  In this short communication, I give a very simple derivation of the Jarzynski
equality, which allows to compute the free energy difference of a body, which
is driven between two equilibrium states $A$ and $B$ by an external
(time-dependent) force, from the probability distribution function of the work
done on the system, regardless of the nature of the transformation (reversible
or irreversible) between the states $A$ and $B$. It perhaps throws some light
on the debate recently started by Cohen and coworkers, and perhaps explains why
all the experiments which has been performed in order to test the Jarzynski
equality are successful, even in very defavorable (irreversible) cases (and
more strikingly, when the state $B$ is not an equilibrium one).
