248 THE POPULAR SCIENCE MONTHLY. 



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 di- 

 rection. 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 com- 

 bustion 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 mo- 

 tion, 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 



