AUTHOR'S NOTE In the comparatively short space of five years, the turbine power unit has established itself on military types, and its employment is now being extended to civil aircraft. Already the modern gas turbine is available in units of far greater power than are obtainable with orthodox reciprocating engines. After the successful early stage of development of turbine-jet units, the industry is now engaged on a second phase comprising turbines driving air-screws, which give improved performance at take-off and at normal flying altitudes. A renewed concentration on pure jet units may occur in the course of a few years when pressurised cabins have been perfected to enable regular and economical high-altitude flight, and the more complex problems compressibility associated with ultra-high speedcaircraft are finallycovercome. The three previous editions issued during the war were mainly devoted to the fundamentals of jet propulsion, the importance of the metallurgical problems, early projects, patent reviews and notes as to the possible lines of development it this gas turbine then in its infancy. In the interim several of the possibilities discussed have been developed to a practical stage as a result of intensive research and development. Opinion grows that the gas turbine will eventually displace the reciprocating power unit on aircraft, and will extend its influence to other fields of engineering such as marine work, locomotives, and stationary power plants. In this, the fourth edition, a great deal of additional matter and comment have been introduced embracing almost every phase of modern gas turbine design, operation and servicing technique. Quotations from papers delivered by aeronautical and turbine engineers in recent months have been freely cited to provide a broad survey of the trend of thought and progress (pg 8).