apparatus for carrying out the invention will usually comprise a probe means for measuring and or controlling the distance of the probe from the surface to be interrogated and means for applying controlled pressure to a liquid inside the probe the applied pressure produces a flow of liquid through the probe tip and in turn generates a related pressure at the interrogated surface the term u2018 interrogate u2019 is intended to refer to the ability to monitor changes at the surface of a structure e g to detect structural changes on or at the surface at a single position or as the probe scans the surface the pressure applied to the surface will if the surface is sufficiently pliable cause the surface to move positive pressure ie flow through the probe towards the surface has the effect of pushing the surface away from the probe increasing the separation between the surface and probe a negative pressure draws the surface towards the probe decreasing the separation however as it is intended that the apparatus will constantly control the distance of the probe from the surface any movement of the surface is followed by a corresponding movement of the probe as a result of this the inter relationship between the liquid.