Bourdon Gauge, an instrument for measuring gaseous or liquid pressure, very extensively adopted in mechanical engineering. If a bent tube be closed at one end, and then be subjected to internal fluid pressure applied at the other end, greater than that of the atmosphere outside, the tube will tend to straighten itself. An opposite effect manifests itself when the internal pressure is reduced below that of the atmosphere. If one end of the tube be fixed, the motion of the other end may be made to record the difference between external and internal pressures. This is the principle of the Bourdon gauge.