The professional engineer, in writing a book for young engineers, is likely to forget that the novice is unfamiliar with many terms which are like daily bread to him. The present writers have tried to avoid that pitfall, and to define each term as it naturally needs definition. More over, the description of parts and the definitions of terms have preceded any suggestions on operation, the authors believing that the young engineer should become thor oughly familiar with his engine and its manner of work ing, before he is told what is best to do and not to do. If he is forced on too fast he is likely to get mixed. The test questions at the end of Chapter III. Will show how perfectly the preceding pages have been mastered, and the student is not ready to go on till he can answer all these questions readily. The system of questions and answers has its uses and its limitations. The authors have tried to use that sys tem where it would do most good, and employ the straight narrative discussion method where questions could not help and would only interrupt the progress of thought. Little technical matter has been introduced, and that only for practical purposes. The authors have had traction engines in mind for the most part, but the directions will apply equally well to any kind of steam engine.