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From the PREFACE TO THE FIRST EDITION. This little book was begun some four or five years since, when the want of any English text-book of machine design had become very apparent to the author. Its progress has been delayed by the pressure of other work, and it has been completed in rather restricted intervals of leisure. Nevertheless, no labour has been spared in condensing into the smallest compass the information at the author's disposal, and in endeavouring to render the treatment of the subject simple and clear. If the student's path has in any degree been rendered easy, it is because a good deal of labour has been expended on the roadway. Long experience has enabled engineers to proportion special machines in a very perfect way, and no great improvement can be expected from a theoretical study of the strength of their different parts. The empirical rules current in the drawing office, are sufficient for the construction of ordinary machines. The present treatise is not merely a collection of such rules. Its primary object is to explain the principles which are available as guides in machine construction. So far as it succeeds in this, it will place the draughtsman in the best position to make use of the facts which come under his notice in the workshops and the drawing office, and will enable him to apply that experience in dealing with new materials, with new forms of construction, and with novel conditions of force and speed. In addition, this text-book contains a selection of practical rules and empirical proportions, for various parts of machines. These are not intended to override the draughtsman's own judgment and experience. All such rules have a more or less limited application, and the most that can be done is, to indicate how such matters can best be dealt with. The author has endeavoured to avoid excessive minuteness in giving the empirical proportions of machine parts, and he has usually left a certain range of choice open. That which appears least desirable, in a text-book of this kind, is to reduce designing to mere rule of thumb. In good designing, judgment, foresight, knowledge and science must be constantly brought to bear. Many rules for machine design are rational in form, but are affected by arbitrary coefficients, intended to allow for contingencies which are neglected. In such cases, the arbitrary part of the rule has, in this treatise, generally been distinguished from the rational part. Thus, in dealing with shafting, the diameter is determined by the law of torsional resistance, but the coefficients, in the rules ordinarily given, are made up of two parts, one belonging to the rational formula for torsional resistance, the other, an arbitrary factor of safety, which is intended to make allowance for the undetermined bending action. In this treatise, the two parts of the coefficient are kept separate, so that it may be seen what amount of undetermined straining action is actually allowed for.

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