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XL VII. On a Universal Tendency in Nature to the Dissipation of 

 Mechanical Energy. By Prof. W. Thomson*. 



THE object of the present communication is to call attention 

 to the remarkable consequences which follow from Carnot's 

 proposition, that there is an absolute waste of mechanical energy 

 available to man when heat is allowed to pass from one body to 

 another at a lower temperature, by any means not fulfilling his 

 criterion of a " perfect thermo-dynamic engine," established, on 

 a new foundation, in the dynamical theory of heat. As it is 

 most certain that Creative Power alone can either call into exist- 

 ence or annihilate mechanical energy, the " waste" referred to 

 cannot be annihilation, but must be some transformation of 

 energyf. To explain the nature of this transformation, it is 

 convenient, in the first ]3lace, to divide stores of mechanical 

 energy into two classes — statical and dynamical. A quantity of 

 weights at a height, ready to descend and do work when wanted, 

 an electrified body, a quantity of fuel, contain stores of mechani- 

 cal energy of the statical kind. Masses of matter in motion, a 

 volume of space through which undulations of light or rachant 

 heat are passing, a body having thermal motions among its 

 particles (that is, not infinitely cold), contain stores of mechani- 

 cal energy of the dynamical kind. 



The following propositions are laid down regarding the dissi- 

 pation of mechanical energy from a given store, and the restora- 

 tion of it to its primitive condition. They are necessary conse- 

 quences of the axiom, " It is impossible, by means of inanimate 

 material agency, to derive mechanical effect from any portion of 

 matter by cooling it below the temperature of the coldest of the 

 surrounding objects." (Dyuam. Th. of Heat, § 12.) 



I. When heat is created by a reversible process (so that the 

 mechanical energy thus spent may be restored to its primitive 

 condition), there is also a transference from a cold body to a hot 

 body of a quantity of heat bearing to the quantity created a defi- 

 nite proportion depending on the temperatures of the two bodies. 



II. When heat is created by any unreversible process (such as 

 friction), there is a dissipation of mechanical energy, and a full 

 restoration of it to its primitive condition is impossible. 



III. Wlien heat is diffused by conduction, there is a dissipation 

 of mechanical energy, and perfect restoration is impossible. 



IV. When radiant heat or light is absorbed, otherwise than in 



* From the Proceedings of the Royal Society of Edinburgh for April 1 9, 

 1852. Communicated by the Author. 



t See the Author's previous paper on the Dynamical Theory of Heat, 

 §22. 



