a hydraulic jar apparatus to be disposed in a drilling string embodying inner and outer telescopically arranged elements . overlapping portions of the elements provide an annual chamber confining an operating liquid by an annular seal fixed to the outer element at the lower end of the chamber and an annular polly pack seal fixed to the outer element at the upper end of the chamber . a piston is extended radially from the inner element into the chamber and the chamber is divided by a cylinder on the outer element into low and high pressure sections . impact shoulders are provided on the elements in axially opposed relation to produce a jarring blow and the elements are telescopically coupled by a hexagonal spline sub assembly .