this book is a review of recent developments in the application of variational methods to the study of the thermodynamics of continuous media. it contains a rich variety of examples from nonequilibrium thermodynamics, fluid dynamics, and chemistry, and should be quite useful as an exposition of the power of variational methods in these areas. the scope is not as broad as the title might suggest. the presentation of the theory of variational methods is brief and not very precise, and there is little mention of the much wider range of applications in other branches of engineering. hydraulic control systems by h. e. merrett is a comprehensive treatise on hydraulic and hydroelectronic control systems. the book fills a gap in the literature by presenting a carefully arranged and complete course in hydraulic control systems. despite its high academic level, the book is very valuable for practicing engineers because it contains precise indications for the design of components and entire systems. the author uses the graphical form imaginatively and avoids submerging the reader in a large number of equations. the arrangement of material in the book contributes greatly to the presenting of a clear picture of the subject. an important part of the book is devoted to a description of components of hydraulic servo-control systems, inclusive of their performance characteristics, and proceeding from the simple to the more sophisticated. chapter 10 contains a study of nonlinearities which are caused by shaped gain, backlash, and friction, and ends with a discussion of the problem of instability. pressure and flow-control valves are important elements of a loop. in chapter 11, the author describes those which use feedback in their operation and thus constitute a servo-loop. the book is concluded with a treatment of hydraulic power supplies, discussing their interaction with the control process as well as heat dissipation, not forgetting to draw the reader's attention to the importance of contamination and filtration—the seemingly trivial factors which must have caused him (and your reviewer) an inordinate amount of trouble.this book is a review of recent developments in the application of variational methods to the study of the thermodynamics of continuous media. it contains a rich variety of examples from nonequilibrium thermodynamics, fluid dynamics, and chemistry, and should be quite useful as an exposition of the power of variational methods in these areas. the scope is not as broad as the title might suggest. the presentation of the theory of variational methods is brief and not very precise, and there is little mention of the much wider range of applications in other branches of engineering. hydraulic control systems by h. e. merrett is a comprehensive treatise on hydraulic and hydroelectronic control systems. the book fills a gap in the literature by presenting a carefully arranged and complete course in hydraulic control systems. despite its high academic level, the book is very valuable for practicing engineers because it contains precise indications for the design of components and entire systems. the author uses the graphical form imaginatively and avoids submerging the reader in a large number of equations. the arrangement of material in the book contributes greatly to the presenting of a clear picture of the subject. an important part of the book is devoted to a description of components of hydraulic servo-control systems, inclusive of their performance characteristics, and proceeding from the simple to the more sophisticated. chapter 10 contains a study of nonlinearities which are caused by shaped gain, backlash, and friction, and ends with a discussion of the problem of instability. pressure and flow-control valves are important elements of a loop. in chapter 11, the author describes those which use feedback in their operation and thus constitute a servo-loop. the book is concluded with a treatment of hydraulic power supplies, discussing their interaction with the control process as well as heat dissipation, not forgetting to draw the reader's attention to the importance of contamination and filtration—the seemingly trivial factors which must have caused him (and your reviewer) an inordinate amount of trouble.