Abstract: A bumping and jarring tool for a well drill string is provided having a telescoping housing and mandrel with an annular hydraulic chamber therebetween divided into two portions. The lower portion is exposed to the ambient hydrostatic well pressure and the upper portion varies in volume as the tool opens and closes. When the tool is opened a jar is produced by a valve between the chamber portions which restricts hydraulic fluid flow between the chamber portions resulting in a pressure drop in the upper portion and by a pressure increasing element in the lower portion which increases the pressure therein while the pressure drops in the upper portion. When the valve permits unrestricted flow again, the sudden return to pressure equilibrium produces the jar stroke.