Nearly seventeen years ago I translated for the Philosophical Magazine the first 'of this series of Memoirs, by Professor Clausius, on the Mechanical Theory of Heat. A short time afterwards the Essay of Professor Helmholtz, Ueber die Erhal tang der Kraft, was placed in my' hands: I translated it, and had it published in the continuation of Taylor's Scientific Memoirs.' It was thus my fortune to introduce to the sci entific public of England the earliest writings of two of the most celebrated contributors to the great theory in question. For many years subsequent to the period here referred to, I was careful to translate, or to have translated, every paper published by these two writers; and the fact that the fol lowing series of these Memoirs is thought worthy of being presented in a collected form to the English public, proves that I did not overestimate their importance. I have been asked by its publisher to write a line or two of introduction to the present volume. This I could not refuse to do though I feel how super?uous it must be; for the name and fame of Professor Clausius stand as high in this country as in his own. My Introduction therefore shall be confined to this brief statement of my relation ship to his writings. They fell into my hands at a time when I knew but little of the Mechanical Theory of Heat. In those days their author was my teacher; and in many respects I am proud to acknowledge him as.