1950. Revised edition. 47 pages. This is an ex-Library book. Pictorial dust jacket over pictorial cloth. Black and white diagrams throughout. Ex library copy with associated labels, inserts, stamps and annotations. Pages remain bright and clean with minimal tanning and foxing. Binding remains firm. Boards have light shelf wear with minor corner bumping and crushing to spine ends. Unclipped jacket has light edge wear with minor chipping and creasing.