[1] 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.
[2] This data was crucial for assessing an engine's efficiency.
Southern became a partner of the firm of Boulton & Watt in 1810.
[1] The use of the instrument was kept as a trade secret for a generation, only becoming public in the 1830s.
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