This Volume concludes the subject of the work with reference principally to Steam Generators and Transmission of Power. The steam generator cannot be much further improved upon. Of the many good types in use, it is a matter for judgment by the experienced engineer to select a type of boiler suitable under given circumstances. The steam boiler and engine combined as a prime mover has, of late years, been subjected to keen rivalry by the Gas Engine. The important question is whether the boiler or gas producer is to be the apparatus for the utilisation of fuel for power generation in the future. It is a significant fact, and one of great importance to all young engineers, that the gas producer is gaining in efficiency and popularity daily, and that small steam engines are not so much used as formerly was the case. In the transmission of power, electricity and compressed air are the chief mediums both have their special sphere of successful application. They are brieﬂy considered as accessories to prime movers. Throughout the whole work an endeavour has been made to present every type of modern and practicable prime mover completely and clearly to the reader. Among them some old types have been given a place. Old machines still find a place where they are appreciated under conditions where newer types would not do so well. The subject of prime movers is one of never ending interest and importance. Electricity and compressed air as transmitters have made it more generally possible to apply power where animal and manual labour was only applicable hitherto. No prime mover of practical service has been omitted. The Author's thanks are due to inventors and manufacturers for much information and many illustrations of great interest on recent improvements.