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Mechanical properties of solid and fluid bodies; the second volume, completing the work, will treat on what Dr Whewell has called the Secondary Mechanical Sciences, namely those relating to Sound, Liglit, and Heat; this volume is already written and will soon be sent to press.

As in former elementary works the plan is adopted of breaking up the subject into numerous short Chapters which are to a great extent independent of each other; thus the attention of the student is required for only a moderate portion at one time, and if he finds a difficulty in thoroughly mastering a particular Chapter he may pass on to the following Chapters, and afterwards recur to the passages not understood at first.

The Examples, which are above 500 in number, form an important part of the work; many of them are original, while the rest have been selected from the Examination papers published by the Universities and other Examining budies. These Examples will be found, it is believed, not too difficult for the use of an early student; while they will afford real exercises to test his knowledge and his power of application. Both the text and the Examples have been arranged with the view of meeting fairly all the difficulties that may occur in the course of study; it is quite possible to give to a work a fallacious appearance of simplicity by omitting every point that requires close attention, and by constructing examples which all resemble a few familiar types, and so may be solved almost without thought. The student however who wishes to master any science, or to pass an examination in it, must be willing to make the exertion which is necessary in order to comprehend the whole of it, and not merely easy selections from it; and he must be prepared to encounter a wide variety of examples and prob'cnis.

An introduction to Natural Philosophy may be used by different classes of readers; some may intend hereafter to devote themselves to the earnest study of the subject either on its theoretical or its experimental side; some may be looking forward to professional occupation with the numerous practical applications of science; while others may seek for such knowledge as will give them an intelligent interest in the phenomena of the world, and in the discoveries and inventions which proceed from the regular cultivators of the subject. To all these classes it is important that the notions at first acquired should be accurate; and I venture to repeat with respect to the present work the hope expressed ten years since with respect to another, namely that the beginner will here find a satisfactory foundation for his future studies, so that afterwards he will only have to increase his knowledge, without rejecting what he originally acquired. An elementary writer may well propose to himself as one of his main objects that those who use his work should have nothing afterwards to unlearn; and this has been recently explicitly recognized by more than one eminent authority.

1. TODHUNTER.

JANUARY, 1877.

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