John Southern (c.1758–1815) was an English engineer, son of Thomas Southern of Derbyshire. He co-invented the indicator diagram with his employer, James Watt, in 1796. Southern became a partner of the firm of Boulton & Watt in 1810. The use of the diagram was kept as a trade secret for a generation, only becoming public in the 1830s.
A Treatise upon Aerostatic Machines, 1785, by John Southern
^ abH. W. Dickinson (31 October 2010). James Watt: Craftsman and Engineer. Cambridge University Press. pp. 131–. ISBN 978-1-108-01223-2.