Abstract: A linear displacement device has a body; a plurality of masses mechanically coupled to the body for reciprocating movement in a common direction with respect to the body; and a mechanism for driving the masses so that they oscillate periodically with different frequencies of oscillation such that; the impulse applied to the body by the oscillating masses, in one direction is in excess of the minimum impulse required to move the body in the one direction, by a greater amount. than the reaction impulse applied to the body in the opposite direction is in excess of the impulse required to move the body in the opposite direction, so that the body will move in the one direction.