during this time tsien wrote "engineering cybernetics" which was published by mcgraw hill in 1954. the book deals with the practice of stabilizing servomechanisms. in its 18 chapters it considers non-interacting controls of many-variable systems, control design by perturbation theory, and von neumann's theory of error control (chapter 18). ezra krendel reviewed the book, stating that it is "difficult to overstate the value of tsien's book to those interested in the overall theory of complex control systems." evidently tsien's approach is primarily practical, as krendel notes that for servomechanisms the "usual linear design criterion of stability is inadequate and other criteria arising from the physics of the problem must be used."