In this paper, we present a new two-dimensional viscometer, and the hydrodynamic
calculations used to obtain the surface viscosities from the measurements. In order to
interpret the experiments, performed with solutions of sodium dodecyl sulfate (SDS)
and also with monolayers of insoluble surfactants, we develop various hydrodynamic
models of soluble Gibbs monolayers and of incompressible Langmuir monolayers,
that describe well the experimental results. In the case of SDS solutions, the calculations
allow the determination of the surface shear viscosity, and its value is in good
agreement with previous studies.