70 THE RUNNING-DOWN OF THE UNIVERSE 



into heat-energy. There is still the same quantity of 

 energy, but even if we could scrape it together and put 

 it through an engine we could not lift the stone back 

 with it. What has happened to make the energy no 

 longer serviceable? 



Looking microscopically at the falling stone we see 

 an enormous multitude of molecules moving downwards 

 with equal and parallel velocities — an organised motion 

 like the march of a regiment. We have to notice two 

 things, the energy and the organisation of the energy. 

 To return to its original height the stone must preserve 

 both of them. 



When the stone falls on a sufficiently elastic surface 

 the motion may be reversed without destroying the 

 organisation. Each molecule is turned backwards and the 

 whole array retires in good order to the starting-point — 



The famous Duke of York 



With twenty thousand men, 

 He marched them up to the top of the hill 



And marched them down again. 



History is not made that way. But what usually happens 

 at the impact is that the molecules suffer more or less 

 random collisions and rebound in all directions. They 

 no longer conspire to make progress in any one direc- 

 tion; they have lost their organisation. Afterwards they 

 continue to collide with one another and keep changing 

 their directions of motion, but they never again find a 

 common purpose. Organisation cannot be brought 

 about by continued shuffling. And so, although the 

 energy remains quantitatively sufficient (apart from un- 

 avoidable leakage which we suppose made good), it 

 cannot lift the stone back. To restore the stone we must 

 supply extraneous energy which has the required 

 amount of organisation. 



