John Southern (engineer)

John Southern (c.1758–1815) was an English engineer, son of Thomas Southern of Derbyshire. In 1796 he and his employer James Watt co-invented the Indicator, an instrument for measuring and recording the pressure inside a steam engine cylinder through its stroke. This data was crucial for assessing an engine's efficiency. Southern became a partner of the firm of Boulton & Watt in 1810. The use of the instrument was kept as a trade secret for a generation, only becoming public in the 1830s.