(2.) The object of the present volume will be to deliver, in an easy and familiar style, an historical view of the invention of the steam engine, and an exposition of its structure and operation in the various forms in which it is now used, and of its most important applications in the arts of life, especially in transport by land and water. It is hoped that the details of these subjects may be rendered easily intelligible to all persons of ordinary information, whether urged by that natural and laudable spirit of inquiry awakened by contemplating effects on the material and social condition of our species, so rapid and so memorable as those which have followed the invention of the steam engine, and by the pleasure which results from the perception of the numerous instances of successful contrivances and beautiful applications of science to art which it unfolds,—or impelled by the exigencies of trade or profession to acquire an acquaintance with a machine on which, more than any other, the prosperity of our commercial and manufacturing interests depends. It will be our aim to afford to the former class all the information which they can require; and, if this work be not as comprehensive in its scope, and as minute in its details, as some of the latter may wish, it will at least serve as an easy and convenient introduction to other works more voluminous, costly, and detailed, but less elementary in their matter, and less familiar in their style. In explaining the different forms of steam engine which have been proposed in the course of the progressive improvement [Pg010] of that machine from its early rude and imperfect state to its present comparatively perfect form, it will be necessary to advert to various physical phenomena and mechanical principles, which, however obvious to those who are conversant with matters of science, must necessarily be at least imperfectly known by the great majority of our readers. To refer for information on such topics to other works on Mechanics and general Physics, would be with most readers ineffectual, and with all unsatisfactory. In former editions of the present work, we consigned these necessary general principles of physics and mechanics to a preliminary chapter; but it appears, on the whole, more convenient not to remove the exposition of the principle from the place where its application is required. We shall therefore pause as we proceed, where these difficulties occur, to give such explanation and illustration as may seem best suited to render them intelligible and interesting to the unscientific reader.