IT is the object of the series of lectures included in this volume to point out the enormous effect that the substi tution of mechanical power for hand and animal labor has had on the organization of society and the conditions of living, and by presenting the development of power machinery to show what sort of ideas have produced this result. The effect of power machinery is soon told and readily understood; but it is not so easy, though far more important, to show how the men who are responsible for what has been done have thought and worked, and with What kind of things and with what reasoning those men must deal who are to take up the responsibility for future progress. The bulk of the subject-matter, accordingly, is concerned with the apparatus and machinery for the converting of natural energy in any of its available forms into useful work, together with the physical processes for the execution of which that apparatus was devised.