This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
In Fig. 44 is shown a half-turn platform stairway. The face of the top riser of the lower flight and the bottom riser of the upper flight have a slight curve adjacent to the cylinder. By this method the whole depth of the cylinder is saved in the run of the stairway - an advantage where space is limited.