How Was It Discovered? 41
Packet ships rode higher in the water, were faster ships, and were better crewed than heavy
Rhode Island merchant ships.
Franklin mentioned the report to a Rhode Island merchant captain off-loading cargo in
London. This captain said it was absolutely true and happened because Rhode Island whalers had taught American merchant captains about the Gulf Stream, a 3 mph current that
spread eastward from New York and New England toward England. American captains
knew to curve either north or south on westward trips to avoid fighting this powerful
current.
When Franklin checked, the Gulf Stream didn’t appear on any maps, nor did it appear
in any of the British Navy shipping manuals. Franklin began interviewing merchant and
whaling captains, recording on maps and charts their experience with the Gulf Stream current. W halers, especially, knew the current well because whales tended to congregate along
its edges.
By 1770 Franklin had prepared detailed maps and descriptions of this current. British
Navy and merchant captains, however, didn’t believe him and refused to review his information. By 1773 rising tensions between England and the colonies made Franklin withhold
his new findings from the British.
Franklin began taking regular water temperature readings on every Atlantic Ocean
crossing. By 1783 he had made eight crossings, carefully plotting the exact course his ship
took each time and marking his temperature readings on the ship’s map.
On his last voyage from France to America, Franklin talked the ship’s captain into
tracking the edge of the Gulf Stream current. This slowed the voyage as the ship zigzagged
back and forth using the warm water temperature inside the Gulf Stream and the colder water temperature outside it to trace the current’s boundary.
The captain also allowed Franklin to take both surface and subsurface (20 and 40 fathoms) temperature readings. Franklin was the first to consider the depth (and thus the volume) of an ocean current.
Franklin discovered that the Gulf Stream poured masses of warm water (heat) from the
tropical Caribbean toward northern Europe to warm its climate. He began to study the interaction between wind and current and between ocean currents and weather. Through the
brief papers he wrote describing the Gulf Stream data he had collected, Franklin brought
science’s attention and interest to ocean currents and their effect on global climate.
Franklin’s description of the Gulf Stream was the most detailed available until German
scientist Alexander von Humbolt published his 1814 book about the Gulf Stream based on
his measurements from more than 20 crossings. These two sets of studies represent the beginnings of modern oceanographic study.
Fun Facts: The Gulf Stream is bigger than the combined flow of the
Mississippi, the Nile, the Congo, the Amazon, the Volga, the Yangtze,
and virtually every other major river in the world.