French). I have also great pleasure in acknowledging the help I have received on many occasions from my friend and colleague Prof. Henrici, F.R.S.
The references given in footnotes are mostly those of the original edition; in Chapters IX and X I have added English references where I was able to do so. The longer footnotes I am responsible for, except in cases where I have placed "R" after them. Of the notes at the end of the book I have added those which are placed in square brackets. A few of the notes in the original, which referred to matters with which English readers would probably be unacquainted, or to passages which have been altered in the text, have been shortened or omitted.
The names which Prof. Reuleaux gives to the various mechanisms have in most cases been invented by himself, and in several other instances he has had to coin words to express ideas to which individual distinctness has now first been given. Such names and words I have not tried to translate, but only to replace by equally good English ones, with what success I must leave my readers to judge. I shall be happy to receive suggestions for improvements in this matter. The names have, however, been very carefully considered, and so arranged as to fit in with each other—I venture to hope, therefore, that those who use them for instruction will not alter them without good reason. For the word "centroid," for which I anticipate great usefulness, I have to thank Prof. Clifford.
Prof. Reuleaux uses the word Kinematics in a limited sense, for the Science of constrained motion—that is, motion as it occurs in machines—without reference to the ideas of either time or force (p. 40, &c.), and has therefore called his book Theoretische Kinematik. Grundzüge einer Theorie des Maschinenwesens. Whether this use of the word be advisable or not, it was obviously impossible to adopt it in this country, where it has obtained firm hold in a much more extended, but quite legitimate, sense. While retaining the second part of Prof. Reuleaux's title I have therefore been compelled to change the first part to Kinematics of Machinery. It is very unfortunate that we have as yet no word for the study of motion as change of position merely. Phoronomy, which is used in Germany very nearly in this sense, is very unpreposessing;—I would suggest Metastatics for the purpose, unless
