While representing Pennsylvania in England, and living with Mrs. Stevenson, in Craven Street, London, he made an experiment to prove that vessels move faster in deep than in shallow water. This was generally believed by seafaring men; but Franklin had a wooden trough made with a false bottom by which he could regulate the depth of water, and he put in it a little boat drawn by a string which ran over a pulley at the end of the trough, with a shilling attached for a weight. In this way he succeeded in demonstrating a natural law which, though known to practical men, had never been described in books of science. He took much pains to collect information about the Gulf Stream. This wonderful river in the ocean has been long known, but the first people to observe it closely were the Nantucket whalemen, who found that their game was numerous on the edges of it, but was never seen within its warm waters. In consequence of their more exact knowledge they were able to make faster voyages than other seamen. Franklin learned about it from them, and on his numerous voyages made many observations, which he carefully recorded. He obtained a map of it from one of the whalemen, which he caused to be engraved for the general benefit of navigation on the old London chart then universally used by sailors. But the British captains slighted it, and this, like his other efforts in science, was first appreciated in France. 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.