On a Universal Tendency in Nature to the Dissipation of Mechanical Energy

By Sir William Thomson (Lord Kelvin)

Proceedings of the Royal Society of Edinburgh for April 19, 1852,
also Philosophical Magazine, Oct. 1852.
[This version from Mathematical and Physical Papers, vol. i, art. 59, pp. 511.]

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 existence or annihilate mechanical energy, the “waste” referred to cannot be annihilation, but must be some transformation of energy [1]. To explain the nature of this transformation, it is convenient, in the first place, 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 mechanical energy of the statical kind. Masses of matter in motion, a volume of space through which undulations of light or radiant heat are passing, a body having thermal motions among its particles (that is, not infinitely cold), contain stores of mechanical energy of the dynamical kind.

The following propositions are laid down regarding the dissipation of mechanical energy from a given store, and the restoration of it to its primitive condition. They are necessary consequences 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.” (Dynamical Theory of Heat [Art. XLVIII. above], §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 definite proportion depending on the temperature 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. When heat is diffused by conduction, there is a dissipation of mechanical energy, and perfect restoration is impossible.

IV. When radiant heat of light is absorbed, otherwise than in vegetation, or in chemical action, there is a dissipation of mechanical energy, and perfect restoration is impossible.

In connexion with the second proposition, the question, How far if the loss of power experienced by steam in rushing through narrow steam-pipes compensated, as regards the economy of the engine, by the heat (containing an exact equivalent of mechanical energy) created by the friction? is considered, and the following conclusion is arrived at:—

Let S denote the temperature of the steam (which is nearly the same in the boiler and steam-pipe, and in the cylinder till the expansion within it commences); T the temperature of the condenser; μ the value of Carnot's function, for any temperature t; and R the value of

[equation 1]

Then (1-R)w expresses the greatest amount of mechanical effect that can be economized in the circumstances from a quantity w/J of heat produced by the expenditure of a quantity w of work in friction, whether of the steam in the pipes and entrance ports, or of any solids or fluids in motion in any part of the engine; and the remainder, Rw, is absolutely and irrecoverably wasted, unless some use is made of the heat discharged from the condenser. The value of 1-R has been shown to be not more than about ¼ for the best steam-engines, and we may infer that in them at least three-fourths of the work spent in any kind of friction is utterly wasted.

In connexion with the third proposition, the quantity of work that could by got by equalizing the temperature of all parts of a solid body possessing initially a given non-uniform distribution of heat, if this could be done by means of perfect thermo-dynamic engines without conduction of heat, is investigated. If t be the initial temperature (estimated according to any arbitrary system) at any point xyz of the solid, T the final uniform temperature, and c the thermal capacity of unity of volume of the solid the required mechanical effect is of course equal to

[equation 2]

being simply the mechanical equivalent of the amount of heat put out of existence. Hence the problem becomes reduced to that of the determination of T. The following solution is obtained:—

[equation 3]

If the system of thermometry adopted [2] be such that μ = J / (t + a), that is if we agree to call J/μ - a the temperature of a body, for which μ is the value of Carnot's function (a and J being constants), the preceding expression becomes

[equation 4]

The following general conclusions are drawn from the propositions stated above, and known facts with reference to the mechanics of animal and vegetable bodies:—

1. There is at present in the material world a universal tendency to the dissipation of mechanical energy.

2. Any restoration of mechanical energy, without more than an equivalent of dissipation, is impossible in inanimate material processes, and is probably never effected by means of organized matter, either endowed with vegetable life or subject to the will of an animated creature.

3. Within a finite period of time past, the earth must have been, and within a finite period of time to come the earth must again be, unfit for the habitation of man as at present constituted, unless operations have been, or are to be performed, which are impossible under the laws to which the known operations going on at present in the material world are subject.


  • [1] See the Author's previous paper on the Dynamical Theory of Heat, §22 [Art. XLVIII. above].

  • [2] According to “Mayer's hypothesis,” this system coincides with that in which equal differences of temperature are defined as those with which the same mass of air under constant pressure has equal differences of volume, provided J be the mechanical equivalent of the thermal unit, and a-1 the coefficient of the expansion of air.