In the 1600's the British Navy ships were often getting lost or running aground on rocks because they had no way of reliably knowing where they were. In 1715 a large prize was set up by Parliament (the Board of Longitude) which was open to anyone who could find a proven way to determine the latitude and longitude at sea. This navigational story has been lively recounted in Dava Sobel's wonderful book - Longitude [1,2] - which was later made into a great film starring Jeremy Irons and Michael Gambon .
Latitude can be determined relatively simply from observations of the sun, moon or stars but the longitude was much harder to determine. What was needed was a reliable way of determining the time difference between an agreed reference point (say Greenwich in London) and wherever you happen to be. Once you have the time difference you can get the longitude because each hour difference represents (360/24 =) 15 degrees of longitude. If the ship's time is found to be earlier than Greenwich the ship is west and if later it must be east of Greenwich.
Many proposals for finding the longitude were suggested the best of which advanced several areas of science. Some ideas however were fairly crazy and perhaps the strangest and most crackpot suggestion was a method called the 'Powder of Sympathy' suggested by Sir Kenelm Digby (1603-1665). Sir Kenelm started out as what could be loosely termed a pirate but he had a keen and clever mind and later retired to study alchemy and was even a founding member of the Royal Society.
In the film 'Longitude', Steven Fry plays the eccentric Sir Kenelm who claims that his miraculous substance - the Powder of Sympathy (apparently green vitriol; ferrous sulphate, Iron (II) sulphate) - could heal at a distance . In use all one had to do was to sprinkle some of the powder onto an article of clothing that used to belong to the injured person and they would start to heal. Digby's suggestion was to use the 'Powders of Sympathy' as a method to determine longitude. To quote Sobel on Digby: "The daft idea to apply Digby's powder to the longitude problem follows naturally enough to the prepared mind: send aboard a wounded dog as a ship sets sail. Leave ashore a trusted individual to dip the dog's bandage into the sympathy solution every day at noon. The dog would perforce yelp in reaction, and thereby provide the captain a time cue. The dog's cry would mean, "The Sun is upon the meridian in London", so allowing the difference in time to be immediately determined giving the longitude!
Needless to say the 'Powder of Sympathy' never succeeded in winning the coveted Longitude prize or even for that matter any kind of prize or success. One has to remember that this was a time before most people had any kind of science education and so to many the strange logic of the Powder of Sympathy story might even have sounded feasible. One wonders how many poor dogs suffered in the trials.
 Longitude - the illustrated book: Dava Sobel and W J H Andrews, Fourth Estate Ltd, 1998. ISBN 1 84115 233 1
 Longitude – on audio tapes: Dava Sobel, HarperCollins. ISBN 0 00 105337 X
 Longitude - the DVD film: 2002, Granada Film Production, Channel 4
(see 'Longitude lunatics' selection of the 'play scenes' section of the DVD)
How teachers can use these articles in a lesson
Why Hollywood Science
Open University Hollywood Science web site
Call for clips - do you have a film clip that needs investigating?
home | diary | whats on | CSC summary | latest news