1922. Sixth edition. 80 pages. No dust jacket. Red cloth covered boards with gilt. Black and white diagrams throughout. Pages with some foxing and tanning, particularly to endpapers and textblock edges. Binding remains firm. Boards have moderate shelf wear with mild bumping and fraying to corners and crushing and fraying to spine ends. All surfaces tanned and sunned, particularly spine. Gilt lettering to spine is slightly dulled.