1971. Revised Edition. 93 pages. No dust jacket. Red cloth covered boards. Black and white diagrams. Book has a slight smoky odour. Pages with some foxing and tanning, particularly to endpapers and textblock edges. Binding remains firm. Minor pencil inscriptions to front free endpaper and small bookshop label. Boards have moderate shelf wear with mild bumping and fraying to corners and crushing and fraying to spine ends. Some marks and staining to surfaces. Some light pen tracing around gilt title.