Antoine Augustin Cournot (diagram popularized by Alfred Marshall)
Core Principles & Analysis
Cournot's Recherches (1838) drew the first demand curve and Jenkin added supply in 1870; Marshall's Principles (1890) popularized the crossing-curves diagram.
Prices rise when quantity demanded exceeds quantity supplied at the going price, signalling scarcity to both sides.
The elasticity of each curve determines how much a shock moves price versus quantity.