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Theories and experiments

Franklin was, along with his contemporary Leonhard Euler, the only major scientist who supported Christiaan Huygens 's wave theory of light, which was basically ignored by the rest of the scientific community. In the 18th century, Isaac Newton 's corpuscular theory was held to be true; it took Thomas Young's well-known slit experiment in 1803 to persuade most scientists to believe Huygens's theory.

On October 21, 1743, according to the popular myth, a storm moving from the southwest denied Franklin the opportunity of witnessing a lunar eclipse. He was said to have noted that the prevailing winds were actually from the northeast, contrary to what he had expected. In correspondence with his brother, he learned that the same storm had not reached Boston until after the eclipse, despite the fact that Boston is to the northeast of Philadelphia. He deduced that storms do not always travel in the direction of the prevailing wind, a concept that greatly influenced meteorology. After the Icelandic volcanic eruption of Laki in 1783, and the subsequent harsh European winter of 1784, Franklin made observations on the causal nature of these two seemingly separate events. He wrote about them in a lecture series.

Though Franklin is famously associated with kites from his lightning experiments, he has also been noted by many for using kites to pull humans and ships across waterways. George Pocock in the book A Treatise on The Aeropleustic Art, or Navigation in the Air, by means of Kites, or Buoyant Sails noted being inspired by Benjamin Franklin's traction of his body by kite power across a waterway.

Franklin noted a principle of refrigeration by observing that on a very hot day, he stayed cooler in a wet shirt in a breeze than he did in a dry one. To understand this phenomenon more clearly, he conducted experiments. In 1758 on a warm day in Cambridge, England, he and fellow scientist John Hadley experimented by continually wetting the ball of a mercury thermometer with ether and using bellows to evaporate the ether. With each subsequent evaporation, the thermometer read a lower temperature, eventually reaching 7 °F (−14 °C). Another thermometer showed that the room temperature was constant at 65 °F (18 °C). In his letter Cooling by Evaporation, Franklin noted that, "One may see the possibility of freezing a man to death on a warm summer's day."

In 1761, Franklin wrote a letter to Mary Stevenson describing his experiments on the relationship between color and heat absorption. He found that darker color clothes got hotter when exposed to sunlight than lighter color clothes, an early demonstration of black body thermal radiation. One experiment he performed consisted of placing square pieces of cloth of various color out in the snow on a sunny day. He waited some time and then measured that the black pieces sank furthest into the snow of all the colors, indicating that they got the hottest and melted the most snow.

According to Michael Faraday, Franklin's experiments on the non-conduction of ice are worth mentioning, although the law of the general effect of liquefaction on electrolytes is not attributed to Franklin. However, as reported in 1836 by Franklin's great-grandson Alexander Dallas Bache of the University of Pennsylvania, the law of the effect of heat on the conduction of bodies otherwise non-conductors, for example, glass, could be attributed to Franklin. Franklin wrote, "... A certain quantity of heat will make some bodies good conductors, that will not otherwise conduct..." and again, "... And water, though naturally a good conductor, will not conduct well when frozen into ice."

While traveling on a ship, Franklin had observed that the wake of a ship was diminished when the cooks scuttled their greasy water. He studied the effects on a large pond in Clapham Common, London. "I fetched out a cruet of oil and dropt a little of it on the water... though not more than a teaspoon full, produced an instant calm over a space of several yards square." He later used the trick to "calm the waters" by carrying "a little oil in the hollow joint of cane."