount Chimborazo in Peru, but they didn't get it to work, partly because it was hard, partly because they were fighting. So, it was shelved. Thirty years later, the Royal Astronomer, Nevil Maskelyne, took it up again in Britain. Dava Sobel, in her book *Longitude*, paints Maskelyne as a fool and a villain, not appreciating the genius of John Harrison, the clockmaker. Maybe she's right. But we should thank Maskelyne for other things, like coming up with a plan to weigh the Earth.
Maskelyne knew the key was to find a mountain that was regular in shape, so you could estimate its mass. He convinced the Royal Society to hire someone to check out the British Isles for such a mountain. And he knew just the guy: astronomer and surveyor Charles Mason. Maskelyne and Mason had become friends 11 years earlier when they measured a big astronomical event: the transit of Venus. Edmund Halley had suggested years earlier that if you measured this from different places on Earth, you could use triangulation to figure out the distance to the sun.
Too bad the transit of Venus only comes in pairs, eight years apart, with a century or more between them. Halley wouldn't see it. It didn't happen in the 20th century. But the idea lived on. By 1761, almost 20 years after Halley died, the scientific world was ready.
Driven by a spirit of hardship, scientists went to over 100 places, like Siberia, China, South Africa, Indonesia, and the wilds of Wisconsin. France sent 32 observers, Britain sent 18, and there were observers from Sweden, Russia, Italy, Germany, and Iceland.
It was the first international scientific effort. But it was hard. Many observers met war, disease, or shipwreck. Some got to their destination only to find their instruments broken or warped by the heat. The French seemed destined for bad luck again. Jean Chappe rode in carriages, boats, and sleds for months to get to Siberia, carefully protecting his instruments. He was stopped by a flooded river just before he got there, caused by rare spring rains. The locals blamed him because they'd seen him point strange devices at the sky. Chappe barely escaped with his life, and he didn't get any meaningful measurements.
Even worse off was Guillaume Le Gentil, whose story Timothy Ferris tells in his book *Coming of Age in the Milky Way*. Le Gentil left France a year early to observe the transit in India, but ran into setbacks and was still at sea on the day of the transit, which was the worst place to be, since the measurements needed to be stable.
Le Gentil didn't give up. He kept going to India and waited for the next transit in 1769. He had eight years to prepare, so he built a great observatory, tested his instruments over and over, and got everything perfect. On June 4, 1769, the day of the transit, he woke up to a sunny morning, but right as Venus was crossing the sun, a cloud blocked the sun for three hours, 14 minutes, and seven seconds. By the time the cloud cleared, the transit was over.
Disheartened, Le Gentil packed up his instruments and headed for the nearest port. He caught dysentery and was bedridden for a year. Still weak, he boarded a ship that almost wrecked in a hurricane off the coast of Africa. After 11 and a half years away, he finally got home with nothing to show for it, only to find that his relatives had declared him dead and taken all his property.
The disappointments of the 18 British observers didn't seem so bad by comparison. Mason partnered with a young surveyor named Jeremiah Dixon. They got along and became lasting partners. They were supposed to go to India, then Sumatra, to chart the transit. But their ship was attacked by a French warship the night after they set sail.
Even though the scientists were in an international spirit of cooperation, the countries weren't.
Mason and Dixon sent a message to the Royal Society saying it seemed dangerous at sea, and they didn't know if they should cancel. They got a scathing reply telling them they'd taken the money, the country and science were counting on them, and they'd be disgraced if they didn't go through with it.
They changed their minds and kept going, but then they heard that Sumatra had fallen to the French. So, they ended up observing the transit at the Cape of Good Hope, but it didn't go well. On the way home, they stopped at St. Helena, a lonely island in the Atlantic, and met Maskelyne, whose observations had been ruined by clouds. Mason and Maskelyne became good friends. They made tide charts and spent a few happy, even meaningful, weeks together.
Soon after, Maskelyne went back to England and became Royal Astronomer, while Mason and Dixon went on to spend four years in America. They surveyed 244 miles of dangerous wilderness to settle a boundary dispute between William Penn and Lord Baltimore, and their colonies, Pennsylvania and Maryland. That resulted in the Mason-Dixon line. Later, that line became a symbolic divide between slave and free states in America. Although their main job was the line, they also made several astronomical observations. One time, they made the most accurate measurement of a degree of latitude in that century. They got way more praise for that in England than for solving the border dispute between those spoiled aristocrats. Back in Europe, Maskelyne and his German and French colleagues had to conclude that the 1761 transit observations were mostly a failure. Ironically, one problem was that there were too many observations. Putting them together often showed contradictions. The one person who successfully charted the transit was an unknown Yorkshire captain named James Cook. He watched the 1769 transit from a sunny mountaintop in Tahiti. Then he mapped Australia and claimed it as a British colony. When he got back home, he heard that a French astronomer, Joseph Lalande, had calculated that the average distance from Earth to the sun was a little over 93 million miles. In the 19th century, there were two more transits. From those, astronomers got a distance of 93,000,000 miles, and that stuck. Now, we know the exact distance is 93,005,978,706.91 miles. Earth finally had a location in space.
Mason and Dixon went back to England as scientific heroes. But, for some reason, their partnership fell apart. It's weird how little we know about these guys, considering they were often involved in important 18th-century scientific events. No photos. Few written records. About Dixon, the *Dictionary of National Biography* just says he was "said to have been born in a coal mine," and lets you imagine what that means. It says he died in Durham in 1777. Nothing else besides his name and partnership with Mason.
We know a little more about Mason. In 1772, he was asked by Maskelyne to find a mountain to measure gravitational deflection. He reported that they needed a mountain in the central Scottish Highlands, near Loch Tay, called Schiehallion. But he wouldn't spend a summer measuring it. He never went back. The next thing we know, in 1786, he showed up mysteriously in Philadelphia with his wife and eight kids, looking poor and pathetic. He hadn't been back to America since the survey 18 years before, and he didn't have any reason to be there, no friends or patrons to greet him. He died a few weeks later.
Because Mason wouldn't measure the mountain, Maskelyne did it himself. In the summer of 1774, he spent four months in a remote Scottish valley, directing a team of surveyors from a tent. They took hundreds of measurements from every possible position. To figure out the mountain's mass from all that data took a lot of boring calculations. A mathematician named Charles Hutton did the work. The surveyors filled maps with dozens of numbers, each one showing the height of a spot on the mountain. Hutton realized that if you just connected the points of equal height with a pencil, it would look more organized. You could see the mountain's shape and slope right away. So, he invented contour lines.
From the Schiehallion measurements, Hutton calculated that the Earth weighed 5,000,000,000,000,000,000 tons. From that, they could figure out the mass of all the main objects in the solar system, including the sun. From this one experiment, we knew the masses of the Earth, sun, moon, and other planets and their moons. Plus, we invented contour lines. Not bad for one summer.
Not everyone was happy with the results. The problem with the Schiehallion experiment was that they didn't know the mountain's real density, so they couldn't get a precise number. Hutton just assumed the mountain had the density of ordinary stone, about 2.5 times the density of water, which was just a guess.
One guy focused on this problem. He was a country parson named John Michell from a remote village in Yorkshire. Despite his location, Michell was a well-respected scientific thinker in the 18th century.
He figured out the wave nature of earthquakes, did a lot of creative research on magnetism and gravity, and imagined black holes 200 years before anyone else. That's pretty cool. Even Newton didn't make that leap. When William Herschel, the German musician, decided his real interest was astronomy, he went to Michell for advice on how to make telescopes. The planetary science community's been grateful ever since.
In 1781, Herschel became the first person in modern times to discover a planet. He wanted to name it after King George, but that didn't go over well. It became Uranus.
But Michell's cleverest and most influential achievement was an instrument he designed and built himself to measure the mass of the Earth. Sadly, he didn't get to do the experiment before he died. The experiment and equipment went to a reclusive London scientist named Henry Cavendish.
Cavendish was a character. Born into a wealthy, powerful family. He was the most talented and eccentric scientist of his time. Several writers have written about him. He was so shy, "almost to a disease," one writer said. He was uncomfortable around people, even his housekeeper. He communicated with her through notes.
One time, he opened his door to find an Austrian admirer standing on his porch. The Austrian was so excited, praising him like crazy. Cavendish was stunned. He couldn't take it. He ran down the path, out the gate, and left his front door open. It took hours to convince him to come back home.
Sometimes he ventured into society, especially the weekly scientific gatherings hosted by the naturalist Joseph Banks, but Banks always told the other guests not to approach Cavendish or even look at him. If they wanted his opinion, they should wander near him, like they didn't mean to, and "speak as if nobody was there." If they talked about science, they might get a mumbled reply, but more often they'd hear an angry squeak, which he always seemed to emit, and turn to find nobody there, Cavendish having fled to a quieter corner.
Cavendish had the money and the reclusive personality to turn his house in Clapham into a big lab and explore every corner of physics without interruption. Electricity, heat, gravity, gases, anything about the properties of matter. The late 18th century was a time when people were really interested in the basic stuff of the universe, especially gases and electricity. They were starting to figure out how to work with them, often with more enthusiasm than sense. In America, Benjamin Franklin famously flew a kite in a thunderstorm, risking his life. In France, a chemist, Pilatre de Rozier, held a mouthful of hydrogen over a flame to test its flammability. He proved that hydrogen was flammable, and eyebrows weren't a permanent feature. Cavendish did a lot of experiments, gradually increasing the electricity he gave himself, carefully noting the increasing pain until he could barely hold the quill he was writing with, but sometimes he couldn't even keep his senses.
Over his life, Cavendish made a lot of important discoveries. He was the first to isolate hydrogen. The first to make water by combining hydrogen and oxygen. But everything he did was eccentric. He would often mention experiments in his published works that he'd never told anyone about, which annoyed his fellow scientists. But, despite his secrecy, he wasn't just imitating Newton. He wanted to surpass him. His experiments on electrical conductivity were a century ahead of their time, but no one knew it until after that century had passed. A lot of his achievements weren't known until the late 19th century, when James Clerk Maxwell, a physicist at Cambridge, edited Cavendish's papers. Until then, he did the work, but almost always got no credit.
Cavendish discovered or predicted the law of conservation of energy, Ohm's law, Dalton's law of partial pressures, Richter's law of reciprocal proportions, Charles's law of gases, and the laws of electrical attraction. He also left clues that led directly to the discovery of a group of elements called the inert gases. Some were hard to get. The last wasn't discovered until 1962. But what we're interested in now is Cavendish's last famous experiment. In the late summer of 1797, at the age of 67, he turned his attention to the boxes of equipment that John Michell had left him.
When it was put together, Michell's instrument looked like an 18th-century nautilus weight-training machine. It had weights, pendulums, shafts, and torsion wires. The core of the instrument was two 350-pound lead balls suspended near two smaller spheres. The idea was to measure the gravitational deflection that the big balls caused in the small ones. That would allow the first measurement of a hard-to-find force, the universal constant of gravitation. From that, they could guess the Earth's weight.
To physicists, mass and weight are different. Your mass never changes, no matter where you go. Your weight changes depending on how far you are from the center of something massive, like Earth. If you go to the moon, you weigh less, but your mass stays the same. On Earth, mass and weight are basically the same, so we use them as synonyms, at least outside of class.
Gravity keeps planets in orbit, makes things fall down. It's easy to think of it as a strong force, but it isn't. It's only strong on a big scale. One big thing, like the sun, holding onto another big thing, like the Earth. On a basic level, gravity's tiny. Every time you pick up a book or a coin, you overcome the gravity of the whole planet without even trying. Cavendish wanted to measure gravity at that small level.
Precision was key. The room with the equipment had to be free of disturbances. So, Cavendish stayed in the next room and used a telescope to watch through a peephole. It took a year of making detailed and unrelated measurements. Finally, Cavendish finished and said the Earth weighed a little over 13,000,000,000,000,000,000,000,000 pounds. Or 6,000,000,000,000,000,000,000 metric tons.
Scientists today have instruments that can measure the weight of a bacterium. Instruments so sensitive that someone yawning 80 feet away can mess up the readings. But they haven't made major changes to Cavendish's 1797 measurements. The best estimate of the Earth's weight right now is 5,972,500,000,000,000,000,000 metric tons, only about 1% different from Cavendish's number.
It's interesting that it all just confirmed Newton's guess from 110 years before. And there's no sign that Newton did any experiments at all.
Anyway, by the end of the 18th century, scientists knew the shape and size of the Earth, plus how far away it was from the sun and the other planets. Even the reclusive Cavendish had figured out their weight. So, you'd think figuring out the Earth's age would be easy, right? They had all the data they needed. But it wasn't. We didn't figure out the age of our planet until we could split atoms, invent TVs, nylon, and instant coffee.
To find out why, we gotta go north, to Scotland, and visit a remarkable person.
A person almost nobody's ever heard of, who just invented a new field of study: geology.