
The Beginnings 21
such that, when run in reverse, it must take more work to restore the original thermal
agency. However, it is clear that perfectly reversible engines permit the construction
of perpetual machines; but they can not perform any useful work. In practice, perfect
reversibility is anyway not possible to achieve, and even perpetual machines of this
limited kind are not possible.
Quite obviously, the reverse process should first of all be a physically realizable
process. Taking the water wheel as the example, clearly the reverse process, i.e of
pumping water from a lower level to a higher level, is certainly physically realizable.
Now if the ...