'The term 'natural philosophy' was used by Newton, and is still used in British Universities, to denote the investigation of laws in the material world, and the deduction of results not directly observed.' This definition, from the Preface to the second edition of 1879, defines the proposed scope of the work: the two volumes reissued here are the only completed part of a survey of the entirety of the physical sciences by Lord Kelvin and his fellow Scot, Peter Guthrie Tait, first published in 1867. Although the partnership ceased after eighteen years of collaboration, the published books, containing chapters on kinematics, dynamics and statics, had a great influence on the development of physics in the second half of the nineteenth century.