Abstract: A tool for enhancing the impact of either an upward or downward blow delivered by a single-acting jar to an object stuck in a well bore includes first and second tubular members telescopically arranged and vertically reciprocable with respect to one another to form an annular space between them, upper, lower and intermediate seal rings between equal diameter portions of the tubular members, and a ring carried by the second tubular member within the annular space between the intermediate seal ring and each of the upper and lower seal rings to form upper and lower pressure and charging chambers on the opposite side of each ring, the rings being of such construction that, with each charging chamber filled with a compressible fluid, movement of the second tubular member in one vertical direction will compress the fluid in the upper and lower pressure chambers and thus enhance the impact of a jar in the same vertical direction, and permit flow therepast, in response to movement of the second tubular member in the