Can any one explain what Feynman means when he is talking of a reversible machine in the chapter four of his lectures? What exactly is a reversible machine? Please explain this paragraph : "If when we have lifted and lowered a lot of weights and restored the machine back to its original condition,we find that the net result is to have lifted a weight, then we have a perpetual motion machine. A weight lifting machine has three units on one balance pan and one unit on another. However in order to get it actually to work, we must lift a little weight of off the left pan. (three units is in left pan and one unit in right). On the other hand,we could lift a one unit weight by lowering three unit weight, if we cheat a little by lifting a little weight off the other pan. Of course we realize that with any actual lifting machine we must add a little extra to get it run. Ideal machines do not require anything extra. A machine we actually use can be in a sense almost reversible: that is if it will lift the weight of three by lowering the weight of one then it will also lift nearly the same amount by lowering the weight of three. We imagine that there are two classes of machines those that are not reversible and those that are reversible. We have a reversible machine A which lowers one unit weight by one unit distance and at same time lifts three unit weight by a distance X. We have another machine B, but not reversible which also lowers one unit weight by a unit distance and lifts the three unit weight by distance Y. We can prove Y can not be greater than X. Suppose if Y were greater than X, we could lower the weight from Y to X, obtaining free power and use reversible machine A, running backwards to lower the three unit weight by a distance X and lift one unit weight by distance one unit. This will put one unit weight back where it was before and leave both machines ready to be used again. We would have a perpetual motion if Y were higher than X" please explain the above!