Fig. 1 Major James Rennell F.R.S. In a time when travel between Europe and the American continent was still by sailing vessels, the position of the Gulf Stream was of vital importance to the mariners. In order to map the Gulf Stream, the British Admiralty set out to produce accurate maps on which the position of the Gulf Stream was charted. This task was assigned to Major James Rennell. Rennell's numerous naval friends, among which the Hydrographer of the Navy Alexander Dalrymple, Admiral Beaufort, and experienced and reliable navigators like William Bligh and Mathew Flinders, furnished him with data in order to chart the currents of the Atlantic Ocean. Rennell extensively describes his currents charts in the covering book. There are five charts, the first and second charts are for the eastern and western Atlantic ocean, respectively. The third gives the `South-African current and counter-current from the Atlantic to Indian Ocean', the fourth gives the `connecting currents between the Atlantic and Indian ocean' and the fifth focuses on the Gulf Stream area. On all charts, observations of SST, currents and numerous observations of wind strength and direction are plotted. Along the coasts and over banks, water depths are given and a few sightings of icebergs and data on the release and retrieval of bottles and (parts of) ship wrecks are entered. Interspersed in the text, Rennell gives the captain and/or ship name who contributed to the SST measurements in his compilation.
Original versions of the Rennell Charts are from the UK Hydrographic Office and are available here in digitized form. The help of Marc Terlien in digitizing the maps is was invaluable. The charts can be reached here.