Abstract: This invention relates to a mobile drilling rig used for deep wells. The mast structure can be made considerably lighter than previously by translation of the drilling string lifting pressure against the bottom of the mast rather than being supported by the mast itself. A hydraulic cylinder extending upwardly within the raised mast bears against the bottom of the mast, and has a pulley at its top over which a lifting chain passes. One end of the chain is linked to the bottom of the mast, the other to the top drive. As the cylinder extends, the top drive rises, lifting the drilling string, but with virtually the entire lifting force against the bottom of the mast.