In existographies, John Southern (1758-1815) was an engineering assistant and employee to engineers Brit Matthew Boulton and Scot James Watt notable for his 1796 invention of the indicator diagram, a modification of the earlier circa 1790 invention of the indicator, a device for measuring steam pressure in the piston and cylinder.

In 1782, Matthew Boulton (1728-1809) introduced Southern, then aged 24, an aspiring musician of sorts, to James Watt, who thereafter became Watt’s assistant and right-hand man, becoming partner in 1810. (Ѻ)

Indicator
The ‘indicator’ was invented by engineers Matthew Boulton and James Watt in circa 1790 and evidence for its use is found in a long series of 1793 experiments of Watt’s friend George Lee at the Philips & Lee cotton spinning factory, which had bought a Watt rotative engine in 1792. In his experiments, Lee had used an indicator to measure the highest and lowest pressures reached in the cylinder.

Indicator diagram
In early 1796, Southern invented the indicator diagram; a version which is shown below: (Ѻ)

On 12 April 1796, after hearing of Southern’s invention of the indicator diagram method, Lee wrote:

“I am like a man parch’d with thirst in the expectation of relief, or a woman dying to hear (or tell) a secret—to know Southern’s mode of determining power.”


Diagram of the indicator (vertical part), which gives a reading of pressure, invented by James Watt and Matthew Boulton in 1790, and the sliding board (horizontal part) and pencil tracer (attached to the indicator), invented by Southern in 1796, which makes an 'indicator diagram', the pressure-volume graph (drawn on the sheet).

Diagram of the indicator (vertical part), which gives a reading of pressure, invented by James Watt and Matthew Boulton in 1790, and the sliding board (horizontal part) and pencil tracer (attached to the indicator), invented by Southern in 1796, which makes an 'indicator diagram', the pressure-volume graph (drawn on the sheet).

Southern's invented device was that whereby a pencil replaced the pointer on the indicator and traced a line, recording the pressure, on a paper which moved backward and forward in pace with the piston. The line traced out was a graph, representing a plot of the pressure against the displacement of the piston. A week later, after being told more about the invention, he commented:

“Southern’s scheme is highly scientific and ingenious but will require nicety in the execution—a good experiment upon a pump engine would not be amiss.”

Watt and Southern's device, afterwards given the name ‘indicator diagram’, were said to have quickly realized that the indicator diagram could be used to give a direct measure of an engine’s power. [2]

Thermodynamics
Southern's indicator diagram method was utilized in the 1834 paper "Memoir on the Motive Power of Heat" by French engineer Emile Clapeyron in such a way as to put the 1824 verbal description of the Carnot cycle into a more rigorous graphical methodology as to how the pressure volume work the working body of the steam engine does in one cycle. This paper then came into the hands of William Thomson (1848) and from him into the hands of Rudolf Clausius (1850) after which the science of thermodynamics was founded. [3]

Education
Little seems to be known about Southern. He authored the 1785 book A Treatise on Aerostatic Machines and contributed to the 1814 Encyclopedia Britannica and steam and steam engines. [3] Southern, in latter years, was described as one of Watt's good friends.

Quotes
The following are related about quotes:

“The indicator was invented by Watt and the sliding board and tracer were afterwords added by Southern.”
— Thomas Tredgold (1838) [1]

References
1. (a) Okil, John. (1938). Autographic Indicators for Internal Combustion Engines (pg. 10). Arnold.
(b) Tredgold, Thomas. (1927). The Steam Engine: Comprising and Account if its Invention and Progressive Improvement. London: J. Taylor.
2. Hills, Richard L. (1989). Power from Steam: A History of the Stationary Steam Engine (Southern, 12+ pgs.; esp. pg. 92; George Lee, pg. 91). Publisher.
3. Baird, D. (1989). “Instruments on the Cusp of Science and Technology: The Indicator Diagram”, in: Knowledge and Society (pgs. 107-). JAI Press.
4. (a) Southern, John (1785-1818) (engineer) – WorldCat Identities.
(b) Robinson, John, Southern, John, Watt, James, and Brewster, David. (1818). “The Articles Steam and Steam Engines written for the Encyclopedia Britannica” (link), John Murray.

External links
● John Southern (engineer) – Wikipedia.