At the time, the most accurate, reliable timekeepers were regulator or pendulum clocks, but these proved useless at sea because they got thrown off-balance by the rolling waves and the 0.2 percent variation in gravity around the globe. In 1675, the inventor of the pendulum clock, Christiaan Huygens, came up with a portable clock that used a balance wheel and a spring for regulation, but it was still not accurate enough to use for navigation. This invention, however, paved the way for pocket watches and wristwatches.

The British government was so determined to find the solution to this problem that in 1714, it offered a prize of 20,000 pounds (the equivalent of millions of dollars today) to an inventor who could come up with such seaworthy devices, dubbed “marine chronometers” by inventor Jeremy Thacker. A carpenter from Yorkshire, John Harrison, then made it his life’s mission to come up with such a clock.

Harrison’s first marine chronometers, H1 and H2, completed in 1735 and 1741 respectively, each used a pair of counter-oscillating weighted beams, which were connected by springs. While these were not affected by the ship’s motion, they were sensitive to centrifugal force, making them too imprecise for navigation. His H3, attempted in 1759, introduced circular balances, a bimetallic strip, and caged roller bearings—inventions that are still used today.

But Harrison wasn’t able to claim the 20,000-pound prize until 1761, when he ditched the circular balances for a fast-beating balance wheel that had a balance spring with a bimetallic rim for temperature control. His prize-winning H4 was much smaller than his other inventions; only 5 inches in diameter, it resembled a large pocket watch.

Then, in 1766 in France, Pierre Le Roy created a marine chronometer using his major clockwork innovation, the detent escapement, with a temperature-compensated balance and a hairspring. Both France’s Ferdinand Berthoud and Thomas Mudge came up with their own marine chronometers, but the device wasn’t perfected until 1780, when Brits Thomas Earnshaw and John Arnold patented a streamlined chronometer with a detached spring detent escapement. These are the most precise portable mechanical clocks ever made, losing around 0.1 second per day, and allowing navigators to determine their ship's position within 4,600 feet after sailing for a full month. Earnshaw and Arnold, fierce competitors, each made about 1,000 chronometers.

At first, these new marine chronometers were so costly few ships’ captains could afford them. Part of the reason chronometers were so expensive is that they often used hard gemstones like ruby or sapphire for the ball bearings to diminish the wear on the escapement. They were also enclosed in a brass case head in gimbals in a mahogany box so that the chronometer stayed level even as the ship swayed.

But for the British Royal Navy, outfitting its fleet with chronometers became a priority by 1825, giving the United Kingdom a navigational advantage on the high seas. The Navy ships preparing for a long journey would dock on the River Thames at Greenwich, and wait for a time ball on a tower to drop at exactly 1 p.m., to set their chronometers. That’s how Greenwich Merdian became the international starting point for measuring time zones.

In the United States, coiled springs were not easy to come by, so most clockmakers built weight-driven clocks until the early 19th century. In the mid-1820s, a few hundred of spring-driven clocks, made with imported springs, were produced by Curtiss & Clark in Plymouth, Connecticut, but they did not sell as well as the cheap wooden clocks available at the time.

But it wasn’t long before the concept of portable clocks, particularly those using oscillating balance-wheel clocks that would operate while moving on uneven surfaces or in different positions, caught on. The earliest balance-wheel clocks in the U.S., were called “marine clocks,” to boast about their potential use at sea, regardless whether they were employed on actual ships.

Famous clockmaker Eli Terry received a patent in 1845 that inspired his son Terry to develop a variety of these marine clocks for mass production, but they were not a hit with the public until 1847. That same year Charles Kirk of Bristol, Connecticut, received a patent for a portable clock with a two-pallet escapement. Around 1890, the marine-clock movement evolved into the same movement that would be used almost universally in mechanical alarm clocks in the 20th century.

Prior to 1900, though, the U.S. Navy was still using the 8-chime ship’s bell code from the days of Columbus. Sailors on watch often relied on their pocket watches and then struck the bells themselves. In the late 1800s, clockmakers set about trying to make clocks that could strike bells in certain increments without disrupting the operation of the time movement. In particular, clocks that could mark the standard mariner’s watches were known as “ship’s bells clocks.”

Seth Thomas Clock Company produced an early ship’s bell clock, which resembled a kitchen clock in a tin can or wood case; meanwhile, Tiffany Makers of New York crafted a limited number of high-quality ship’s bell clocks, but these were only available to members of the elite New York Yacht Club.

At Boston Clock Company, formerly the Harvard Clock Company, John S. Negus and Joseph Eastman started developing their tandem wind movement in the late 1880s, and this invention was patented in 1893, a year before Boston went out of business. Eastman took this concept and formed the Vermont Clock Company with George D. McMillen, which got into an intense competition with Walter K. Menns and the Cheslea Clock Company between 1897 and 1900 to mass-produce the top-quality striking ship’s bell clocks.

An important but rare Boston model called the Locomotive seems to have inspired Chelsea Clock Company when it introduced its line of “marine” clocks in 1897. A year later, Chelsea patented a movement by George W. Adams, which the firm called 4L. This patent, modified by Menns for ship’s bell clocks and assigned to Chelsea owner Charles L. Pearson, was the foundation of Chelsea’s iconic ship’s bell movement, which turned out to be more reliable than the Vermont clocks and earned the Chelsea clocks the nickname, “Timekeepers of the Sea.” Chelsea ship’s bell clocks are still produced today, with brass workings and a patented mahogany case.

Real marine chronometers were also handcrafted continually into the 1970s, by companies such as Mercer of St. Albans. Swiss watch companies Ulysse Nardin attempted to create interchangeable parts for chronometers at the turn of the 20th century, but it wasn’t until the beginning of World War II that Hamilton Watch Company began to mass-manufacture high-quality chronometers for the U.S. Navy and other Allied ships.

Ships today use the satellite-based Global Positioning System for navigation, but international mariner certifications for deck officers like Master, Chief Mate, and Officer in Charge of Navigational Watch require the ability to apply celestial navigation using modern marine chronometers, which are quartz clocks corrected regularly by GPS or radio time signals.

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