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G - 87 : Toulon and the Longitude Problem


A Royal Navy Ship of the Line in Story Weather (Artist Impression from 17th Century)


We write the year 1707. A British Fleet of 21 ships is on the way home from the Mediterranean. It had participated in the long-lasting War of Spanish Succession (1701 – 1715). Its operation was of mixed success. Gibraltar - conquered in 1704 - could be defended and a large part of the French Mediterranean fleet was destroyed. But the siege of Toulon had failed. There were not enough soldiers to take the heavily fortified town. Most of Toulon’s massive, Vauban-style bastions still stand today We definitely see them during our second week of sailing this summer.

The Siege of Toulon 1707 failed due to the lack of Ground Troops from Savoy


Three days before reaching the English home ports, on 22 October 1707, disaster strikes. In foul weather, several ships hit the rocks of Scilly island. Four ships break apart and sink immediately, one of them the HMS Association, Admiral Shovell’s flagship. Almost 2’000 sailors drown – the precise number is not known. Amongst the drowned is the Admiral himself. His body is washed ashore the following day – as so many others. It is the Royal Navy’s greatest loss of life in history.

The HMS Association hitting the Rocks near Scilly Island in October 1707


The cause of the disaster was manyfold but boils down to navigational error. The fleet believed it was 200 nautical miles west of Scilly. The 21 ships had run into one gale after another soon after leaving Gibraltar. Their course was selected carefully – far away from the dangerous French coast. Taking navigational readings in such bad weather was challenging though. The ships’ compasses were less reliable than today's. With an overcast sky, the longitude position was a guess. The absence of precise maps was another factor. Sailing the oceans was dangerous in the 17th century.


The Course of Admiral Shovell's Fleet in 1707. The

filled Dot shows where they believed they were. The

empty Dot the Actual Position


But since Christopher Columbus discovered America (1492) and Vasco da Gama found a way to India (1498), more and more ships had taken to the sea. The name “Age of Discovery” was not coined by accident. Less than 100 years after the first Atlantic crossing, King Philip II from Spain tried to invade England to dethrone Queen Elizabeth. His Great Armada counted 150 ships with 20’000 men on board. The invasion ended in disaster. English fireships and bad weather destroyed the Spanish Fleer. Many ships were smashed to pieces on England’s rocky shores. It was the year 1588.

The Great Spanish Armada of 150 Ships assembled in the Channel in 1588


Despite such failures, Spain, France, England and the Netherlands built large number of ships in the 17th century. The promise of colonial riches was too tempting. These nations understood how Venice and Genoa had become wealthy and thought it was now their turn. By combining global trading with growing sugar, coffee and cotton in their colonial estates overseas, they intended to become even richer. The key to all was mastering the sea.

The Admiralty Map of the Scilly Island from 1920 includes Water Depth. The Maps in the 17th Century did not. The Blue Circle indicates where the HMS Association sunk.


On the open ocean, 17th century ships navigated safely. Navigational techniques were still rudimentary but did the job. Staying close to coastal waters was dangerous though. The first maritime maps only outlined shore lines but said nothing about the ocean’s depth. The ships relied on measuring depth with a weighted line. It was measured in fathom, the equivalent of three feet. The method was slow, dangerous and often unreliable.


Long-distance fleet deployments had an important Achilles heel. They could not stay away for too long. The wooden ships need maintenance. Barnacles which slow speed needed to be removed. The hulls needed to be regularly caulked. Water and food run out after two to three months. More importantly, sailors fell ill with scurvy if not regularly provided with fresh fruits. In a nutshell, fleets had to regularly return home. The long journeys to and back from their area of operation increased the risk of navigational errors.


To mitigate these risks, the Royal Navy adopted a two-prong strategy:


  • Establish Naval Bases globally to maintain fleets far away from home. It is no accident that the Royal Navy conquered Menorca with Port Mahon in 1708, only a year after the Scilly disaster. The port served as Mediterranean station for close to hundred years. It had a wharf, a hospital and many supply depots.

The Royal Navy's Logistics Infrastructure was built on the unpopulated Northern Side of the Mahon Bay (on this photo to the right) - it is still there!


  • Improve the accuracy in navigation. After Scilly, all compasses of the surviving 17 ships were inspected. Only 3 out of 101 were in proper working order. Royal Navy minimum standards were imposed. Also, a program to improve sea charts was launched. Henceforth every ship of the Royal Navy had to make measurements to improve maritime cartography. Most interesting though was the Longitude Act of 1714 which promised 20'000.- pounds sterling to anybody who could solve the longitude problem.

Map of France from 1684 showing the Improvements due to better Measuring Techniques (light shade = old Map; darker shade = improved Map)


If time could precisely be kept for several months, a ship could compare the zenith time (12.00 h) of its current position with the reference zenith time at home. When the sun reached zenith at the ship’s position and the reference clock showed 14.00 h, the ship was 2 hours or 30 degrees west. Building such precise chronometers was a challenge though. Watchmakers knew that clocks became unreliable in changing humidity and temperature. Metal shapes and strength vary. The calibration of a clock becomes undone. But they could do little about it. Clocks on a ship were thus unreliable and not used for navigation.


John Harrison with his famous Chronometer H4


All this changed with improvements in metallurgy which allowed John Harrison (1693 – 1776), a passionate watch maker, to make in 1761 his famous H4. It was the result of 30 years of experimentation during which his clocks became more and more accurate. On the final sea trial, a trip from Portsmouth to Barbados and back, his H4 chronometer convinced. It kept time within 39 seconds which translated into a longitude error of just 10 miles. The story is wonderfully told in Dava Sobol’s book “Longitude”, published in 1999.


To Everybody's Amazement, the H4 was Pocket Size


A bit earlier, in 1731, the Royal Navy also introduced John Hadley’s navigational sextant which allowed the precise measurement of the sun’s, the moon’s and other stars’ position. It opened the way to an alternative way of measuring longitude - by comparing the sun’s and the moon’s position. The Method of Lunar Distances was on the same sea trial as Harrison’s H4 and also produced satisfactory results. The H4 was three times more accurate though. Harrison won the award for the Longitude challenge.


The famous English Mathematitian and Astronomer John

Hadley invented the modern Sextant - technical picture

of his first model from 1731

Using time to determine our location is a method we now use so often that we hardly think about it. It is the basis of GPS which powers interactive maps but also artillery systems. The efforts to use time for navigation started with Shovell’s fleet returning from Toulon. Wonder what inspiration Toulon has in store for us.


With John Harrison's Chronometer on board, Admiral

Shovell would never have hit the Scelly Island



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