In the early part of the nineteenth century students of light and heat began to demonstrate that these two natural forces were different manifestations of one universal medium. This in turn led to the thought that possibly these forces, instead of being absolutely distinct, could be converted one into the other. This idea was confirmed in various experimental ways. Sir Humphrey Davy, about the end of the eighteenth century, rubbed together two pieces of ice until they were nearly melted. Precautions had been taken that no heat could be abstracted from the outside by the ice. The only tenable conclusion was that the energy expended in rubbing, had been converted into heat, which had melted the ice. About the same time, Count Rumford, a distinguished American, was superintending the boring of a cannon at the arsenal at Munich, and was forcibly struck with the heating of the iron due to this process. He, like Davy, believed that the energy of the boring instruments had been converted into the heat.[A] [Footnote A: The Conservation of Heat--Stewart, pp. 38, 39.] From 1843 to 1849, Dr. Joule of Manchester, England, published the results of experiments on the relation between mechanical energy and heat. Dr. Joule attached a fixed weight to a string which was passed over a pulley, while the other end was connected with paddles moving in water. As the weight descended, the paddles were caused to revolve; and it was observed that, as the weight fell and the paddles revolved, the water became warmer and warmer. Dr. Joule found further that for each foot of fall, the same amount of heat energy was given to the water. In fact, he determined that when a pound weight falls seven hundred and seventy two feet it gives out energy enough to raise the temperature of one pound of water one degree Fahrenheit.[A] This experiment, frequently repeated, gave the same result and established largely the law of the convertibility of energy. [Footnote A: The Conservation of Energy--Stewart, pp. 44, 45. Recent Advances in Physical Science--Tait, pp. 63, 65.] About the same time, it was shown that light can be converted into heat; and later it was proved that electricity may be changed into heat or light. In all these cases it was found that the amount of energy changed was exactly equal to the amount of energy produced. Thus, by countless experiments, it was finally determined that energy is indestructible; that, when any form of energy disappears, it reappears immediately in another form. This is the law of the persistence of force or energy. In more recent days, it has been suggested that all known forces are variations of a great universal force, which may or may not be known. The very nature of force or energy is not understood. In the language of Spencer, "By the persistence of force, we really mean the persistence of some cause which transcends our knowledge and conception."[A] [Footnote A: First Principles, Spencer, 4th ed., p. 200.]