Joule then set to work to determine, in the most accurate manner possible, the number of foot-pounds of work which, if entirely converted into heat, would raise one pound of water through 1 deg. Fahr. The best known of his experiments is that in which he caused a paddle to revolve by means of a falling weight, and thereby to churn a quantity of water contained in a cylindrical vessel, the rotation of the water being prevented by fixed vanes. In these experiments he allowed for the work done outside the vessel of water or calorimeter, for the buoyancy of the air on the descending weight, and for the energy still retained by the weight when it struck the floor. From the results obtained he deduced 772 foot-pounds as the mechanical equivalent of heat. Expressed in terms of the Centigrade scale, Joule's equivalent, that is, the number of foot-pounds of work in the latitude of Manchester, which, if entirely converted into heat, will raise one pound of water 1 deg. C., is 1390. Joule's experiments show that the same amount of energy always corresponds to, and can be converted into, the same amount of heat, and that no transformations, electrical or other, can ever increase or diminish this quantity. Maxwell expressed this principle as follows:-- _The energy of a system is a quantity which can neither be increased nor diminished by any actions taking place between the parts of the system, though it may be transformed into any of the forms of which energy is susceptible._ This is the great principle of the conservation of energy which is applicable equally to all branches of science.