Text-books on Mechanics are of three sorts. Many teachers, school-committees, and parents wish to add a taste of Mechanics to the smatterings of twenty or thirty different subjects which constitute "liberal education," as understood in American high schools and colleges. For this purpose it is of the first importance that the text-book should be brief, for the time to be devoted to it is very short; secondly, it must divest the subject of every perplexity and difficulty, that it may be readily understood by all young persons, though of small capacity and less application. Such a text-book can contain nothing beyond the statement, without proof, of the more important principles, illustrated by familiar examples, and simple explanations of the commonest phenomena of motion, and of the machines and mechanical forces used in the arts. To a few it seems that more light comes into a room through two or three broad windows, though they be all on one side, than through fifty bull's-eyes, scattered on every wall. But the many prefer bull's-eyes,--fifty narrow, distorted glimpses in as many directions, rather than a broad, clear view of the heavens and the earth in one direction. Hence superficial, scanty text-books on science are the only ones which are popular and salable. The thorough study of Mechanics is, or should be, an essential part of the training of an architect, an engineer, or a machinist; and there are several text-books, like Weisbach's Mechanics and Engineering, intended for students preparing for any of these professions, which are complete mathematical treatises upon the subject. Such text-books are invaluable; they become standard works, and win for their authors a well-deserved reputation.