In H. G. Wells's novel The First Men in the Moon professor Cavor invents a material - cavorite - which can shield against gravity just as a conductor can shield from electrostatic fields. Electrostatic shielding is possible because there are both positive and negative charges in nature. So gravitational shielding requires the existence of matter possessing negative mass. The object of this question is to show that the existence of particles of negative mass would not only enable us to shield against gravity but also to extract limitless amounts of energy from the interaction of the two sorts of mass.