1921. Sixth Edition. 138 pages. No dust jacket. Red cloth with gilt lettering. Contains a portrait of the author to frontispiece and 5 diagrams. Pages are moderately tanned with mild foxing. Heavy tanning to front pastedown. Notable cracking to gutters. Pencil inscription to front endpaper. Mild thumb-marking present. Pencil annotations to p.1-8. Mild droplet staining to text block edges. Boards have mild edgewear with corner crushing and mild marking to boards. Heavy tanning to spine, which has mild crushing and tearing to ends. Book has forward lean.