of piston 140 is acted upon by the ambient string pressure through annular passage 127 which communicates with bores 132 and 134 the jar 10 as a whole is exposed to the ambient temperature at that depth in the well bore increased pressure will naturally tend to move equalizing piston 140 upwardly compressing the fluid in jar 10 increasing temperature will tend to expand the fluid in jar 10 moving equalizing piston 140 in a downward direction as the string moves through the well varying temperatures and pressures will move equalizing piston back and forth always maintaining the fluid on both sides of metering assembly 200 at the same pressure to ensure that a pressure buildup on one side of the metering assembly 200 or the other which buildup could diminish the jarring effect of the tool or possibly rupture a seal an increase in pressure in lower reservoir chamber 142 will result in a bleedoff to intermediate chamber 60 through radial passage 212 forcing o ring 216 away from the mouth thereof while the viscosity of the fluid in jar 10 will vary somewhat with temperature the presence of equalizing piston pressurizing the fluid will ensure substantial uniformity of jarring force and there is no substantial variation of time from initiation of an upward pull on mandrel assembly 90 to time of impact of hammer element 110 on anvil element 46 due to.