The 200‐atmos mercury manometer described in previous papers is studied experimentally and revised, especially by removing the mercury reservoirs from the measuring system. A rotating loaded piston is used to provide a constant test pressure, and details as to its construction and use are given, as also for the toluene pump, valves, unions, Invar tape, and cathetometers. The temperature control of the manometer and the management of its fluids are described. The barostat piston was known to vary 2 to 3 parts per 104in. cross section. By means of a screw‐driven plunger the barostat was held manually at the same vertical position during each of a series of 8 readings of the barostat pressure. The maximum divergence from the mean of the series is 0.83 part per 104, and the average of the absolute values of the divergences from the mean is 0.30 part per 10. It is concluded that the manometer can be relied on to measure a pressure of about 200 atmos to within 1 part in 104.