This would explain why the horizontal measurement h (in the drawing below) is about 8” greater than the vertical measurement v. These measurements were made using an electromagnetic device which emits a high frequency signal. Part of this signal is reflected back from the rear surface of the stone at a rate which is constant for that particular material. The time the signal takes to return to the source is proportional to the thickness of the rock. The measurements obtained by this technique are no more than two percent in error.