On 24th March, 1693, John Harrison, a self-educated English carpenter and clockmaker was born.
Harrison is perhaps most famous for inventing the marine chronometer, a long-sought after device which helped solve the problem of calculating longitude whilst at sea.
Harrison was born in Foulby, West Riding, Yorkshire, the eldest of five children. His step father worked as a carpenter at Nostell Priory. It is believed that around 1700, the family moved to Barrow upon Humber, due to his father’s work. Harrison himself followed in the man’s footsteps and became a carpenter, building and repairing clocks. And if legend is to be believed, at the age of six when he was in bed due to smallpox, he was given a watch to amuse himself with, and spent hours listening to it and studying its moving parts. A fascination with music led to him becoming a choirmaster for Barrow parish church.
At the age of 20, in 1713, Harrison built his first longcase clock, the mechanism was made entirely of wood. Three of his clocks survive to this day and are housed in a variety of museums across the UK.
Harrison’s many skills allowed him to drastically improve the performance of the pendulum clock. He invented the gridiron pendulum consisting of alternating brass and iron rods assembled so that the thermal expansions and contractions cancelled one another out.
As Harrison’s legend grew, the demand for him to do something, anything to fix an issue that could help Britain and her navy grew. That issue was the longitude problem.
Longitude fixes the location of a place on Earth, east or west of a north-south line called the prime meridian. It is given an angular measurement. Knowledge of a ship’s east-west position was and is essential when approaching land. Not having this knowledge led to an insane amount of shipwrecks, which was not acceptable in an age when trade and navigation was increasing rapidly.
Whilst there had been many proposals as to how to determine longitude during a sea voyage, none of the proposals had ever amounted to much.
Harrison, therefore had a challenge ahead of him.
He produced a reliable clock that could keep the time of the reference place, but found himself thwarted by the elements.
It took Harrison five years to build his first sea clock. He demonstrated it to members of the Royal Society who then spoke on his behalf before the Board of Longitude. The clock was considered the first proposal considered worthy by the Board, and Harrison was instructed to test it out as soon as possible. This he did in 1736, when he sailed to Lisbon aboard the HMS Centurion and returned on the HMS Orford. The clock lost time on the outward voyage, but performed well on the return, with the captain and sailing master of the Orford praising its design.
The Board was impressed enough to grant Harrison £500 for further development, which encouraged him to move to London in 1737, and develop an instrument known to history as H2, which was a more compact and rugged version. In 1741, H2 was ready to be deployed, but the outbreak of the War of Austrian Succession saw the mechanism deemed too important to risk falling into enemy hands.
Harrison also suddenly abandoned work on the second machine when he discovered a flaw with the concept of the bar balances. He hadn’t recognised that the period of oscillation of the bar balances could be affected by the yawning action of the ship. It was this that led to the adoption of circular balances in the Third Sea Clock.
The Board was so impressed they granted him another £500, and whilst waiting for the war to end, he started work on H3.
Harrison then spent seventeen years working on H3 but found that despite his best efforts it did not perform exactly how he would have liked it to. The problem was that because Harrison did not fully understand the physics behind the springs used to control the balance wheels, the timing of the wheels were not isochronous, a characteristic that affected its accuracy. This was not just a Harrison problem though as the engineering world as a whole were not able to fully understand the properties of the springs for such applications. However, the whole experience was a valuable lesson for Harrison as he learned much from its construction. And it left two enduring legacies, the bimetallic strip and the caged roller bearing.
Following thirty years of experimentation, Harrison found that some of the watches made by Thomas Mudge kept time as accurately as his huge sea clocks. It’s possible that Mudge was able to achieve this due to the availability of the new crucible steel produced by Benjamin Huntsman which enabled harder pinions and a tougher more highly polished cylinder escapement to be produced. Harrison realised that a mere watch could be made accurate enough for the task and was far more practical than a huge sea clock. He decided to redesign the concept of the watch as a timekeeping device, basing his design on ‘sound’ scientific principles.
In the early 1750s, Harrison had designed a precision watch for his own use, this watch incorporated novel frictional rest escapement and contained compensation for temperature variations and contained the first miniature going fusee which enabled the watch to continue running whilst being wound. These features made the ‘Jeffreys’ watch a success, which Harrison incorporated into the design of two new timekeepers.
Harrison’s first sea watch, H4, is housed in a silver pair cases 13 cm in diameter. The clock’s movement was highly complex for the period and resembled a larger version of the then current conventional movement. A coiled steel spring inside a brass mainspring barrel provided 30 hours of power. This was covered by a fusee barrel which pulled a chain wrapped around a conically shaped pulley known as the fusee. The fusee is topped by the winding square which requires a separate key. The great wheel attached to the base of this fusee transmits power to the rest of the movement. The fusee contains the maintaining power, a mechanism for keeping H4 going whilst being wound.
Following the construction of the watch, the Board of Longitude ordered that it be trialled on a voyage from Portsmouth to Kingston, Jamaica. It was placed aboard the HMS Deptford, accompanied by Harrison and his son William. The watch was tested before departure by Robertson, Master of the Academy at Portsmouth, who reported that on 6th November, 1761, at noon it was three seconds slow, having lost 24 seconds in 9 days on mean solar time. The daily rate of the watch was therefore fixed as losing 24/9 seconds per day.
After accounting for this and at the conclusion of the voyage, it was found that the watch was five seconds slow compared to the known longitude of Kingston. When Harrison junior returned to England on the HMS Merlin, he came to report a successful outcome. The Board however demanded another trial, which raised outrage from the Harrisons before they agreed to go on another trip to Bridgetown, Barbados.
On this second voyage, a second observer was asked to judge the voyage through the Lunar Distance System. Once again the watch proved to be extremely accurate keeping time to within 39 seconds. The lunar measures were also fairly good at 30 miles, but required considerable work and calculation in order to be used. At a meeting of the Board in 1765, the results were presented, but again were attributed to luck. Once again the matter reached Parliament who offered 10,000 pounds in advance and the other half once Harrison turned over the design to other watchmakers to duplicate.
Harrison decided to enlist the help of King George III. Having obtained an audience with the King, Harrison pitched his case and found the King receptive. The King had even tested H5 himself at the palace and found it to be accurate to one third of one second per day. The King advised Harrison to petition Parliament for the full prize, after threatening to appear in person himself to dress them down. Finally, in 1773, Harrison received a monetary reward to the tune of £8000. But never received the official award.
Harrison’s reward would come later, after his death, when Captain James Cook used H4 on his second and third voyages and found it to be incredibly accurate. Others used H4 to chart similar courses and also found it to be incredibly accurate. His designs were later copied by others, in a simplified form, and as the cost of production became cheaper, they were more readily available for consumption, improving our understanding of the seas and longitude, making journeys safer.
Leave a Reply