more delicate observations and experiments, which we may suppose made by one who can perceive and handle the individual molecules which we deal with only in large masses."
In the theory of dynamics we say that action and reaction are equal; and if a body be arrested in the course of any motion, and sent back on its path with exactly the same velocity, it will retrace its path, and at any point of that path it will have exactly the same velocity and the same energy as when it passed through it in the opposite direction. But, practically, we are unable to realize such a law, because of the resistances we meet with in friction, electric induction, etc., so that any series of actions taking place in nature is not a reversible one, or the mechanical energy spent can never be wholly restored to its primitive condition. But as a reversible process is the only one which can maintain itself for all time, it follows that our earth is gradually lowering its energy from high to lower classes; the ultimate form to which all must be reduced being that which has its source not in the position or motion of masses, but of molecules.
If this process of transformation were not continually going on, our fires would cease to burn, our machines stop working, and all animal and vegetable life would perish. "The whole of active life is simply transformations of energy." Wherever two particles of matter fall together, whenever a drop of rain falls to the earth, whenever an atom of carbon combines with an atom of oxygen in the furnace, we must look upon it as so much energy let down, the greater part of which is dissipated and lost to human good. But it may easily be seen that this degradation of energy is not restricted to the earth alone, for among natural forces we recognize—
1. The energy of fuel, under which we include the energy of food, as being simply the fuel of an animated machine.
2. The energy of a head of water.
3. The natural motions of air and water.
4. The tides and trade-winds.
5. The very inconsiderable mechanical effect derived from the combustion of native sulphur, and from meteoric sources. The first three are wholly due to the sun, and the fourth in part, so that by far the greater amount of our available energy is derived from the sun. "For it is he who separates the carbon from the oxygen of the carbonic acid and enables them to combine again, whether in the furnace of the steam-engine or in the animal body." It is he who sets the air in motion, and raises up the water whose fall is to turn the wheels of our mills. We are thus receiving a constant supply of energy from the sun, and his must be diminished in a corresponding degree.
But, not content with what we receive, we dig down into the bowels of the earth and exhaust our own. There can be no doubt as to the final result to which this universal tendency points. Long after the earth has become uninhabitable, it may be, the kinetic energy of
