A Short History of Nearly Everything: Chapter 4 Summary & Analysis

In 1735, a group of French scientists led by Pierre Bouguer and Charles Marie de la Condamine attempt to trek through the Andes Mountains to work out Earth’s circumference by measuring a 200-mile stretch of land near the Equator. The trip is a disaster: the locals pelt the scientists with stones, their doctor is stabbed and killed in a lover’s quarrel, several scientists die of illness, and one runs off with a teenager, never to return. They also have to wait eight months in Lima, Peru for permits, because authorities don’t believe their reasoning for why they need to go all the way to the Andes to do their calculations.

The answer to the question of why these scientists need to go to the Equator goes back to Edmond Halley, a sea captain, scientist, and mathematician who makes many inventions and scientific contributions in his lifetime (the famed Halley’s Comet is named after him). After making a dinner bet in 1638, Halley becomes obsessed with finding out why Earth’s orbit is elliptical, and he seeks out Isaac Newton’s advice. Newton tells Halley that he’s actually already figured out why the orbit is elliptical—but he forgot where he wrote down the explanation. Bryson says that this is like finding a cure for cancer and forgetting where you noted it down. On Halley’s pressing, Newton sits down to recalculate the formula and ends up writing and publishing a book called Principia, which changes the face of science forever.

Newton’s book identifies the laws of motion and it contains the discovery of gravity. The laws of motion are: (1) an object moves in same the direction toward which it’s pushed, (2) an object will move in a straight line if undisturbed by other forces, (3) every action has an equal and opposite reaction, and (4) every object in the universe pulls others toward it. This fourth law means that every object has a gravitational pull that’s proportional to its mass and the inverse of the squared distance from the object it pulls. This means that if the distance between two objects is doubled, the gravitational pull between them becomes four times weaker. Controversially, Principia also claims that Earth is a slightly squashed sphere—slightly flatter at the poles but wider at the Equator. 

If Newton is right, it means that prior calculations about Earth’s circumference and mass are wrong, because, up until this point, scientists assumed that Earth was a perfect sphere. Robert Norwood makes one such calculation a few years earlier by walking 208 miles from the Tower of London to York measuring the distance on the ground with a chain. Norwood wants to know the width (or circumference) on Earth’s surface that one degree of a circle captures, if that degree were to originate from Earth’s center and extend out toward the surface (like a slice of pie). Norwood calculates this distance as 110.72 kilometers, while French astronomer also Jean Picard uses a different geometric method to estimate slightly more accurately that it’s 110.46 kilometers.

In 1669, however, father and son team Giovanni and Jacques Cassini dispute Newton’s claim that Earth is slightly flattened (like a tangerine) and argue that it’s actually slightly elongated (like an egg). To settle the dispute, two expeditions are sent off from France: one (led by Bouguer and Condamine) to measure the circumference of a degree at the Equator and another to Scandinavia to measure the circumference of a degree near the North Pole. If the circumference of a degree at the Equator is longer than elsewhere, Newton’s hypothesis is correct. Nine-and-a-half grueling years later, Bouguer and Condamine discover that Newton was indeed right. Even worse for them, the French team trekking in Scandinavia worked it out and beat them to the punch.

Newton also argues in Principia that a plumb bomb hung near a mountain will tilt toward the mountain, which Nevile Maskelyne and Charles Mason attempt to prove in the 1770s. Ten years earlier, however, they have a different challenge: to measure the passage of Venus across the sun (known as a “transit”) so that they can calculate Venus’s distance from the sun.  Halley had wanted to do that himself, but Venus didn’t transit in his lifetime. Venus’s peculiar orbit means that the planet passes across the sun (or “transits”), then passes again eight years later, but then it disappears for 125 years.

One of the first international collaborative scientific efforts was a series of expeditions across the world to measure the transit of Venus from multiple places. The expeditions, however, are ill-fated. Jeanne Chappe’s journey to Siberia is halted by a flooded river just before he reaches his destination, and Guillame Le Gentil travels a year to India only to have a cloud block the transit from view at just the wrong moment. Another ship, carrying Charles Mason and Jeremiah Dixon (who famously plot the Mason-Dixon line in the American wilderness a few years later) is attacked by the French before they reach their destination in Sumatra.

Eventually, Maskeleyne’s team—who compare the various measurements of Venus’s transit—conclude the 1761 effort failed due to too many conflicting measurements. Eight years later, in 1769, British explorer James Cook completes the task from Tahiti, before claiming Australia as a British colony. Using Cook’s measurements of Venus’s transit, a French astronomer named Joseph Lalande calculates that Earth is 150 million kilometers from the sun.

Once that issue is resolved, Maskeleyne turns back to the issue of plumb bombs tilting toward mountains. In 1774, he makes a lengthy survey—full of complicated measurements—of Scotland’s Schiehallion mountain to test Newton’s hypothesis. Maskeleyne’s measurements prove essential to the ongoing quest to figure out Earth’s mass: Charles Hutton ends up using them to estimate Earth’s mass (at 5,000 million million tons) and to deduce the mass of the sun and other planets. Incidentally, he also invents contour lines (which connect points of equal height on a mountain) when he makes a diagram of Maskeleyne’s measurements. 

Twenty-three years later, a pathologically shy Henry Cavendish (who is terrified of people looking at him or speaking to him in public and only communicates with his household servants through notes) figures out a more accurate measurement of Earth’s mass. He estimates it to be six billion trillion metric tons (each metric ton weighs 1000 kilograms), which he deduces using a curious machine invented by a country parson named John Michell, who leaves it to Cavendish in his will.

Incidentally, Cavendish also discovers at least five scientific laws later coined by other scientists, as well as electrical conductivity and a way to discover the noble gases. However, Cavendish is too shy to publish his findings in his lifetime. Despite vast advances in technology, no scientist has since improved upon Cavendish’s 1797 measurements. Bryson says that by the late 1700s, we thus knew Earth’s precise dimensions and accurate distances between the planets and the sun.