consumption of heat, but by its transportation from a hot body to a cold one. Thus, by the fall of heat from a higher to a lower temperature, work could be done in the same way that work could be done by allowing water to fall from a higher to a lower level. The quantity of water which reaches the lower level is exactly the same as that which leaves the higher level, as none of the water is destroyed in the fall. He argued, therefore, that the work produced by a heat-engine was produced in a similar manner, the quantity of heat which reaches the condenser being supposed to be equal to that which left the source. Thus the work was done by the heat flowing from a hot body to a cold one, and, in doing this work, it lost its momentum like falling water, and was brought to rest. One of the most important points noted by Carnot is the necessity that, in all engines which derive work from heat, there must be two bodies at different temperatures, that is, a source and a condenser, which correspond to a hot and cold body, so that there may be the passage of heat from the hot to the cold body. In order to get work out of heat it is absolutely necessary to have a hotter and a colder body. From this reasoning we learn, therefore, that work is obtained from heat by using up the heat of the hotter body, part of which is converted into actual work, while part is absorbed by the colder body. So that wherever we have two bodies at different temperatures, according to the second law of thermodynamics, there we have the power of doing work by the transmission of heat, from the body of higher to the one of lower temperature. That Carnot ultimately came to believe in the dynamical theory of heat, is proved by the following passage taken from his notes on the Motive Power of Heat: "It would be ridiculous to suppose that it is an emission of matter, while the light which accompanies it could only be a movement. Could a motion produce matter? No! undoubtedly, it can only produce a motion. Heat is then the result of motion. It is plain then that it could be produced by the consumption of motive power, and that it could produce this power. Heat is then simply motive power, or rather motion which has changed its form. It is a movement among the particles of bodies. Wherever there is a destruction of motive power, there is at the same time production of heat in quantity exactly proportional to the quantity of motive power destroyed. Reciprocally, whenever there is destruction of heat there is production of motive power."