  We observed Cyg X-1 with {\em RXTE} contiguously over its 5.6-day binary
orbit. The source was found to be in the hard state throughout the observation.
Many intensity dips were detected in the X-ray light curves. We found that the
dips fell into two distinct categories based on their spectral properties. One
type exhibits strong energy-dependent attenuation of X-ray emission at low
energies during a dip, which is characteristic of photoelectric absorption, but
the other type shows nearly energy-independent attenuation. While the first
type of dips are likely caused by density enhancement in an inhomogeneous wind
of the companion star, as previous studies have indicated, the second type
might be due to partial obscuration of an extended X-ray emitting region by
optically thick ``clumps'' in the accretion flow. It is also possible that the
latter are caused by a momentary decrease in the X-ray luminosity of the
source, due, for instance, to a decrease in the mass accretion rate, or by
Thomson scattering in highly ionized ``clumps''. We discuss the implications of
these scenarios.
