1. A jar apparatus comprising: a mandrel member telescopically disposed within a housing member, said members being relatively movable between extended and contracted positions; means for connecting said members in a pipe string; coengageable impact surfaces adapted to engage one another in at least one of said contracted and extended positions; first chamber means between said members; hydraulic means enclosed by said first chamber means for releasably retarding relative movement of said members; second chamber means between said members separate from said first chamber means; means on said mandrel and housing members enclosed by said second chamber means for slidably and corotatively coupling said members together; said first chamber means being filled with a hydraulic fluid having a substantially constant viscosity over a wide temperature range; said second chamber means being filled with a lubricant adapted to prevent wear on said members; and balancing means for each of said first and second chamber means to equalize pressures therein with the hydrostatic pressure of the fluids in a well bore.

2. A jar apparatus comprising: an inner tubular member telescopically disposed within an outer tubular member; means for connecting each of said members to a pipe string; limit means for stopping telescoping movement of said members in longitUdinally spaced positions; first chamber means between said members filled with a hydraulic fluid; hydraulically operable means in said first chamber for retarding relative movement only over an initial portion of the relative movement of said members, said hydraulic means then enabling free telescoping movement over the remaining portion of such movement; second chamber means between said members filled with a lubricant; spline means totally disposed within said second chamber means for preventing rotation of one member relative to the other; and seal means between said members at the ends of each of said chambers, the respective seal means for each chamber being located on the same seal diameter to provide constant volume for the respective chambers during telescopic movement of said members.

5. A well jar apparatus comprising: a housing assembly having a cylinder section, longitudinally spaced seal sections and a spline section; a mandrel assembly telescopically disposed within said housing assembly and including a cylinder section and a spline section; coengageable impact surfaces on said housing and mandrel assembly adapted to be bought together to deliver an impact blow to an object stuck in a well bore; first chamber means between said cylinder sections; hydraulic fluid filling said first chamber means; first seal means on one of said seal sections sealingly slidable on one of said cylinder sections for closing one end of said first chamber means; first pressure transmitting means sealingly slidable between said cylinder sections for closing the other end of said first chamber means and for equalizing the fluid pressure in said first chamber means with hydrostatic pressure of fluids in a well bore; hydraulically operable means in said first chamber means having portions cooperating with said cylinder sections for retarding relative longitudinal movement only over an initial portion of the movement of said mandrel and housing assemblies; second chamber means between said mandrel and housing assemblies; a lubricant filling said second chamber means; second seal means on the other of said seal sections sealingly slidable on one of said spline sections for closing one end of said second chamber means; second pressure transmitting means sealingly slidable between said spline sections and closing the other end of said second chamber means, said second pressure transmitting means functioning to equalize fluid pressure in said second chamber means with the hydrostatic pressure of fluids in a well bore; and longitudinally extending splines on said spline sections in said second chamber means for preventing relative rotation of said mandrel and housing assemblies while enabling relative longitudinal movement thereof.