Su Song was best known for his horology treatise written in 1092, which described and illustrated in great detail his hydraulic-powered, tall astronomical clock tower built in Kaifeng. The clock tower featured large astronomical instruments of the armillary sphere and celestial globe, both driven by an early intermittently working escapement mechanism (similarly to the western verge escapement of true mechanical clocks appeared in medieval clockworks, derived from ancient clockworks of classical times). Su's tower featured a rotating gear wheel with 133 clock jack mannequins who were timed to rotate past shuttered windows while ringing gongs and bells, banging drums, and presenting announcement plaques. In his printed book, Su published a celestial atlas of five star charts. These star charts feature a cylindrical projection similar to Mercator projection, the latter being a cartographic innovation of Gerardus Mercator in 1569.