  We have used the Fourier cross spectra of Cyg X-1, as obtained with BATSE
during a period of almost 2000 days, to estimate the phase (or time) lags
between X-ray flux variations in the 20-50 keV and the 50-100 keV bands as a
function of Fourier frequency, nu. We find that these lag spectra do not depend
on source state, as measured by the fractional rms variations of the X-ray
flux. For frequencies well below the Nyquist frequency the variations in the
50-100 keV band lag those in the 20-50 keV band by a time interval proportional
to nu^{-0.8}. Binning effects cause the observed hard X-ray time lags to
decrease to zero at the Nyquist frequency. Some previous results on time lags
were affected by these binning effects. At photon energies above 10 keV the
time lags are approximately proportional to the logarithm of the ratio the
energies of the passbands used.
