I found this story on Futility Closet's blog here. The original source was Carl Van Doren's Benjamin Franklin (1938). He writes:
"the sage was not too grave to play a joke on his friends. One day, when they were walking in the park at Wycombe, he said that he could quiet the waves on a small stream which was being whipped by the wind. He went two hundred paces above where the others stood, made some magic passes over the water, and waved his bamboo cane three times in the air.
"The waves gradually sank and the stream became as smooth as a mirror. After they had marvelled Franklin explained. He carried oil in the hollow joint of his cane, and a few drops of it spreading on the water had caused the miracle."
So . . . what, Benjamin Franklin just carries OIL in his can, on the off-chance he needs to be perceived to do a minor miracle? If so, why explain it? I suppose Benjamin Franklin needs no explanation. We've already discovered that he's a magnificent bastard. And we've already looked at the weird stuff people used to carry around in their walking sticks. Let's just all be grateful he wasn't toting around anything worse.