If thirty years ago the author had fully realized the multiplicity and complexity of the problems which arise in designing machinery, the present treatise would probably not have been written. If he had now for the first time to face the task of writing it, he would no doubt take the view that for an adequate scientific treatment of the subject a much larger treatise would be necessary. However, at the suggestion of the late Professor Merrifield the author undertook to write a small but systematic treatise on Machine Design, and although he was shortly afterwards engaged in the onerous duties of a new Professorship the work was completed There are now so many aids to the study of the application of scientific principles to all branches of engineering practice, and so much of engineering experience has been made accessible, that the difficulty of dealing with the subject at the time this book was written will hardly be recognized now. That it has so long continued in demand may perhaps be taken to prove that it met a want and has proved useful. Perhaps on the whole it has been not the less useful that it was not originally projected on a more ambitious scale.

In 1890 the work was revised, considerable additions were made, and the chapters relating to engine details published separately. Since that time knowledge has advanced, and further revision became desirable. As in the previous revision, no change has been made in the plan or arrangement of the book, but about a hundred pages have been added, though this does not fully represent the amount of alteration. By omitting some parts and condensing others room has been found for extensive additions, and it is hoped that the modifications add considerably to the completeness, accuracy, and clearness of statement of the work.

This treatise was intended to occupy a distinct field between works on Applied Mechanics and empirical books of rules and collections of examples of machine details. In common with the former, a summary of scientific rules applicable in the design of machines is given, but with only so much of explanation as is necessary to make them intelligible to readers already acquainted with the principles of mechanics. To some extent standard proportions and examples of details are given, and empirical rules for cases which cannot be treated scientifically. But a great part of the work, and in the author's opinion the most useful part, deals with the region between applied mechanics and rule of thumb, viz. the discussion of the various considerations which should be present in the mind of the designer in applying scientific principles on the one hand, and which limit the reliance on purely practical experience on the other.

In an admirable Address to Section G of the British Association for the Advancement of Science, in 1894, Professor A. B. W. Kennedy said that an engineer is a man who is continually being called on to make up his mind. It may be only as to the size of a bolt, it may be as to the type of the Forth Bridge. But whatever it is, once it is settled it is decided irrevocably - it is translated into steel and iron and copper, and cannot be revoked by an act passed in another session. The matters are too complex to be dealt with mathematically or even physically. Even if they were not, there are few engineers who would have the special capacity to handle them. From this point of view the whole use of college training, of workshop practice, of practical experience is to provide the engineer later on with the means of critically examining each question as it comes up.

That statement puts very nearly what should be the function of a treatise on Machine Design like the present, in aiding the draughtsman in his daily work. What the author aimed at was, to gather together from science and practical experience the materials for critical examination of the problems which arise in machine designing, and, within such limitations as a short treatise permitted, to exemplify the process of criticism applied to the problems most frequently presented, whether in applying the formulae of applied mechanics or in following precedents. At every step the designer has to stop and weigh various considerations, the estimate of loads, the factor of safety, the limiting stress, the allowance for friction and wear, and many others. It is the multiplicity of these which render all practical problems in a sense indefinite, and from a purely scientific standpoint insoluble. But the recognition of them and the power of according to them their due relative importance in different circumstances is the distinction of the competent designer of machines.

A more consistent and scientific system of units could easily be adopted, but it would involve a departure from the modes of reckoning current in the workshop. Thu fault of many of the terms commonly used in the workshop and in books dealing with the subject of the strength of materials is, that they are applied to express both the forces acting on a structure and the deformations which are produced. Thus, compression means in ordinary usage either the stress acting on a bar or the strain due to its action. There is a further ambiguity arising from the use of the same words for a quantity and an intensity. Thus elongation and compression are used either for the whole deformation or for the deformation per unit of length.

The "present volume contains Chapters VI. and XII. to XVII. of the older editions of “Machine Design” but these chapters have been revised throughout and more than half the present volume is entirely new. It deals chiefly, though not exclusively, with steam-engine details. The subjects discussed form, it is believed, a definitely related group, and involve the application of mechanical principles to cases on the whole more complex than those examined in the first part of this treatise.

The general data of the strength of materials and the laws relating to the dependence of dimensions on straining actions are given in Part I. But in the cases to which these laws are applied in Part I. the straining actions are generally steady or statical, or may for practical purposes be treated as being so. In the design of Transmissive Machinery, which is treated of in Part I., in addition to questions of strength, kinematical problems are those most commonly presented In Part II., which relates to machines the object of which is to supply mechanical energy for doing work, the parts of which move at varying speeds, and are subjected to varying forces of inertia which are too large to be neglected, dynamical principles find a more frequent application.

A treatise such as this has a double function. It is partly a collection of designs, data, and rules, of use in the workshop and engineering drawing-office. But it differs from mere collections of precedents or pocket-books in this, that the reasons for different arrangements or proportions are, as far as possible, discussed in connection with the examples given. In the less complicated cases fairly definite rules can be given based on scientific principles, as a guide to the practical engineer. In the more complicated cases, where conditions are too indefinite for any precise theory, at least an indication can be given of the extent to which approximate solutions are to be distrusted. It is not the object of machine science to reduce every problem of construction to a rule of thumb. Rather it furnishes limits within which a free judgment, based on experience, has to be used and alternatives amongst which choice is to be made.

It is another function of such a book as this to serve as a text-book for engineering students, furnishing a series of practical problems. By working through such problems a student becomes acquainted with the various requirements involved in machine construction, and the considerations which guide an engineer in designing machines.

It is with a view to students that some problems are treated here in a fairly complete and systematic way, although necessarily with less completeness than in special treatises on particular parts of engineering. Such special treatises are generally too detailed to be convenient for students' use, and to understand them a larger knowledge of practical details is required than a student has time to master. It was specially from an experience of the want of some account of valve gears better suited to students than the large and admirable monographs already published that the author was led to extend the chapters on that subject. The best special monographs are intended to explain the use of a single method in solving all problems. But for a student's purpose an acquaintance with different methods is desirable. No one method is equally convenient in all cases, and initially it is useful to look at the same problem from different points of view.