  We report our measurement of the rate of change of period with time dP/dt for
the 215 s periodicity in the pulsating white dwarf G 117-B15A, the most stable
optical clock known. After 31 years of observations, we have finally obtained a
4 sigma measurement dP/dt_observed = (4.27 +/- 0.80) x 10^{-15} s/s. Taking
into account the proper-motion effect of dP/dt_pm = (7.0 +/- 2.0) x 10^{-16}
s/s, we obtain a rate of change of period with time of dP/dt = (3.57 +/- 0.82)
x 10^{-15} s/s. This value is consistent with the cooling rate in our white
dwarf models only for cores of C or C/O. With the refinement of the models, the
observed rate of period change can be used to accurately measure the ratio of
C/O in the core of the white dwarf.
