He has been called the discoverer of the temperature of the Gulf Stream; but this statement is somewhat misleading. That the stream was warmer than the surrounding ocean seems to have been long known; but Franklin was the first to take its temperature at different points with a thermometer. He did this most systematically on several of his voyages, even when suffering severely from sea-sickness, and thus suggested the use of the thermometer in investigating ocean currents. He first took these temperatures in 1775, and the next year Dr. Charles Blagden, of the British army, took them while on the voyage to America with troops to suppress the Revolution. He and Franklin are ranked together as the first to show the value of an instrument which is now universally used in ocean experiments as well as in the practical navigation of ships.[19]
In the same careful manner he collected all that was known of the effect of oil in stilling waves by making the surface so smooth and slippery that the wind cannot act on it. So fascinated was he with this investigation that he had a cane made with a little receptacle for oil in the head of it, and when walking in the country in England experimented on every pond he passed. But it would be long to tell of all he wrote on light and heat, the _vis inertiæ_ of matter, magnetism, rainfall, evaporation, and the aurora borealis.
One of the discomforts of colonial times, when large open fireplaces were so common, was a smoky chimney. Franklin's attention was drawn to this question about the time that he invented the Pennsylvania fireplaces, and he made an exhaustive study of the nature of smoke and heated air. He became very skilful in correcting defects in the chimneys of his friends' houses, and while he was in England noblemen and distinguished people often sought his aid. It was not, however, until 1785, near the close of his life, that he put his knowledge in writing in a letter to Dr. Ingenhausz, physician to the Emperor of Austria. The letter was published and extensively circulated as the best summary of all that was known on this important question. It is as fresh and interesting to-day as when it was written, and well worth reading, because it explains so charmingly the philosophy of some phenomena of common occurrence which modern books of science are not at much pains to make clear.