In this paper we describe a formal language for a class of logical expressions. We then present a Finite State Machine for recognition and for evaluation of this language. The main interest of the language is its historical characteristic. This language invented by the british scholar W. Stanley JEVONS in 1865 is probably the earliest language where expressions were evaluated by a Finite State Machine. The two outstanding contributions were the use of a machinery to evaluate formulas and the evaluation of formulas with variables by several evaluations with constants. The contribution of this paper is to present this ancient evaluation process in a contemporaneaous framework, i.e. formal languages and automata.
