In 1743, Captain George Anson and HMS Centurion ventured into the unimaginably vast Pacific to hunt down a single Spanish ship, the Nuestra Señora de Covadonga. Better known as the Acapulco galleon, she carried silver between the Philippines and Mexico each year and was the richest treasure ship in the world. Despite all the millions of square miles of ocean, the ships met and fought each other. Anson triumphed, captured the treasure, and returned home to a hero’s welcome. Curiously, his success resulted from the primitive state of navigation at the time.
Cartographers had long divided their charts with gridlines to aid navigation. Latitude lines run parallel to the equator, perpendicularly crossing longitude lines, which intersect at the poles. Finding a ship’s latitude is a relatively straightforward task, depending mainly on measuring a noontime altitude of the sun. The equipment to make such measurements requires little more than a good eye and a steady hand.
In Anson’s day, the same could not be said for finding a ship’s longitude. No practical method was available, forcing mariners to rely on “dead reckoning.” In this method, each change of course and speed is meticulously plotted on a chart, but the plot is prone to numerous inaccuracies. Historical methods used to calculate speed and direction were crude, and ocean currents were difficult to measure and account for. All of this introduced errors that compounded the longer the ship went without sighting a known landmark. Locations plotted on charts inevitably parted from reality, sometimes with disastrous consequences.
Sea captains therefore learned to navigate with what certainties they could get—properly charted coastlines and lines of latitude. They would follow a coastline to a point known to correspond with a destination on the far side of an ocean and—wind permitting—“sail the parallel,” i.e., follow the desired line of latitude. Once landfall was achieved on the far side, the coastline could be followed once more.
The method was safe—but also predictable, a major disadvantage in war. Anson did not know the location of his quarry, but he knew her destination, Acapulco, was at latitude 17 degrees North. He found his Spanish opponent, because he knew where to look.
Even as Anson sought his prize in the distant Pacific, John Harrison, the son of an English carpenter, labored at his workbench on a solution to the longitude problem that would free ships to travel wherever the wind would serve.
Time was at the root of his solution, because the earth takes exactly 24 hours to rotate. Noon is the moment when the sun reaches its maximum altitude above the horizon. By using an onboard chronometer to track the time at a place with a known longitude—say, Greenwich—a navigator could compare local noon with time at the fixed point and, hence, calculate how far around the world he had traveled—his longitude, in other words. A difference of exactly one hour, for example, would be 1/24 of the way around the world, or 15 degrees.
The British Parliament had passed the Longitude Act in 1714, offering up to £20,000 for a “practicable and useful” solution to calculate longitude at sea and reduce losses of ships and lives to errors in navigation. Accurate clocks existed in 1714, with weights to power them and pendulums to regulate them. They worked well in church towers but were useless on board heaving, pitching ships. Land-based clocks could be corrected each day with solar observations, but the clock that Harrison envisioned would have to keep correct time without any such adjustment for months and even years.
It also would have to cope with the wide variety of conditions to be expected at sea, whether on board a whaler at the fringes of the Arctic or an East Indiaman crossing the tropics. Changes of temperature would make the metal parts of the mechanism expand or contract; changes in humidity would affect the lubricants in the clock. The problems to be overcome were legion.
But Harrison had extraordinary determination. Self-taught, he had made his first clock before he turned 20, and by the time the prize was announced, he had produced several timepieces of unusual accuracy. Harrison would spend 43 years on the list of engineering challenges required of a marine chronometer and would be an old man before he had solved them all.
For power, Harrison replaced weights with springs. Balance wheels replaced pendulums as regulators. Laminated strips of dissimilar metals resisted changes in temperature, while jewels and self-lubricating lignum vitae wood made his mechanisms close to frictionless.
He finished his first marine chronometer (H1) in 1737. It weighed 75 pounds and required a case four feet square. His fourth, H4, was finished in 1760 and resembled a large pocket watch. The marvel of the age, it was presented to the Royal Society, admired by the King, and feted across Europe. A party of French horologists including Ferdinand Berthoud, visited London to study the masterpiece. Berthoud would produce his nation’s first marine chronometers.
Larcum Kendall, another watchmaker, made a copy of H4 (imaginatively called K1) that accompanied James Cook on his second voyage of discovery. The great explorer reported that the “watch exceeded expectations . . . [and] has been our faithful guide through all vicissitudes of climates.”
K1 had taken two years to make and had cost £450. (For comparison, a Royal Navy frigate cost about £14,000.) Given that a ship ideally should carry more than one chronometer—to spot any malfunctions—equipping the fleet with the new technology proved a considerable challenge, one beyond Harrison. Well into his 70s, he continued to devote himself to improving his life’s work. His H5 chronometer was unveiled in 1770 and would be his last.
The challenge of mass-producing marine chronometers fell to the next generation of watchmakers, such as John Arnold and Thomas Earnshaw. These men of business streamlined designs, farmed out production of the components, and industrialized production. By the 1780s, a boxed Earnshaw cost as little as £65, and the numbers in use grew steadily.
The Admiralty encouraged adoption of the new technology by offering to fund a second device for any captain who purchased the first himself. In 1737, H1 was the sole marine chronometer in the world. By 1815 there were more than 5,000, and most oceangoing ships had them by the middle of the century, some in prodigious numbers. Charles Darwin’s HMS Beagle set off on her scientific expedition in 1831 carrying 22. Precise time measurement continues to dominate navigation today through GPS, banishing uncertainty over longitude forever, and saving countless lives.
Looking at H4 today, in its glass case at Greenwich, it can be hard to think of the device as helping shape the modern world. Yet behind its enamel face are technologies that still surround us. The bimetallic strips that compensate for changes in climate lie at the heart of devices from thermostats to refrigerators. The caged-ball bearings that Harrison developed are present in most machines with moving parts.
But John Harrison’s true legacy was to give us faith in what technology could achieve. His heirs are the giants of the industrial world that came after him, such as Isambard Kingdom Brunel and Thomas Edison, engineers and inventors who knew that if machines could solve a problem as intractable as longitude, then they could power vehicles, fly through the air, perform mathematical calculations, and take humanity to the moon.