Edward Norton Lorenz derived his equations from Saltzman's free convection equations,
simplified as to get the essence of the nonperiodic behaviour noticed about their variations. Written in 1962 and published in 1963, his original paper contains all the main characters of the chaos theory  although this theory didn't exist at the time : the use of a computer, simplifications of the equations, sensivity to initial conditions, use of Poincaré's map, etc ... Original Lorenz paper  Deterministic nonperiodic flow (1963)  PDF 
