We study the role of defects in the flow properties of a thermotropic smectic (in Cano wedge geometry) by in situ deformation experiments in an optical polarizing microscope. The motion of the edge dislocation array is studied for imposed strain rates and their climb velocity is measured. At a stress close to 3000 dyn/ cm2a jerky climb of edges is observed implying the existence of screw dislocations which hinder the free motion of the former. At a stress close to 5000 dyn/ cm2a viscous motion of edges is evidenced for which we confirm the validity of the Orowan relation, established for solid crystals.