Abstract: An acceleration sensor uses a capsule of mercury filling a rigid enclosure with pressure sensors responsive to inertial forces exerted by the mercury on the walls of the enclosure when it is caused to accelerate. The enclosure may have a flexible lining on its walls or contain a flexible bag for holding the mercury. Means are provided for producing a bias pressure in the mercury. The enclosure may be cubic or tetrahedral, for example, and the outputs of the pressure sensors additively combined to produce outputs representing accelerations in required orthogonal directions when these differ from the directions to which the sensors are responsive. A rotational acceleration sensor is also described.