The idea of conservation laws has been around almost as long as powdered wigs, arising out of the ideas presented by Isaac Newton after that whole apple incident. For years and years, scientists debated what exactly it was that was being conserved—they knew something had to stay constant, but it was hard to get a consensus on just what, exactly.

Some championed "motion,'' a shorthand for the product of mass and velocity, while others fought for vis viva, an idea introduced by Gottfried Wilhelm Leibniz which was described by the product of mass and velocity squared (sound familiar?).

Perhaps the biggest sticking point in the entirety of the argument for conservation of energy, however, was energy's pesky tendency to up and disappear from a system, apparently not conserved at all. Things would heat up, they would make noise—but eventually they would slow down and stop moving.

Enter a man named James Prescott Joule. The heir to a rather vast brewery fortune, Joule preferred to spend his time doing one of two things:

1) growing a bountiful and glorious beard, and
2) science.

Perhaps growing up around the workings of a brewery had an effect on the young Joule, though, because it was Joule who devised the crucial experiment proving what he called "the mechanical equivalence of heat''—that is, that heat is just another form of mechanical energy. Debate over.

How'd he do it? Well, meticulously.

Having spent his youth watching machines churn endless vats of hops, Joule finally decided to apply his knowledge to the problem of conservation of energy. He rigged up a large weight (like, 700 pounds large) to a pulley, wrapped the rope holding the weight around a paddle that sat submerged in a bucket of water, and then dropped the weight.

Joule's Apparatus
(Source)

As the weight fell, it turned the paddle, which started moving the water. This heated the water up a tiny bit—and Joule measured that increase in temperature, proving it was exactly equal to the work gravity had done lowering his weight.

"But James!'' the other physicists cried, "we can't believe this!''

"Well, read my paper,'' Joule said, putting on his favorite sunglasses. "I showed my work.''

YEEEAAAAAHHHHHHHHH