The Library Shelf

One of the most significant features of post‐war engineering has undoubtedly been the rapid growth of automatic control techniques and their widespread application. The importance of this development can be gauged not only by its technical possibilities but also by the fact that it is associated with its own theoretical tools and language with which to re‐examine old problems as well as to study new ones. Closed and open loop systems, feedback, transfer functions, and the use of operational calculus are rapidly becoming familiar elements in the speech and theoretical usage of engineers; and they are being accompanied by important changes in engineering philosophy. The functions of automatic control theory, or cybernetics, as it is sometimes called, were laid by Norbert‐Wiener and his colleagues, who were also responsible for the development of the related subject of Information Theory. An important problem of the latter topic is that of the response of an oscillatory system to a random input disturbance or noise. The successful solution of this problem has again had widespread applications, since essentially the same problem arises in many branches of engineering.