The elementary conception of a steam engine — The essential elements of steam engines-description of a simple form of modern steam engine and boiler — Distribution of steam by an ordinary slide valve — The use of the ?y wheel — Various purposes for which steam engines are employed — Importance of the accurate study of the engine in all its bearings — The natural subdivisions of the subject. The complete study of the steam engine is, in its nature, somewhat complex, involving as it does an acquaintance with the sciences of heat, of chemistry, and of pure and applied mechanics, as well as a knowledge of the theory of mechanism and the strength of materials. It is proposed, therefore, to begin this work by showing, in a very simple case, how steam can be used to do work, and then to pro ceed to describe an actual steam engine of the most modern construction, but one which at the same time is remarkably free from complexity. When studying this description, the student will soon find out how it is that the perfect know ledge of the steam engine involves an acquaintance with so many branches of science; and the order in which these subjects must be studied, so far as they bear on the matter in hand, will naturally be suggested by the description.