These lectures have been rewritten and considerably extended, and have been given a form which is more appropriate to this method of presentation of the subject. The account of the gradual development of the philosophy of the steam-engine has been extended and considerably changed, both in arrangement and in method. That part in which the direction of improvement during the past history of the steam-engine, the course which it is to-day taking, and the direction and limitation of that improvement in the future, are traced, has been somewhat modified to accord with the character of the revised work. The author has consulted a large number of authors in the course of his work, and is very greatly indebted to several earlier writers. Of these, Stuart[1] is entitled to particular mention. His "History" is the earliest deserving the name; and his "Anecdotes" are of exceedingly great interest and of equally great historical value. The artistic and curious little sketches at the end of each chapter are from John Stuart, as are, usually, the drawings of the older forms of engines. [1] "History of the Steam-Engine," London, 1824. "Anecdotes of the Steam-Engine," London, 1829. Greenwood's excellent translation of Hero, as edited by Bennett Woodcroft (London, 1851), can be consulted by those who are curious to learn more of that interesting old Greek treatise. Some valuable matter is from Farey,[2] who gives the most extended account extant of Newcomen's and Watt's engines. The reader who desires to know more of the life of Worcester, and more of the details of his work, will find in the very complete biography of Dircks[3] all that he can wish to learn of that great but unfortunate inventor. Smiles's admirably written biography of Watt[4] gives an equally interesting and complete account of the great mechanic and of his partners; and Muirhead[5] furnishes us with a still more detailed account of his inventions. [2] "Treatise on the Steam-Engine," London, 1827. [3] "Life, Times, and Scientific Labors of the Second Marquis of Worcester," London, 1865. [4] "Lives of Boulton and Watt," London, 1865. [5] "Life of James Watt," D. Appleton & Co., New York, 1859. "Mechanical Inventions of James Watt," London, 1854. For an account of the life and work of John Elder, the great pioneer in the introduction of the now standard double-cylinder, or "compound," engine, the student can consult a little biographical sketch by Prof. Rankine, published soon after the death of Elder. The only published sketch of the history of the science of thermo-dynamics, which plays so large a part of the philosophy of the steam-engine, is that of Prof. Tait--a most valuable monograph.