During his voyage to Philadelphia Franklin made the observations on the sea that appear above under April 10. On May 16 he promised in a letter to Priestley to communicate to him “a valuable philosophical discovery” that he had made on the voyage; years later William Temple Franklin conjectured, in a note on that letter, that the discovery was related to variations in the temperature of sea water. This draft justifies a further conjecture, that Franklin started to redeem his promise but broke off after a short introduction; and outside evidence suggests that what he was about to explain was how to recognize the presence of the Gulf Stream. Part of the evidence is in his April observations: at two periods on the crossing, close together, considerable seaweed appeared, phosphorescence vanished, and the temperature of the water rose. The rest of the evidence is what followed this fragment when he embodied it in his “Maritime Observations,” completed in 1785: he described the same three changes in the sea as indications of the Gulf Stream.4 By the time he disembarked from the Pennsylvania Packet, it seems reasonable to conclude, he had a method for identifying the great current; and soon afterward he set out to impart his discovery to Priestley. Then he stopped. Perhaps he was unwilling to commit himself without further tests, which he made years later; perhaps he was distracted by the press of business. In any case a decade passed before he resumed his inquiry into the Gulf Stream.