Abstract: A jarring and bumping tool for a drill string is described. The tool includes an inner mandrel and an outer housing telescopically coupled together and defining therebetween an annular chamber for hydraulic fluid. A floating piston assembly is located in said chamber and is moved along in the chamber by a piston displacing device carried by the mandrel. A vertical strain applied to the mandrel when the tool is in use causes the said device to lift the piston assembly. The pressure of hydraulic fluid in the said chamber cushions movement of the mandrel until the piston assembly enters an enlarged portion of the hydraulic cylinder, whereupon the hydraulic pressure is released, allowing abutment faces on the mandrel and housing to slam together and apply an upward jar to the drill string. When the mandrel is returned downwardly, the piston displacing device draws the piston assembly down in the cylinder until movement of the assembly is arrested.