Can mechanical energy vanish into thin air?

In 1845, James Prescott Joule published his celebrated experiment that established the mechanical equivalent of heat. He connected the falling of a weight to the rotation of paddles inside a water container, and measured the increase of temperature of the stirred water. Thereby he found a direct relation between the loss of potential energy by the descending weight and the heat transferred to the water. This result encouraged the contemplation of heat as a form of energy against the opposing theory of the caloric, a supposedly indestructible fluid flowing between bodies that exchange heat. In due time, after this and other similar experiments, the conservation of energy (including heat transfer) was recognized and sanctioned as the first law of thermodynamics. Here, Moreno explains why this is so, and how is it possible that the purported elastic energy of stretched rubber can disappear leaving no trace of being converted into heat or other forms of energy.