NO attempt has been made to present in this book an extensive description of all the existing types of compressors or of the count less appliances using compressed air. The author's chief aim was to provide the student, who is interested in technical ques tions concerning the operation as well as the construction of compressors and air engines, with a background of understand ing that will enable him, not only to solve the many theoretical problems connected therewith, but to make independent research into the seemingly unlimited possibilities of compressed air. The territory still unexplored is vast and full of promises to the intrepid explorer who enters the field with a thorough knowledge of all the truths discovered, as well as the pitfalls encountered, by those who have gone before him. The numerous, carefully selected problems constitute what the author believes to be one of the strong features of this book. If ever any doubt lingers in the student's mind as to the mean ing of certain principles or laws presented in the text and their practical application, a numerical problem will, as a rule, re move the doubt and make clear the meaning. Moreover, such problems make the student familiar with actual quantities, never revealed by mere formulas; quantities which are Often startling to the uninitiated and impress him with the practical value of such formulas more forcibly then the mere text can do.