Navigating by thermometer

Never leave port without your chronometer, sextant, tables, and Nautical Almanac. And don’t forget to carefully stow your navigational thermometer.

As early as 1770 it was suggested by Benjamin Franklin that Fahrenheit’s thermometer might assist the navigator in finding his way across the seas. While on diplomatic missions to France and Britain, and constantly suffering from seasickness, Franklin never failed to make and record temperature observations when his ship crossed that mysterious "river in the ocean" called the Gulf Stream.

During November 1789, Jonathan Williams (Commandant of the Corps of Engineers) pored over Franklin’s meticulous notes and compared them with his own data gathered over a period of years. His research and Franklin’s records led him to conclude that a navigator could discover when his vessel entered the Gulf Stream "by attentively examining the temperature of the sea," the water of the surrounding sea being colder than that of the Gulf Stream.

In 1799 Williams published a book entitled Thermometrical Navigation, in which he urged ship masters when approaching the North American coast to have available two thermometers, one attached to a shaded area on deck to measure air temperature and one trailed in the sea behind the vessel for water temperature. By measuring the temperature of the air and water as often as every two hours, the navigator, according to Williams, could tell when his ship entered and exited the Stream.

This knowledge would not only give him a good estimate of his longitude but would also allow him to make allowance for the distance his ship was set northeastward by multiplying the time the vessel was in the Stream by the velocity of the current. Williams suggested that this procedure could shorten voyage time from one to five days and was particularly useful in checking DR position if the navigator had been unable to make celestial observations for several days.

Perhaps, in the interest of keeping old navigational techniques alive, the Marion/Bermuda Race organizers might in future offer a "time bonus" to those entrants who decide to find their way to Bermuda by thermometer alone.

contributed by J. Gregory Dill

By Ocean Navigator