  In this paper we consider the viability of these non-gravitational torques to
take the gas off the plane. We show that magnetically generated warps are
clearly flawed because they would wrap up into a spiral in less than two or
three galactic rotations. The inclusion of any magnetic diffusivity to dilute
the wrapping effect, causes the amplitude of the warp to damp. We also consider
observational consequences of the accretion of an intergalactic plane-parallel
flow at infinity. We have computed the amplitude and warp asymmetry in the
accretion model, for a disc embedded in a flattened dark matter halo, including
self-consistently the contribution of the modes with azimuthal wavenumbers m=0
and m=1. This model predicts quite asymmetric warps, maximum gas displacements
on the two sides in the ratio 3:2 for the preferred Galactic parameters,and the
presence of a fraction ~3.5% of U-shaped warps, at least. The azimuthal
dependence of the moment transfer by the ram pressure would produce a strong
asymmetry in the thickness of the HI layer and asymmetric density distributions
in z, in conflict with observational data for the warp in our Galaxy and in
external galaxies. The amount of accretion that is required to explain the
Galactic warp would give gas scaleheights in the far outer disc that are too
small. We conclude that accretion of a flow with no net angular momentum,
cannot be the main and only cause of warps.
