This figure appears in [i]Horologium oscillatorium sive de motu pendulorum ad horologia aptato demonstrationes[/i] by [url=https://www-history.mcs.st-andrews.ac.uk/Biographies/Huygens.html]Christiaan Huygens[/url] (1629-1695), p. 38. In Proposition XIV on the cycloid, Huygens states that the circular arc, BG, has the same length as the line segment, EG. The slider enables the ‘wheel’ to roll along the horizontal plane. Huygens’ work is in the collection of [url=https://www.marshlibrary.ie/catalogue/Record/15618]Marsh’s Library[/url], Dublin.