From Apprentice Corner: Book Review

Longitude by Dava Sobel

Longitude, by Dava Sobel is a short but no less fascinating read. The history revealed and timeline constructed, outline the trials and tribulations of mariners, scientists and astronomers alike in their quest to accurately measure longitude.

The concept of using a grid like system of coordinates to plot and chart any spot on the earth’s surface is owed in part to Ptolemy in his work Geographica of the 2nd century. These lines, latitude and longitude respectively, thereby can be used to not only chart but also navigate the open ocean with no landfall in sight. Latitude, the lines circling the girth of the earth, is calculated simply with skyward indicators in the positioning of the stars or sun. Longitude however began to be calculated utilising time comparison, given the angular nature of the lines as they circle the earth pole to pole. The mariners must have two ’times’ for their calculations, that of their current location (native time) and that of the pre-determined port of known longitudinal location. They needed a watch that they could carry on the boat and accurately and consistently tell the time of their destination port. Cue one of the greatest problems that persisted over almost 16 centuries to stump even the greatest minds, and the man who triumphed in creating a solution to which there was no contest and the ramifications that we still use today.

The catalyst that formed the tipping point for the longitude problem was the 1707 tragedy. Admiral Sir Clowdesley Shovel was the head of a British fleet that, due to miscalculations that were prevalent at the time, dashed and sunk four warships against the rock of Scilly Isles and lost more than 1600 lives to the mistake. In response a parliamentary committee was formed and the parliament in 1714 passed the 'Longitude Act' offering £20,000 for a viable solution. The wining solution was one that had accuracy within a singular half degree, approximately 30miles, on a journey from England to the West Indies.

The great many ideas for solutions across the centuries were either mechanical (clock) or astronomical. Ranging from celestial patterns, tracking the moon's path across the sky in relation to stars, or the four known moons of Jupiter. The mechanical clock method relied on the time difference equation with a clock bearing the destination port time and the local time noted at noon, thereby calculating for every hour apart, there is 15 degrees of longitude between the ship and destination.

Newton himself noted a small watch would be ideal however he saw the possibility of an astronomical solution as more giving. It wasn't until 1727 when a self-educated village man of carpentry and watchmaking skills caught wind of the prize potential. By 1730 this man, named John Harrison, already had substance enough plans to share them with Edmund Halley, a board member who encouraged Harrison and connected him with another Society member who specialised in clockmaking. Five years later Harrison had a clock (H1) that tested well on its maiden voyage of Lisbon. Harrison, taken by the possibilities of his own idea built another chronometer, namely H2, in just 5 years. His second clock was given more accolades than the first however Harrison being a man of not science but watchmaking and carpentry, was self-driven to achieve not the best but his best, and this he took twenty years to build another chronometer, H3.

As Harrison's instruments became more accurate and reliably more suitable, so did the contending astronomical solutions,  just as the intrigue did for with more scientific minds wanted their idea not Harrison's to succeed. Chiefly against Harrison was Nevil Maskelyne, Astronomer Royal, and he pursued testing on Harrison's clocks and at each point attempted to thwart his efforts in the name of having a more scientific solution despite the brilliance of Harrison's chronometers. Despite this after completion of the third watch and receiving of the Copley Gold Medal (1749), a fourth pocket version was completed, tested and proven on the West Indies trip that was requirement. Despite this board did not immediately award the prize as they had in Harrison's absence of making, become focused on other methods that proved not as accurate but still viable. 1765 was when they finally awarded only half the total prize and moved the goal posts again for Harrison, and demanded he create duplicates to prove the replicability of his watches, as well as ordering a secondary watchmaker to produce a duplicate. Harrison would not receive the final prize amount until 1773. The true prize More than 5,000 ship chronometers would be used by 1815.

Marine chronometers are now regarded and appreciated worldwide as priceless collectors items. After reading Longitude, I got to see a marine chronometer from 1841. The craftsmanship behind the chronometer really made me want to develop the skills so that I can one day sit in front of a historical timepiece not just in admiration, but know how to disassemble, adjust, reassemble, and if needed make a replacement part. This is a watchmaker's dream.                         

Dent No. 1508 Marine Chronometer: 172 years young - and it is still keeping a perfect time!

Dent No. 1508 Ships Chronometer, signed "Dent, 82 strand LONDON". Purchased by Admiralty in March 1841. Used as navigation instrument on HMS 'Virago' - 1847, HMS 'Plover' - 1869, HMS ' Sattelite' - 1886, HMS 'Carysfort'. Serviced in 1879 Cape Observatory, 1895 Cape of Good Hope, 1895 by Mercer (famous Marine chronometer himself) , 1902 in Sydney, 1919 in Liverpool. Sold by auction in 1920. In private collection since 1983.

Edward John DENT (1790-1853) manufacturer of Marine Chronometer and astronomical clocks was one of the most famous Watchmakers in England. in 1828. his ship chronometer No. 114 won first prize in the Premium Trials with a variation of 1/2 second in 12 months, and the Admiralty awarded him a prize of 300 pound. His chronometers were used on many explorations in Polar regions and tropics. No. 1800 was owned by David Livingstone (1836) during his African exploration. In 1842 Dent was granted the Royal Errant as "Chronometer Maker to the Queen and HRH Prince Albert".

He was the maker of great clock of House of Parliament, known as Big Ben.

Archive record from The Old Royal observatory, National Maritime museum, Greenwich.

No. 1508 is one of only the handful survived ships chronometer actually made by Dent. The 2A and 259 have been lost. and other 3 older were "exchanged" which most likely means returned to Dent's workshop.