THE appearance of the fourth edition of this book will do much to dispel the impression, prevalent in some quarters, that the technique of radio direction finding has been rendered obsolete by the introduction and development of radar in recent years. As the author explains in his introductory chapter, there are several fields in which direction finding will still have commitments for many years to come. The relative simplicity of the equipment required to give guidance to ships and aircraft, and particularly to those in distress, makes it an invaluable aid to navigation ; in another field, the network of direction finders provides the only means so far available of locating radio transmitting stations, and such information is frequently required under peacetime conditions as well as in time of war. Wireless Direction Finding By R. Keen. Fourth edition. Pp. xii + 1059. (London: Iliffe and Sons, Ltd., 1947.) 45s. net.