Forward's extensive work in the field of gravitational wave detection included the invention of the rotating cruciform gravity gradiometer or 'Forward Mass Detector', for Lunar Mascon (mass concentration) measurements. The gravity gradiometer is described in the well-known textbook "Gravitation" by Misner, Thorne & Wheeler. The principle behind it is quite simple; getting the implementation right is tricky. Essentially, two beams are crossed over and connected with an axle through their crossing point. They are held at right angles to each other by springs. They have heavy masses at the ends of the beams, and the whole assembly spun around the common axle at high speed. The angle between the beams is measured continuously, and if it varies with a period half that of the rotation period, it means that the detector is experiencing a measurable gravitational field gradient.