<P> Meanwhile, in 1843, James Prescott Joule independently discovered the mechanical equivalent in a series of experiments . In the most famous, now called the "Joule apparatus", a descending weight attached to a string caused a paddle immersed in water to rotate . He showed that the gravitational potential energy lost by the weight in descending was equal to the internal energy gained by the water through friction with the paddle . </P> <P> Over the period 1840--1843, similar work was carried out by engineer Ludwig A. Colding, although it was little known outside his native Denmark . </P> <P> Both Joule's and Mayer's work suffered from resistance and neglect but it was Joule's that eventually drew the wider recognition . </P> <P> In 1844, William Robert Grove postulated a relationship between mechanics, heat, light, electricity and magnetism by treating them all as manifestations of a single "force" (energy in modern terms). In 1846, Grove published his theories in his book The Correlation of Physical Forces . In 1847, drawing on the earlier work of Joule, Sadi Carnot and Émile Clapeyron, Hermann von Helmholtz arrived at conclusions similar to Grove's and published his theories in his book Über die Erhaltung der Kraft (On the Conservation of Force, 1847). The general modern acceptance of the principle stems from this publication . </P>

Who invented the theory of conservation of energy