Lost at sea: could you find your way using alchemy?

In the 1600s British Navy ships were often getting lost or running aground on rocks because they had no way of reliably knowing where they were. So insurmountable was the problem thought to be that in 1715 the British parliament, through the Board of Longitude, put up a large prize - of the order of ca £3m in today's currency - 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 film starring Jeremy Irons and Michael Gambon.³ 

Now Jonathan Hare asks – Lost at sea: could you find your way using alchemy?