  The geometry of canal hypersurfaces of an n-dimensional conformal space C^n
is studied. Such hypersurfaces are envelopes of r-parameter families of
hyperspheres, 1 \leq r \leq n-2. In the present paper the conditions that
characterize canal hypersurfaces, and which were known earlier, are made more
precise. The main attention is given to the study of the Darboux maps of canal
hypersurfaces in the de Sitter space M_1^{n+1} and the projective space
P^{n+1}. To canal hypersurfaces there correspond r-dimensional spacelike
tangentially nondegenerate submanifolds in M_1^{n+1} and tangentially
degenerate hypersurfaces of rank r in P^{n+1}. In this connection the problem
of existence of singular points on canal hypersurfaces is considered.
