The age of discovery relived through the clocks of John Harrison

The statue of Captain Cook rises at the foot of Greenwich Hill. Cloaked in his uniform of bronze, the explorer contemplates the meridian which took its name from this pleasant London countryside bordering the River Thames. The Royal Observatory is home to the navigation tools that enabled James Cook - and other famous navigators of his time - to secure British dominion of the seas. Cook called them his "faithful friends".

They were marine clocks invented by a self-taught horologist, a stubborn Lincolnshire man whose finely crafted pieces were equalled only by the coarseness of his manners: John Harrison.

A "science week" took place in Britain in early March to pay tribute to the mechanical marvels of John Harrison. Hordes of students and curious visitors filed past the four clocks preserved at the Greenwich Royal Observatory.

They were all set to beat in unison for the occasion. The choir of copper, steel, wood and rubies had not been heard in ages. Given the fragility of the historical pieces, it will be some time before this is repeated. "At least not in your lifetime" quipped the guards to young and old visitors alike. 

Born in 1693, John Harrison began by learning his father's joinery skills. By the age of 20, already extraordinarily nimble-fingered, he was designing his first clocks. They are singular in that they were made entirely from wood. With no background in clock making, and without an education to speak of, Harrison quickly made a name for himself as a rebellious and bold technician. 

An example: in the 1720s, serious problems were posed by the poor quality of the gear oil. Instead of trying to improve it, Harrison decided to do without it. Thanks to the invention of a new type of escapement and the use of a dense and oily tropical wood, the young man designed a clock that worked without any lubricant, something never before seen.

Three centuries later, mounted in a stable tower on Lord Yarborough's Brocklesby Park Estate (Lincolnshire), the timekeeper still works unperturbed. It was at this time that John Harrison heard about the reward promised by the King to whichever scientist was able to solve an enigma: the precise determination of longitude. A fabulous sum was promised: 20,000 pounds, today's equivalent of 2.5 million francs.

As the cause of fatal shipwrecks, delays in the delivery of cargo, and diseases on board vessels, the inaccuracy with which longitude was then measured was a constant source of concern.

No reliable calculation of the east-west coordinates seemed to exist. People did have recourse to the movements of the moon, but the calculation was too long, too tedious and too dependent on weather conditions. In theory, since longitude is also a coefficient of time, a clock might be a useful aid to navigators. The Earth, with a circumference of 360 degrees, makes one complete rotation every 24 hours.

Each hour of time is thus equal to 15 degrees of rotation, or 15 degrees of difference in longitude. However, an accurate clock is needed to do full justice to this fine piece of mathematics. The annoying thing, as Sir Isaac Newton, himself a connoisseur, sighed, is that "no such watch has yet been made...".

The public reward promised by the king stipulated that a timekeeper should not vary by more than 2.8 seconds a day. In the early 18th century, small portable clocks lost or gained more than a minute each day. The invention also needed to be immune to rolling, gravity, dampness and temperature differences. The task was thought to be impossible. John Harrison nevertheless rose to the challenge.

As the large wall clocks and other regulators made by him had a constant rhythm (varying by one second every 24 hours), the Englishman attempted to adjust his large movements to maritime conditions. He dispensed with lubricant, engineered balance wheels immune to gravity and bimetallic parts that offset temperature differences. In the process, he invented the ancestor of the ball-bearing. Financed in part by the authorities of the day, John Harrison spent thirty years developing three successive prototypes. In 1753, the Englishman realised that he had taken a wrong turn. It dawned on him that a small chronometer fitted with a high-frequency oscillator would create a much more stable unit than a cumbersome and heavy portable marine clock. The mother of all precision watches, the fourth prototype (known as "H4") was completed in 1759. In the deepest recesses of the mechanism, certain precious stones of the "H4" timekeeper are so finely cut that no one can fathom how they were worked, not even today. After several sea trials, and a series of the vicissitudes due in large part to the inventor's bad character, the Longitude Prize was awarded to John Harrison in 1773. It would take him 50 years to finally accomplish this major work of the 18th century. The clockmaker died three years later, in 1776, at the age of 83. 

Le Temps
Luc Debraine
3 avril 2002