More heat is carried to the cylinder of a steam-engine than is carried from it. The piston does work at every stroke and this work comes from the heat that disappears. Every stroke of the piston annihilates heat. Careful experiments show that the heat destroyed and the work performed are in strict agreement with Joule's equivalent. With a given supply of fuel, the engine will give out less heat when it is made to work hard than when it runs without doing much work.