  The hydrodynamic regime of charge transport has been recently realized in
high-quality conductors. In the hydrodynamic as well as in the Ohmic regimes
the main part of the Hall resistance of a long sample is determined by the
balance between the Lorentz force and the electric force, acting on conduction
electrons. Experimentally observed deviations of the Hall resistance in
hydrodynamic samples from such the ''standard'' value are usually associated
with the Hall viscosity term in the Navier-Stokes equation. In this work we
theoretically study the Hall effect in a Poiseuille flow of a two-dimensional
electron fluid. We show that the near-edge semiballistic layers with the width
of the order of the inter-particle mean free path, which inevitably appear near
sample edges, give the contribution to the Hall resistance which is comparable
with the bulk contribution from the Hall viscosity. In this way, the measured
deviations of the Hall resistance from the ''standard'' one in hydrodynamic
samples by the usual contact techniques should be associated with both the Hall
viscosity in the bulk and the semiballistic effects in the near-edge layers
