IT has been the aim of the author to prepare a book for all who are interested in gas engines — students, draughts~ men, engineers, as well as the men who operate gas engines of any kind, and wish to become better acquainted with the theory and the why of many things. The book should be of special interest to the technical student, and was, in fact, first prepared for the engineering classes at the Michigan Agricultural College, since no suit able text — book could be found. The reading-matter throughout has been arranged care fully and with a definite object in view. The large number of figures illustrating the text have been made as simple as possible. It has also been the aim of the author to make the treatment clear and concise, and for this reason every paragraph should be studied — not merely read over. It is hoped that this book will enable every earnest stu dent to acquire a foundation upon which he may eventually build a broad and comprehensive knowledge of the subject. Acknowledgment is due Professor L. L. Appleyard for his kindly criticism and assistance in reading the proofs.