  A scalar potential model (SPM) was developed from considerations of galaxy
clusters and of redshift. The SPM is applied to HI rotation curves (RCs) and RC
asymmetry of spiral galaxies. The resulting model adds the force of a scalar
potential of the host galaxy and of neighboring galaxies to the Newtonian
rotation velocity equation. The RC is partitioned radially into regions. The
form of the equation for each parameter of each region is the same with
differing proportionality constants. Integer values of each equation are
determined empirically for each galaxy. Among the sample galaxies, the global
properties of galaxies of B band luminosity, of position, and of orientation
determine the RC and RC asymmetry. The Source of the scalar field acts as a
monopole at distances of a few kpc from the centre of spiral galaxies. The
scalar potential field causes Newtonian mechanics to considerably underestimate
the mass in galaxies, which is the ``missing mass problem''. The SPM is
consistent with RC and RC asymmetry observations of the sample spiral galaxies.
