The subjection of the core to a hydraulic pressure of four tons to the **square inch** and an electric pressure of 5000 volts from an alternating-current transformer has been adopted, by one manufacturer at least, to secure the detection of masked faults which might develop themselves after submergence.

Let the boiler pressure be 175 lb per **square inch**.

Taking 85% of this, the maximum mean effective pressure would be 149 lb per **square inch**.

The engine can only exert this large tractive force so long as the mean pressure is maintained at 149 lb per **square inch**.

When the initial pressure is 100 lb per **square inch** by the gauge the thermal efficiency drops to about nearly 15% with the same back pressure.