SIR ROBERT BALL'S “Theory of Screws” is one of the most notable modern extensions of theoretical dynamics. It is based on Poinsot's discovery that every set of forces, regarded as acting on a rigid body, is reducible to a force along one definite line and a couple round the line; combined with Chasles's discovery that every instantaneous motion of a rigid body is reducible to rotation round one definite line and translation along it—in other words to a screwing motion. The modes of reduction in the two cases are strictly analogous; a force along a line being the analogue of a rotation round the line, and translation in any direction being the analogue of a couple whose axis has this direction. A Treatise on the Theory of Screws. By Sir Robert Stawell Ball Pp. xix + 544. (Cambridge University Press, 1900.)